Lens design for low-parallax panoramic camera systems

Abstract

A lens format for a low-parallax imager includes multiple imaging lens elements arranged to capture adjacent fields of view. The imaging lens elements include multiple lens element groups for directing incident light to an imaging surface. The lens elements control the distance between an entrance pupil placed behind the imaging surface and a low-parallax point to reduce parallax error between adjacent imaging lens elements.

Description

[Technical Field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This disclosure claims the benefit of priority to U.S. Provisional Patent Application No. 62 / 865,741, entitled "Opto-Mechanics of Panoramic Capture Devices with Abutting Cameras," filed June 24, 2019; U.S. Provisional Patent Application No. 62 / 952,973, entitled "Opto-Mechanics of Panoramic Capture Devices with Abutting Cameras," filed December 23, 2019; U.S. Provisional Patent Application No. 62 / 952,983, entitled "Multi-camera Panoramic Image Capture Devices with a Faceted Dome," filed December 23, 2019; and U.S. Provisional Patent Application No. 62 / 972,532, entitled "Integrated Depth Sensing and Panoramic Camera System," filed February 10, 2020, each of which is incorporated by reference in its entirety.

[0002] The present disclosure provides a method for detecting multiple adjacent Oh The present disclosure also relates to a camera lens design that captures incident light from a polygonal field of view to form a polygonal image. [Background technology]

[0003] Panoramic cameras have substantial value due to their ability to simultaneously capture wide-angle images. The earliest such example is the fisheye lens, an ultra-wide-angle lens that produces strong visual distortion while capturing a wide panoramic or hemispherical image. While the field of view (FOV) of a fisheye lens is typically between 100 and 180 degrees, this approach has been extended to even wider angles, including a range of 220 to 270 degrees, such as that proposed by Y. Shimizu in U.S. Patent 3,524,697. Alternatively, there are mirror- or reflection-based cameras that capture circular panoramic images, such as the system proposed by P. Greguss in U.S. Patent 4,930,864. While these technologies continue to evolve, they struggle to provide full hemispherical or spherical images at the resolution and image quality currently required by modern applications.

[0004] As another alternative, panoramic multi-camera devices, in which multiple cameras are arranged around or around the circumference of a sphere, are becoming increasingly popular. However, in most of these systems, including those described in U.S. Patents 9,451,162 and 9,911,454 to A. Van Hoff et al., both of Jaunt Inc., the cameras are sparsely distributed on the exterior surface of the device. To capture a complete 360-degree panoramic image, including the gaps or seams between adjacent individual cameras, the cameras then have extended FOVs that overlap with each other. In some cases, as much as 50% of the camera FOV or resolution may be used for overlap between cameras, which also creates substantial parallax differences between the captured images. Parallax is the visual perception that the position or orientation of an object appears different when viewed from different directions. Then, in subsequent image processing, both the excess image overlap and parallax differences complicate and significantly slow efforts to properly combine, tile, or stitch together acceptable images from images captured by adjacent cameras.

[0005] There are also panoramic multi-camera devices in which multiple cameras are arranged around or around the circumference of a sphere, with adjacent cameras touching along part or all of their adjacent edges. As an example, U.S. Patent 7,515,177 to K. Yoshikawa describes an imaging device with multiple adjacent image pickup units (cameras). Images are collected from cameras with overlapping fields of view to compensate for mechanical errors.

[0006] Opportunities remain for improving the optical design and functionality of imaging lens systems that can be used in low-parallax panoramic multi-camera devices. Potential optical improvements may also have direct and indirect benefits or synergistic effects on the optomechanical design of the individual camera lens systems and the device as a whole, particularly related to issues at or near the seams between adjacent cameras. [Brief explanation of the drawings]

[0007] [Figure 1] 1 shows a portion of a multi-camera device capture device, and specifically the 3D views of its two adjacent cameras. [Figure 2A] 1 shows a cross section of a portion of a camera lens assembly, including lens elements and ray paths. [Figure 2B] 1 shows a cross section of a portion of a camera lens assembly, including lens elements and ray paths. [Figure 3] 1 shows a cross-sectional view of a portion of a typical multi-camera capture device, illustrating FOV overlap, field of view, overlap, seams, and blind areas. [Figure 4] 1 shows two polyhedral shapes, a regular dodecahedron and a truncated icosahedron, for which multi-camera capture devices can be designed and manufactured. [Figure 5A] Figure 5B shows the optical geometry for the field of view for adjacent hexagonal and pentagonal lenses, as might occur in a device with a truncated icosahedral geometry. Figure 5B shows the enlarged region of Figure 5A in greater detail. [Figure 5B]Figure 5B shows the optical geometry for the field of view for adjacent hexagonal and pentagonal lenses, as might occur in a device with a truncated icosahedral geometry. Figure 5B shows the enlarged region of Figure 5A in greater detail. [Figure 5C] FIG. 5C shows an example of a paraxial NP point or low parallax (LP) quantity located near both the entrance pupil and the device center. [Figure 5D] The parallax difference for two adjacent cameras is shown relative to the perspective center. [Figure 5E] 1 shows the front color at the edge of the outer compressor lens element. [Figure 6] 1 shows a distortion correction curve plotted on a graph showing the percentage of distortion versus fractional field. [Figure 7] The fields of view for adjacent cameras are shown, including both core and extended fields of view (FOV), both of which can be useful for the design of optimized panoramic multi-camera capture devices. [Figure 8A] 1 shows a cross-sectional view of an improved imaging lens system with reduced parallax. [Figure 8B] 8B shows a graph of calculation residuals versus perspective center difference for the lens of FIG. 8A. [Figure 8C-1] 8A provides lens prescription data for the lens of FIG. 8A. [Figure 8C-2] 8A provides lens prescription data for the lens of FIG. 8A. [Figure 8D] The calculated resolution for the lens of FIG. 8A is provided as a graph of the MTF. [Figure 8E] 8A provides a graph of the calculated astigmatism and distortion at the image plane for the lens of FIG. 8A. [Figure 8F] 8B provides a graph of calculated relative illuminance for the lens of FIG. 8A. [Figure 8G] 8B provides an illustration of a front collar for the lens of FIG. 8A. [Figure 9A] 1 provides a cross-sectional view of an alternative low parallax lens design. [Figure 9B]9B shows a cross-sectional view of an enlarged portion of FIG. 9A. [Figure 10A] 10 provides a cross-sectional view of a second alternative low parallax lens design. [Figure 10B] 10B provides an alternative cross-sectional view of the lens of FIG. 10A. [Figure 10C] The calculated resolution for the lens of FIG. 10A is provided as a graph of MTF. [Figure 10D] 10A provides a graph of calculated relative illuminance for the lens of FIG. 10A. [Figure 11] 10 provides a cross-sectional view of a third alternative low parallax lens design. [Figure 12] 10 provides a cross-sectional view of a fourth alternative low parallax lens design. [Figure 13] 1 shows a cross-sectional view of an improved optomechanical structure for a multi-camera capture device and a 3D view of its camera channels. [Figure 14] A cross-sectional view of two adjacent stepped edge angle compressor lenses at and near the seam and a magnified view of part (B) thereof are shown. [Figure 15] 1 shows an electronic system diagram for a multi-camera capture device. [Figure 16] 10A-10C show different views of an outer compressor lens element with a lens datum and an associated lens housing with a datum feature. [Figure 17] 1 shows a cross-sectional view of a portion of an assembly of an improved multi-camera panoramic image capture device, and some details thereof; [Figure 18A] 1 shows a series of views of components and features of a lens housing and channel loading support that can be used during assembly of an improved multi-camera panoramic image capture device. [Figure 18B-1] 15 shows a cross-sectional view of an alternative version of the lens housing and its interaction at or near the seam to that shown in FIG. 14. [Figure 18B-2] 18B shows a cross-sectional view of an alternative version of the inter-channel datum to that shown in FIG. 18A. [Figure 19]13 shows a view of another alternative design for mounting the camera channel to the central support. [Figure 20] 1 shows a cross-sectional view of an optical system in which a low parallax imaging lens is combined with a relay optical system. DETAILED DESCRIPTION OF THE INVENTION

[0008] As commonly understood in the field of optics, a lens or lens assembly typically comprises a system or device having multiple lens elements mounted within a lens barrel or housing and working together to generate an optical image. An imaging lens captures a portion of light coming from an object or objects residing in object space at some distance or distances from the lens system. The imaging lens can then form images of these objects at an output "plane," with the images having a finite size determined by magnification, as determined by the focal length of the imaging lens and the conjugate distances to the object(s) and image plane for that focal length. The amount of image light passing through the lens, from object to image, is largely determined by the size of the imaging lens's aperture stop, which is typically quantified by one or more values ​​for numerical aperture (NA) or F-number (F# or F / #).

[0009] The image quality provided by an imaging lens is determined by many characteristics of the lens design, including the choice of optical materials used in the design, the size, shape (or curvature) and thickness of the lens elements, the relative spacing of the lens elements from one another, the spectral bandwidth, polarization, the optical load (power or flux) of the transmitted light, optical diffraction or scattering, and / or lens manufacturing tolerances or errors. Image quality is typically described or quantified in terms of lens aberrations (e.g., spherical, coma, or distortion) or the relative size of the resolved spot provided by the lens, and is often also quantified by the modulation transfer function (MTF).

[0010] In a typical electronic or digital camera, an image sensor is nominally located at the imaging plane. This image sensor is typically a CCD or CMOS device, physically attached to a heat sink or other heat removal means, and also includes electronics that power the sensor and readout and communication circuitry that provides image data to data storage or image processing electronics. Image sensors typically have a color filter array (CFA), such as a Bayer filter, within the device, with color filter pixels aligned with the image pixels to provide an RGB (red, green, blue) pixel array. Alternative filter array patterns, including CYGM (cyan, yellow, green, magenta) filters or RGBW filter arrays (W = white), can alternatively be used. Image sensors typically include a cover glass that can seal or protect the sensor from the environment. The cover glass can also provide UV or IR cut filtering via a thin-film interface or dichroic coating, or this function can be provided on a separate window or external filter. The UV or IR cut filter reduces the level of non-visible light incident on the sensor that is associated with the image light. Alternatively, or additionally, UV and IR cut filtering can be provided by coatings applied to the lens elements, including the outer surface of the outer lens element. The cover glass or filter can also be a UV light absorbing glass, providing UV filtering through a combination of absorption and coating reflectance.

[0011] Note that for many image sensors, image light is incident directly on the sensor pixels. However, many commercially available image sensors also include an integrated lenslet array that overlaps and is aligned with the pixel array. A given lenslet directs a portion of the incident image light toward the corresponding underlying pixel. This approach can reduce optical crosstalk between pixels because the incident image light is more likely to be photoelectrically converted into an electronic signal within the incident pixel than would otherwise be the case. As pixels become progressively smaller, this approach is becoming increasingly popular as a means to improve or maintain image resolution (MTF). Such sensors with integrated lenslet arrays can be used in the improved low-parallax camera (320) of this approach.

[0012] In typical usage, many digital cameras are used by people or remote systems in relative isolation to capture images or photographs of a scene without any dependency or interaction with any other camera devices. In some cases, such as surveillance or security, camera operation can be directed by people or algorithms based on image content seen from another camera that has already captured overlapping, adjacent, or nearby image content. In another example, people capture panoramic images of a scene, such as a landscape scene, with an extended or wide-angle FOV by sequentially capturing a series of adjacent images while manually or automatically moving or panning to assemble the adjacent images. Image processing software, such as Photoshop or Lightroom, can then be used to stitch, mosaice, or tile the adjacent images together to represent a larger, extended scene. Image stitching or photo stitching is the process of combining multiple photographic images with overlapping fields of view to generate a segmented panoramic or high-resolution image. Image quality improvements, including exposure or color correction, can also be applied either in real time, or in a post-processing or image rendering phase, or a combination thereof.

[0013] Unless objects in a scene are directionally illuminated and / or have directional optical responses (e.g., through reflection), the available light is plenoptic, meaning that there is light (λ) traveling in all directions, or nearly so, within a given space or environment. The camera can then sample a subset of this light as image light and use it to provide a generated image showing a given view or perspective of different objects in the scene at one or more times. If the camera is moved to a different nearby location and used to capture another image of that same scene portion, both the apparent perspective and relative positions of the objects change. In the latter case, one object may now partially occlude another, while a previously hidden object becomes at least partially visible. These differences in the apparent positions or orientations of objects are known as parallax. Specifically, parallax is the displacement or difference in the apparent positions of objects viewed along two different lines of sight, measured by the angle or half-angle of inclination between these two lines.

[0014] In stereoscopic image capture or projection systems, dual-view parallax is a cue, along with shading, occlusion, and perspective, that can provide the perception of depth. For example, in a stereoscopic (3D) projection system, pairs of polarized or spectrally encoded images are projected superimposed on a screen and can be viewed by audience members wearing appropriate glasses. The amount of parallax may have an optimal range, outside of which the resulting perception of depth is either too small to be practically noticed by audience members or too large to be properly fused by the human visual system.

[0015] On the other hand, in panoramic image capture applications, parallax differences can be considered an error that can complicate both the stitching and appearance of the images. In the example of manually capturing a series of individual panoramic landscape images, if objects in the scene are sufficiently far apart (e.g., optically infinity), visual differences in perspective or parallax across the images may be too small to notice. Panoramic capture devices integrated with rotating or multi-cameras have the potential to continuously capture real-time image data at high resolution without relying on the uncertainty of manual capture. However, such devices may also introduce errors, including their own binocular parallax, image artifacts, or those of parallax, perspective, and exposure. While the resulting images can often be successfully stitched together using image processing algorithms, input image errors complicate image processing, lengthening processing times, and sometimes leaving visually apparent residual errors.

[0016] To provide context, FIG. 1 illustrates an integrated panoramic multi-camera capture device 100 with two adjacent cameras 120 within a housing 130 designed for reduced-parallax image capture. These cameras are alternatively referred to as image pickup units, or camera channels, or objective lens systems. Each camera 120 has multiple lens elements (see FIG. 2) mounted within a lens barrel or housing 130. Adjacent outer lens elements 137 have adjacent beveled edges 132, positioning one camera channel in close proximity to another, but which may not be touching, and are therefore separated by a gap or seam 160 of finite width. A portion of the available light (λ), or light rays 110, from the scene or object space 105 enters the camera 120 and becomes image light that is captured within a constrained FOV and directed to the image plane, while other light rays do not reach the camera at all. Some light rays 110 propagate into the camera and pass through the lens elements forming as field edge chief rays 170, or marginal rays, while other light rays can potentially propagate through the lens elements, creating stray or ghost light and false bright spots or images. As an example, some light rays (167) incident at a large angle on the outer surface of outer lens element 137 can travel complex paths through the camera's lens elements and create ghost images that are detectable at image plane 150.

[0017] More specifically, FIG. 2A shows a cross section of a portion of a camera 120 having a set of lens elements 135 mounted within a housing (130, not shown) within a portion of an integrated panoramic multi-camera capture device 100. A fan of light rays 110 from object space 105 diverges from on-axis to the off-axis full-field chief ray range and strikes outer lens element 137, where it is refracted and transmitted inward. Refracted and transmitted through further inner lens element 140 and aperture stop 145, this image light 115 converges to a focused image at or near image plane 150, where an image sensor (not shown) is typically located. Lens system 120 of FIG. 2A can also be defined as having a lens format consisting of outer lens element 137, or compressor lens element, and inner lens element 140, the latter of which can also be defined as consisting of a wide-angle lens group before the stop (pre-stop) and an eyepiece-like lens group after the stop (post-stop). This compressor lens element (137) directs the image light 115 sharply inward, compressing the light and allowing the overall lens assembly to provide a short focal length, while also helping to allow the camera lens housing or barrel the room necessary to provide the mechanical features necessary both to hold or mount the lens elements and to properly interact with the barrel or housing of an adjacent camera. Image light passing through the camera lens assembly from the outer lens element 137 to the image plane 150 provides an image having image quality quantifiable by image resolution, image contrast, depth of focus, and other attributes, defined by the optical aberrations (e.g., astigmatism, distortion, or spherical) and chromatic or spectral aberrations encountered by the passing light at each of the lens elements (137, 140) in the camera 120. Figure 2B shows a chief ray 170, or fan of marginal rays, incident along or near the beveled edge 132 of the outer lens element 137 of the camera optics (120) shown in Figure 2A. FIG. 2B also shows a portion of the captured polygonal or asymmetric FOV 125 extending from the optical axis 185 to a line coincident with the edge ray.

[0018] 2A , outer lens element 137 functions as a compressor lens element that redirects passing image light 115 toward second lens element 142, which is the first lens element in a group of inner lens elements 140. In this design, second lens element 142 has a very concave shape, much like the outer lens elements used in fisheye-type imaging lenses. This compressor lens element serves to sharply direct image light 115 inward, or bend the light rays, allowing the overall lens assembly to provide a short focal length, while also allowing the camera lens housing 130 or barrel the room necessary to provide the mechanical features necessary both to hold or mount lens element 135 and to properly interact with an adjacent camera barrel or housing. However, with good lens and optomechanical design, and appropriate sensor selection, camera 120 can be designed with a lens assembly that supports image resolutions of 20-30 pixels / degree, even 110 pixels / degree or more, depending on the application and device configuration.

[0019] The resulting image quality from these cameras is determined by light scattered at surfaces or within lens elements, as well as light reflected or transmitted by each lens surface. Surface transmittance and camera lens system efficiency can be improved by the use of anti-reflection (AR) coatings. Image quality can also be determined by the results of non-image light. Referring back to FIG. 1, another portion of the available light may be primarily reflected by the outer lens element 137. Still other light entering the camera 120 can be blocked or absorbed by a blackened area (not shown) provided at or near the aperture stop, the inner lens barrel surface, the edges of the lens elements, internal baffles or light-trapping features, the field stop, or some combination of other surfaces. Still other light entering the camera can also become stray or ghost light potentially visible at the image plane.

[0020] The overall image quality obtained by multiple adjacent cameras 120 in an improved integrated panoramic multi-camera capture device 100 (e.g., FIG. 1) may also depend on various other factors, including camera-to-camera variations in focal length and / or track length, as well as the magnification provided by the individual cameras. These parameters may vary depending on factors including variations in glass refractive index, variations in lens element thickness and curvature, and variations in lens element mounting. As an example, images that are tiled or mosaiced together from multiple adjacent cameras typically need to be rectified, one after the other, to correct for image size changes due to camera magnification differences (e.g., ±2%).

[0021] The images produced by the multiple cameras in the integrated panoramic multi-camera capture device 100 may vary in image quality and in other respects resulting in mosaicing or tiling of the images. Specifically, the directional pointing or collection of image light through the lens elements to the image sensor of any given camera 120 may vary depending on whether the camera is angularly distorted or asymmetric.

number

[0022] In contrast to the system of FIG. 1, in a typical commercially available panoramic camera, the seams between cameras are complete gaps that can be 30-50 mm wide or even larger. Specifically, as shown in FIG. 3, a panoramic multi-camera capture device 101 may have adjacent cameras 120 or camera channels separated by large gaps or seams 160, leaving a blind spot or area 165 between them where no camera can capture an image. The actual physical seam 160 between adjacent camera channels or outer lens elements 137 (FIGS. 1 and 3) can be measured in various ways: as the actual physical distance between adjacent lens elements or lens housings, as the angular extent of the lost FOV, or as the number of "lost" pixels. However, the optical seam can be even larger, as the distance between the outer chief ray of one camera and another, due to any gaps in light reception caused by vignetting or coating limitations. For example, anti-reflection (AR) coatings are typically not applied to the edges of the optical system, and an offset margin is provided to accommodate the coated aperture (CA).

[0023] To compensate for both camera misalignment and large seams 160 and to reduce the size of the blind area 165, a typical panoramic multi-camera capture system 101 ( FIG. 3 ) has each individual camera 120 capture image light 115 from a wide FOV 125 that provides overlap 127, thereby reducing the blind area 165 and potentially lost image content. As another example, in most commercially available multi-camera capture systems 101, the gaps are 25-50+ mm wide, and the compensated FOV overlap between cameras is similarly large; for example, the portion of the FOV 125 that overlaps and is captured by two adjacent cameras 120 may be 10-50% of the cameras' FOV. The presence of such large image overlap from the shared FOV 125 wastes potential image resolution, increases image processing and image stitching time, and introduces significant image parallax and perspective errors. These errors complicate image stitching because they must be corrected or averaged out during the stitching process. In such systems, disparity is not predictable because it varies as a function of object distance. If the object distance is known, disparity can be predicted for a given field of view and inter-camera spacing. However, because the object distance is typically unknown, disparity errors result, complicating image stitching. Optical flow and general stitching algorithms determine object depth and enable image stitching, but they incur processing and time burdens.

[0024] Similarly, in a panoramic multi-camera capture device 100 of the type shown in FIG. 1 with closely integrated cameras, the width and structure of the seam 160 can be a significant factor in the operation of the overall device. However, the seam can be smaller than in FIG. 3, with the effective optical seam between the FOV edges of two adjacent cameras determined by both optical and mechanical contributions. For example, by using standard optical engineering techniques to construct lens assemblies within a housing, the mechanical width of the seam 160 between the outer lens elements 137 of adjacent cameras can be reduced to 4-6 mm. For example, it is standard practice to mount lens elements within a lens barrel or housing with a minimum radial width of 1-1.5 mm, particularly near the outermost lens element. Then, consider standard coated apertures or coating margins, as well as possible vignetting, entrance pupil aberrations, front color, and chip edges, and attempt to mount adjacent lens assemblies or housings closely together using standard techniques. Therefore, when considering both optics and mechanics, the optical seam width between adjacent lenses can easily be 8-12 mm or more.

[0025] However, with optical and optomechanical designs that allow for much smaller seams, improved versions of the panoramic multi-camera capture device (300) of the type shown in FIG. 1 are possible, with even improved parallax performance. As a first example, with regard to the present technology for improved polygonal cameras, during the initial stages of fabrication of the outer lens elements 137, these lenses may have a circular shape and be AR-coated up to or near their physical edges. If these lenses are subsequently processed to add polygonal shapes that define beveled edges 132 (e.g., FIG. 2B), the result may be that the AR coating essentially extends to the beveled lens edges. The effective optical or coated aperture can then be defined by mechanical attachment or any tolerances to standard edge polishing used in optical system fabrication to avoid edge chipping. With this approach, and a combination of other techniques described below, the optical seam can be reduced to a width of 1-5 mm.

[0026] The use of the improved multi-camera capture device 300 with improved low-parallax cameras 320 is illustrated in the context of Figure 15, which shows, in part, an electronics system diagram. In this example, a dodecahedron-type device has eleven cameras 320 and an electromechanical interface at the twelfth camera position. Image data can be collected from each of the eleven cameras and directed through an interface input-output module and down a cable or cable bundle to a portable computer.

[0027] The primary objective of the present invention is to generate high-quality, low-parallax panoramic images from an improved multi-camera panoramic capture device (300) for use in a system such as that of FIG. 15. This broad goal can be enabled by developing a range of design strategies to inform both optical and optomechanical lens design efforts, as well as optomechanical device design and manufacturing efforts, and strategies for improved image capture and processing. This goal can also be enabled by providing both initial and ongoing camera and device calibration. Broadly speaking, image processing, or image rendering, is a method for generating quality images from raw captured image data that depends on camera intrinsics (geometric factors such as focal length and distortion), camera extrinsics (geometric factors such as camera orientation relative to object space), other camera parameters such as vignetting and transmittance, and lighting parameters such as color and directionality. With respect to the improved multi-camera panoramic capture device 300, knowledge of the camera intrinsics for any given camera 320 in the device is all an aid toward completing a reliable and repeatable tiling of images acquired from multiple adjacent cameras. Accordingly, the following description focuses broadly on providing an optical (camera or objective lens) design that can enable the desired image quality, as well as camera and device assembly approaches, management of key tolerances, camera calibration, knowledge of camera intrinsics and extrinsics, and other factors that can similarly affect resulting device performance. The improved panoramic multi-camera capture device of the present invention can be used to support a wide variety of applications or markets, including cinematic image capture, augmented reality or virtual reality (VR) image capture, surveillance or security imaging, sports or event imaging, mapping or photogrammetry, vehicle navigation, and robotics.

[0028] Before investigating optomechanical means for enabling improved panoramic multi-camera capture devices (300), means are developed for providing improved cameras 120 for use in these systems. Accordingly, the goal includes providing an improved camera (320) with both reduced parallax error and image overlap. In one aspect of this approach, the goal is to reduce residual parallax error for edge chief rays collected individually by each camera in an adjacent pair. Parallax error is defined as the change in parallax with respect to object distance (e.g., chief ray trajectories are slightly different for closer distances from the device (e.g., 3 feet) versus farther distances (e.g., 1 mile)). For example, a goal or target for reduced parallax, or effectively eliminating parallax error or "parallax-free," is that the chief rays of adjacent cameras should deviate from parallelism by ≦0.5-2.0 degrees, and preferably ≦0.01-0.1 degrees. Alternatively, or equivalently, parallax error, evaluated as a perspective error with respect to position on the imaging plane, should be reduced to ≦2 pixels, and preferably ≦0.5 pixels. As another aspect of this approach, the width of the seams 160 between adjacent cameras (e.g., 120, 320, 920) assembled within their own lens housings should be reduced. The goal is to reduce the seam widths, both in terms of their absolute physical width and their optical or effective width. For example, the goal is to reduce the seams 160 between adjacent outer lens elements 137 to have a maximum gap or actual physical seam in the range of only about 0.5-3.0 mm, and then reduce the maximum optical seam width to a range of only about 1-6 mm. By way of example, these reduced seam widths can translate into a reduced angular range of lost FOV of only 0.25-1.0°, or a "lost" number of pixels of only 2-20. For example, for a device that provides 8k pixels around an equirectangular image such as a 360 degree panorama, a loss of only 2-4 pixels at the seams may be acceptable, as residual image artifacts may be difficult to notice.The actual details or numerical target for substantially no parallax error or maximum optical seam width will depend on many factors, including the detailed optomechanical design of the improved camera (320) and the overall device (300), tolerance control, possible tolerances on the center offset distance or amount of extended FOV (215) and the target for low parallax, as well as the overall device specifications (e.g., diameter, sensor resolution or sensor pixels used within the imaging FOV or core FOV 205 (FIG. 7)). A further goal, enabled by some combination of the aforementioned improvements, is for each camera to reliably and quickly provide an output image from an embedded sensor package that is cropped to provide a core FOV image, and each cropped image can then be easily stitched or tiled with cropped images provided by adjacent cameras to easily provide a panoramic output image in real time from the improved multi-camera capture device (300).

[0029] An improved panoramic multi-camera capture device 300, such as those shown in FIGS. 13 and 15, may have multiple cameras arranged around a sphere to capture a 360-degree circular FOV. Alternatively, the panoramic multi-camera capture device may have multiple cameras arranged around a sphere or polyhedron shape. A polyhedron is a three-dimensional solid composed of a collection of polygons joined at their edges. One polyhedron shape, as shown in FIG. 4, is that of a dodecahedron 50, which has 12 sides or faces, each shaped as a regular pentagon 55, and 20 vertices or corners (e.g., vertex 60). A dodecahedron-shaped panoramic multi-camera capture device has cameras with pentagonal outer lens elements that image at a nominal full wide angle of 69.1°. Another shape is that of a truncated icosahedron, similar to a soccer ball, also shown in FIG. 4, which has a combination of 12 regular pentagonal sides or faces, 20 regular hexagonal sides or faces, 60 vertices, and 90 edges. More complex shapes with more sides, such as regular polyhedra, Goldberg polyhedra, or shapes with octagonal sides, or even some irregular polyhedral shapes, may be useful. For example, the Goldberg truncated dodecahedron resembles the truncated icosahedron and has both pentagonal and hexagonal facets, for a total of 42 sides. However, in general, preferred polyhedra for present purposes have hexagonal or pentagonal sides or faces, which are generally rounded and have beveled edges 132 that meet at obtuse angles. Other polyhedral shapes, such as an octahedron or regular icosahedron, can also be used, although they have triangular facets. Polyhedral facets with steeper or sharper corners, such as square or triangular facets, may be easier to manufacture compared to pentagonal and / or hexagonal facets because there are fewer edges to cut to provide polygonal edges on the outermost lens elements to define the captured polygonal FOV. However, due to their sharp corners, more care may be required during cutting, chamfering, and processing of the optics as a result. Additionally, it may be more difficult to design camera lenses and camera lens housings for optical and optomechanical performance with lens facets that have large FOVs and sharp facet angles.Typically, a 360° polygonal camera does not capture a full spherical FOV because at least a portion of one facet is sacrificed to allow for support features and power and communication cabling, such as by a mounting pole. However, if the device communicates wirelessly and is also suspended at the apex by a thin cable, the FOV lost in such physical connections can be reduced.

[0030] 1 and 2B, camera channel 120 resembles a truncated cone, or a portion thereof, where a truncated cone is a geometric solid (usually a cone or pyramid) that exists between one or two parallel planes that intersect it. In that context, the fan of chief rays 170 corresponding to the polygonal edges is refracted by outer compressor lens element 137 to nominally coincide with the edges of the truncated cone of the polyhedral geometry.

[0031] To help illustrate some of the issues related to camera geometry, FIG. 5A shows a cross section of a pentagonal lens 175 capturing a pentagonal FOV 177 and a hexagonal lens 180 capturing a hexagonal FOV 182, representing a pair of adjacent cameras whose outer lens elements have pentagonal and hexagonal shapes, such as might occur in a truncated icosahedron, or soccer-ball-shaped panoramic multi-camera capture device (e.g., 100, 300). The theoretical hexagonal FOV 182 spans a 20.9° half FOV along the side, or a 41.8° full FOV (θ1), with the FOV near the vertices being even larger. The pentagonal FOV 177 supports a 36.55° FOV (θ2) within a circular region, with the FOV closer to the corners or vertices being even larger. Notably, in this cross section, the pentagonal FOV 177 is asymmetric, supporting a 20 degree FOV on one side of the optical axis 185 and only a 16.5 degree FOV on the other side of the optical axis.

[0032] Optical lenses are typically designed using programs such as ZEMAX or Code V. The success of a design typically depends in part on selecting the best or most appropriate lens parameters, identified as operands, for use in a merit function. This is also true when designing lens systems for improved low-parallax multi-camera panoramic capture devices, for which there are several factors that affect performance (including, in particular, parallax) and several parameters that can be optimized individually or collectively to control it. One approach targets optimization of the "NP" point, or more significantly, a variant thereof.

[0033] By way of background, in the field of optics, there is the concept of an entrance pupil, which is the projected image of the aperture stop as seen from object space, or the virtual opening toward which image rays from object space appear to propagate before any refraction by the first lens element. By standard techniques, the location of the entrance pupil can be found by identifying the paraxial chief ray from object space 105 passing through the center of the aperture stop and projecting or extending that object space in a direction toward where it strikes the optical axis 185. In optics, an incident Gaussian or paraxial ray is understood to lie within an angular range of 10° or less from the optical axis, corresponding to a ray directed toward the center of the aperture stop, which also defines the entrance pupil location. Depending on the lens characteristics, the entrance pupil may be larger or smaller than the aperture stop and located in front of or behind the aperture stop.

[0034] In contrast, in the field of low-parallax cameras, there is the concept of a no-parallax (NP) point, or viewpoint center. Conceptually, an "NP point" is associated with a high-FOV chief ray or principal ray incident on or near the outer edge of the outermost lens element, projecting or extending its object space in a direction toward where it strikes the optical axis 185. For example, depending on the design, camera channels in a panoramic multi-camera capture device can support a half-FOV with non-paraxial chief rays at angles >31° for dodecahedron-type systems (FIG. 4) or >20° for truncated icosahedron-type systems (see FIGS. 4 and 5A). This concept of NP point projection has been applied to the design of panoramic multi-camera capture devices in conjunction with expectations for chief ray propagation and parallax control for adjacent optical systems (cameras). It can also be argued that if a camera pivots around the NP point, or multiple cameras appear to rotate around a common NP point, the parallax error is reduced and images can be aligned with little or no parallax error or perspective difference. However, in the field of low-parallax cameras, the NP point is also identified with the entrance pupil, and the axial position of the entrance pupil is estimated using the first-order optics tangent relationship between the projection of the paraxial field angle on the first lens element (see Figures 2A and 2B) and the height of the incident ray.

[0035] Thus, confusingly, in the field of low-parallax camera design, NP points have previously been associated with both the projection of the FOV chief rays' edges and the projection of chief rays that lie within the Gaussian or paraxial region. As can be seen, in practice, they both have value. Specifically, NP points associated with paraxial entrance pupils can be useful in developing initial specifications for designing and describing lenses. NP points associated with non-paraxial edge field chief rays can be useful in targeting and understanding parallax performance and in defining the conical volume or truncated cone that a lens assembly can reside within.

[0036] The projection of these non-paraxial chief rays may miss the entrance pupil defined by the paraxial chief rays due to both lens aberrations and practical geometry-related factors associated with these lens systems. Regarding the former, in well-designed lenses, image quality at the image plane is typically prioritized by limiting the impact of aberrations on resolution, telecentricity, or other attributes. Within a lens system, aberrations at intermediate surfaces, including the aperture stop, can vary widely, with emphasis on the net sum at the image plane. While aberrations at the aperture stop are often somewhat controlled to avoid vignetting, non-paraxial chief rays do not need to pass through the center of the aperture stop or the entrance pupil located along the projection paraxial axis.

[0037] To further illustrate these concepts and enable the design of improved low-parallax lens systems, note that the camera lens system 120 in FIG. 2A shows both a first NP point 190A, corresponding to the entrance pupil defined by the vector projection of a paraxial chief ray from object space 105, and an offset second NP point 190B, corresponding to the vector projection of a non-paraxial chief ray from object space. Both of these ray projections intersect the optical axis 185 at positions behind both the lens system and the image plane 150. As will be explained later, ray behavior between and within the nearby regions of projection points 190A and 190B can be complex, and neither the projection position nor the projection point has a definitive value or size. While the projection of a chief ray intersects the optical axis at a point, the projections of groups of chief rays converge toward the optical axis and intersect at different locations, which may be tightly grouped (e.g., within a few microns or tens of microns), in which case the extent or size of that “point” may depend on the set of nearby chief rays used in the analysis. On the other hand, when designing a low-parallax imaging lens for imaging a large FOV, the axial distance or difference between NP points 190A and 190B provided by the projected paraxial and non-paraxial chief rays can be significantly large (e.g., millimeters). Thus, as also explained below, the axial difference represents a useful means of parallax optimization (e.g., low-parallax budget 188) of lens systems designed for current panoramic capture devices and applications. As can also be seen, the design of the improved device (300) can be optimized to locate the device geometric center, or device center 196, outside of, but close to, this low-parallax budget 188, or alternatively, within, and preferably close to, the non-paraxial chief ray NP point.

[0038] In one embodiment, FIG. 5A shows the projection of the theoretical edge of the field of view (FOV edge 155) past the outer lens elements (lenses 175 and 180) of two adjacent cameras to provide lines pointing toward a common point (190). These lines represent the theoretical limits of complex "conical" optomechanical lens assemblies, which are typically volume-limiting pentagonal or hexagonal cones. Again, ideally, in a parallax-free multi-camera system, the entrance pupils or NP points of two adjacent cameras are co-located. However, to avoid mechanical collisions, features of a given lens assembly, including the sensor package, generally should not protrude outside the camera system's frustum cone and into the conical space of an adjacent lens assembly. However, the actual lens assemblies in a multi-camera panoramic capture device are also separated by a seam 160. Thus, the actual chief ray 170 received at the lens edge, which is inside both the mechanical seam and the physical width or aperture of the attached outer lens element (lenses 175 and 180), when projected generally toward paraxial NP point 190, may instead arrive at offset NP point 192, separated by NP point offset distance 194.

[0039] This can be better understood by considering the expanded region AA proximate to the nominal or ideal point NP 190, as shown in detail in FIG. 5B. Within the hexagonal FOV 182, rays propagating within the Gaussian or paraxial region and passing through the nominal center of the aperture stop (e.g., paraxial ray 173) can be projected to the nominal NP point 190 (corresponding to the entrance pupil) or to an offset NP point 190A at a small NP point difference or offset 193 from the nominal NP point 190. Meanwhile, the actual hexagonal lens edge chief ray 170 associated with the largest inscribed circle within the hexagon can be projected to reach a common offset NP point 192A, which may be at an even larger offset distance (194A). The two adjacent cameras in FIGS. 5A and 5B may or may not also share a co-located NP point (e.g., 190). Distance offsets can arise for a variety of reasons, including geometric considerations between the cameras (adjacent hexagonal and pentagonal cameras), geometric asymmetries within the cameras (e.g., for pentagonal cameras), or due to constraints on the practical width of the seam 160, or due to directional differences between deviating light rays.

[0040] As just mentioned, there are also potential geometric differences in the projection of incident chief rays toward a simplified nominal "NP point" (190). First, incident imaging paths from the corners or vertices or near the mid-edge (mid-chord) of a hexagonal or pentagonal lens may or may not project to a common NP point within the described range between the nominal paraxial NP point 190 and the offset NP point 192B. Also, as shown in FIG. 5B, solely due to the geometric asymmetry of the pentagonal lens, for an actual accepted FOV, associated pairs of edge chief rays 170 and 171 can project to different nominal NP points 192B that can be separated both from the paraxial NP point (190) by offset distance 194B and from each other by offset distance 194C.

[0041] Another issue is that during lens design, the best performance typically occurs on-axis or near-axis (e.g., ≤0.3 normalized field of view), near the optical axis 185. In many lenses, good imaging performance often occurs at or near the field edge by design, in which case optimization weighting is often used to enforce compliance. The worst imaging performance can then occur in the intermediate field (e.g., 0.7-0.8 of the normalized image field height). Reviewing Figures 5A and 5B again, outside the paraxial region, but not as extreme as the edge chief ray (10°<θ<20.9°), intermediate off-axis rays from the intermediate field (θ) can project toward an intermediate NP point between the nominal NP point 190 and the offset NP point 192B. However, other, more extreme, off-axis rays, specifically from the 0.7-0.8 intermediate field, which are more affected by aberrations, can project to an NP point at a location that is more or less offset from the nominal NP point 190 than the offset NP point 192B at the field edge. To accommodate differences in lens design, the non-paraxial offset "NP" point can be dropped either before (closer to the lens) the paraxial NP point (entrance pupil), as proposed in Figure 5B, or after (as shown in Figure 2A).

[0042] This is shown in more detail in Figure 5C, which essentially illustrates a more zoomed-in region AA of Figure 5B, illustrating the effects of vector projected ray paths associated with errant imaging rays converging at or near the paraxial entrance pupil (190) for an imaging lens system designed and optimized using the methods of the present approach. In Figure 5C, the projected ray paths of green errant imaging rays at multiple fields of view from the camera lens system converge within a low parallax volume 188 near one or more "NP" points. Similar illustrations of ray fans can be generated for red or blue light. Projections of paraxial rays 173 can converge at or near the nominal paraxial NP point 190, or an entrance pupil located on the nominal optical axis 185 a distance Z behind the image plane 150. Projections of field edge rays 172, including chief ray 171, converge at or near an offset NP point 192B along the optical axis 185. The NP point 192B can be quantitatively defined, for example, as the center of the plethora of full-field edge rays 172. An alternative offset NP point 192A can be identified, corresponding to the "circle of least confusion," where the paraxial, edge, and intermediate or mid-field rays converge to a minimum spot. These different "NP" points are separated from the paraxial NP point by offset distances 194A and 194B, and from each other by offset distance 194C. Thus, it can be appreciated that the convergent "NP point" for any given practical imaging lens assembly or camera lens supporting a larger paraxial FOV, or an asymmetric FOV, will typically not be a point, but instead may be an offset low-parallax (LP) smudge or quantity 188.

[0043] Within the smudge or low-parallax volume 188, various possible optimal or preferred NP points can be identified. For example, offset NP points corresponding to field edge rays 172 can be emphasized to help provide improved image tiling. Alternative intermediate-field (e.g., 0.6-0.8) NP points (not shown) can also be tracked and optimized. Additionally, the overall size and location of the “LP” smudge or volume 188, or preferred NP points therein (e.g., 192B), can be varied depending on the lens design optimization. Such parameters may also vary from lens to lens for a manufactured lens system of a given design due to manufacturing variations between lens assemblies. While FIG. 5C illustrates these alternative offset “NP points” 192A, B for non-paraxial rays as being located after the paraxial NP point 190 or further away from the lens and image plane, other lenses of this type optimized using the present approach can be provided where similar non-paraxial NP points 192A located at low-parallax volume 188 can occur between the image plane and the paraxial NP point.

[0044] FIG. 5C also shows the location relative to the center of the low-parallax multi-camera panoramic capture device, device center 196. Based on optical considerations, the improved panoramic multi-camera capture device 300 can preferably be optimized to nominally position device center 196 within low-parallax volume 188. The optimized location may include being located at or near either offset NP point 192A or 192B, or within an offset distance 194B between them, to prioritize parallax control for field edge chief rays. The actual location is then determined by parallax optimization, which can be determined by lens optimization for entrance pupil spherical aberration, direct chief ray constraints, or distortion, or a combination thereof. For example, whether spherical aberration is optimized to be over-corrected or under-corrected, and what weighting is used for the field operand in the merit function, can affect the location of the non-paraxial "NP" point for peripheral or intermediate fields. "NP" point location may also be determined by managing manufacturing tolerances and residual variations in lens system manufacturing. The device center 196 can also be positioned close to the low-parallax volume 188, but offset from it by a center offset distance 198. This approach also aids in tolerance control and can provide more space near the device center 196 for cables, circuits, cooling hardware, and related structures. In such cases, adjacent cameras 120 may consequently have offset low-parallax volumes 188 at the "NP" point (FIG. 5D) instead of being co-located (FIGS. 5A, 5B). In this example, if the device center 196 were instead positioned at or close to the paraxial entrance pupil, the NP point 190, one or more of the outer lens elements 137 of the cameras 120 would be substantially smaller than normal, and the desired full FOV would not be achievable. FIG. 10B illustrates a possible positioning of a similar lens system 920 with respect to the offset device center 910.

[0045] Thus, the no-parallax (NP) point is a useful concept to address and can usefully inform panoramic image capture and system design, aiding in the design of low-parallax error lenses. While it is ideal, its limitations must also be understood. Given this description of NP point(s) and LP smudges, understanding ray behavior in this region is crucial in enabling improved low-parallax multi-camera panoramic capture devices (example lens designs to follow; e.g., device 920 in FIG. 8A) to define appropriate parameters or operands to optimize and appropriate target levels of performance to strive for. In the latter case, for example, a low-parallax lens with a track length of 65-70 mm can be designed for LP smudges as wide as 10 mm (e.g., offset distance 194A). However, alternative lens designs where this parameter is further improved may have low-parallax volume 188 with a longitudinal LP smudge width or width along the optical axis (offset 194A) of a few millimeters or less.

[0046] The width and location of the low-parallax volume 188, as well as the vector directions of projection of the various chief rays and their NP point locations within the low-parallax volume, can be controlled during lens optimization by methods using operands associated with fans of chief rays 170 (e.g., FIGS. 2A and 2B). However, the LP smudge or LP volume 188 in FIG. 5C can also be understood as a visualization of the transverse component of the spherical aberration of the entrance pupil, and this parameter can be used in an alternative, but equivalent, design optimization method to using chief ray fans. Specifically, during lens optimization, using, for example, Code V, a lens designer can create a special user-defined function or operand for the transverse component of the spherical aberration of the entrance pupil (e.g., ray height), which can then be used in a variety of ways. For example, the operand value can be calculated as the residual sum of squares (RSS) of values ​​over the entire FOV or a local field of view, using either uniform or non-uniform weighting for the field of view operand. In the latter case of local field of view preference, a value can be calculated for a position at or near the entrance pupil, or somewhere within the low-parallax volume 188, depending on a preference for paraxial, intermediate, or peripheral vision. An equivalent operand could be the width of the circle of least confusion at a plane, such as the plane of offset NP point 192A or the plane of offset NP 192B, as shown in FIG. 5C. The optimization operand can also be calculated with weighting to reduce or limit parallax error across the field of view non-uniformly, with disproportionate weighting favoring peripheral or edge vision over intermediate vision. Alternatively, the optimization operand can be calculated using weighting to provide nominally low parallax error in a nominally uniform manner across the entire field of view (within or across the core FOV 205, as in FIG. 7). That type of optimization can be particularly useful for mapping applications.

[0047] Whether a low-parallax lens design and optimization method uses operands based on chief ray or entrance pupil spherical aberration, the resulting data can also be analyzed with respect to changes in imaging perspective. Specifically, parallax errors for field of view and color can also be analyzed using calculations of the center of perspective (COP), a parameter more directly related to visible image artifacts that are low-parallax quantities, and can be evaluated in terms of image pixel error or difference for imaging objects at two different distances from the camera system. Center of perspective error is essentially the change in chief ray trajectory given multiple object distances—such as an object at close range (3 ft) versus another object at “infinity.”

[0048] In drawing and architecture, perspective is the technique of depicting solid objects on a two-dimensional surface to give a correct impression of their height, width, depth, and location in relation to one another when viewed from a particular point. For example, for drawings using linear or point perspective, objects appear smaller as their distance from the observer increases. Such illustrated objects are also subject to foreshortening, meaning that an object's dimensions along the line of sight appear shorter than its dimensions across the line of sight. Perspective works by representing light passing from the scene through an imaginary rectangle (realized as the plane of the drawing) to the viewer's eye, as if the viewer were looking through a window and depicting what they see directly on the windowpane.

[0049] Perspective is related to both parallax and stereoscopic perception. In stereoscopic image capture or projection using a pair of adjacent optical systems, perspective is a visual cue, along with dual-view parallax, shadowing, and occlusion, that can provide the perception of depth. As previously mentioned, parallax is the visual perception that an object's position or orientation appears different when viewed from different positions. In the case of image capture by a pair of adjacent cameras with at least partially overlapping fields of view, parallax image differences are a cue to stereoscopic perception or an error for panoramic imaging assemblies.

[0050] To capture an image using an optical system, whether a camera or the human eye, the geometry and performance of the optical system affect the usefulness of the resulting image for low-parallax (panoramic) or high-parallax (stereo) perception. Specifically, for an ideal lens, all chief rays from object space point precisely toward the center of the entrance pupil, which coincides with the center of perspective (COP) or center of view for the resulting image. For such an ideal lens, there are no errors in perspective or parallax.

[0051] However, with respect to real lenses, there may be residual parallax error due to both physical and image quality constraints. As previously mentioned, with respect to real lenses, the projections of paraxial chief rays from the first lens element point toward a common point, the entrance pupil, whose location can be determined as an axial distance from the front surface of that first element. On the other hand, with respect to real lenses that capture a FOV large enough to include non-paraxial chief rays, the chief rays in object space may point toward a common location or a nearby amount, but typically toward a location offset from the center of the entrance pupil. While these chief rays do not inherently coincide at a single point, they can be directed through a small, low-parallax volume 188 (e.g., an LP "smudge") with appropriate lens optimization. The longitudinal or axial variation of the rays within the LP smudge can be determined from the position of the chief rays across the optical axis. Ray error can also be measured as the lateral width or axial position of the chief rays within the LP smudge.

[0052] The concept of parallax correction is illustrated in FIG. 5D with respect to the perspective center. First camera lens 120A collects light from object space 105, including light from two outer ray fans 179A and 179B, whose chief ray projections converge toward low-parallax volume 188A, and images it into at least one core FOV. These ray fans may correspond to groups 172 of field-of-view or field-edge rays near the field of view, as shown in FIG. 2B or FIG. 5C. As shown in FIG. 5C, vector projections of such rays from object space, generally directed toward image space, within LP volume 188, favor field-edge rays, so they can cross optical axis 185 beyond the image plane at or near alternative NP point 192B, which can be selected or preferred. However, as also shown in FIG. 5C, such field-edge rays 172 need not intersect optical axis 185 at exactly the same point. These differences, when transformed back into object space 105, translate into small differences in parallax or perspective for the imaging ray bundle or fan within or across the imaging FOV of the camera lens (e.g., core FOV 205, as in FIG. 7).

[0053] A second, adjacent camera lens 120B, shown in FIG. 5D , can provide similar performance, imaging a fan of chief rays 170 from within the core FOV 205 with vector projections of those chief rays converging within a corresponding low-parallax volume 188B, including ray fan 179C. LP volumes 188A and 188B can overlap, coincide, or be offset, depending on factors including the camera geometry and seams between adjacent cameras, lens system manufacturing tolerances and compensators, or whether device center 196 is offset from LP volume 188. The more these LP volumes 188 overlap or coincide, the more the perspective centers of the two lens systems overlap. Ray fan 179B of camera lens 120A and ray fan 179C of camera lens 120B are also nominally parallel to one another, e.g., there is no parallax error between them. However, even if the lens design allows for very little residual parallax error at the edges of the FOV, manufacturing variations between lens systems can magnify the difference.

[0054] Analytically, chief ray data from a real lens can also be expressed in terms of perspective error, including chromatic error, as a function of field of view. Perspective error can then be analyzed as the position error in an image between two objects positioned at different distances or orientations. Perspective error can depend on the choice of COP position, angle within the imaging FOV, and chromatic error. For example, it can be useful to prioritize the COP to minimize green perspective error. Perspective or parallax error can be reduced by optimizing the axial position (Δz) or width of a color within the LP quantity 188 relative to the perspective center for one or more field angles within the imaging FOV. The perspective center can also be graphed and analyzed as a family of curves for Z (axial) intercept position (distance in mm) versus field of view angle, for each color. Alternatively, to get a better idea of ​​what the captured image will look like, the COP can be graphed and analyzed as a family of curves for the camera system as parallax error in image pixels, for each color, versus the field of view.

[0055] During the design of a camera lens system, a goal may be to limit the parallax error to a few pixels or less for imaging within the core FOV 205 (FIG. 7). Alternatively, it may be preferable to limit the parallax error, for example, to the outer edges of the core FOV and to extended FOV regions (if provided), particularly in the peripheral field. If the residual parallax error for a camera is thus small enough, the parallax difference, seen as the perspective error between two adjacent cameras near their shared seam 160 or in seam-related regions of the extended FOV where the imaging overlaps, may similarly be limited to a few pixels or less (e.g., ≦3-4 pixels). Depending on the lens design, device design, and application, it may be possible and preferable to further reduce the parallax error for the lens system, as measured by perspective error, to ≦0.5 pixels for the entire core FOV, the peripheral field, or both. If these residual parallax errors for each of two adjacent cameras are small enough, the images can be acquired, cropped, and easily tiled while correcting or hiding image artifacts from any remaining seams 160 or blind areas 165.

[0056] Continuing with the design of the type of panoramic camera of FIG. 1, lens optimization methods and parameter selection can be important to enable an improved low-parallax multi-camera panoramic capture device (300) with multiple adjacent cameras. A camera lens 120, or system of lens elements 135, such as that of FIG. 2A, can be used as a starting point. The camera lens has compressor lens element(s) and inner lens element 140, which can also be defined as consisting of a wide-angle lens group in front of the aperture and an eyepiece-like lens group behind the aperture. In designing such a lens to reduce parallax error, it can be useful to consider how the paraxial-to-nonparaxial chief ray fan 125 (see FIG. 2A), or edge chief ray fan 170 (see FIG. 2B), or localized collection of field edge rays 172 (see FIG. 5C) or 179A, B (see FIG. 5D) are imaged by the camera lens assembly. While it is possible to optimize a lens design by using a set of merit function operands for a collection or set of chief rays (e.g., 31 defined rays), the optimization process can become cumbersome. Alternatively, in pursuit of the design of an improved low-parallax multi-camera panoramic capture device (300), it has been determined that improved performance can also be obtained by using a reduced set of ray parameters or operands that emphasize the lateral component of spherical aberration at the entrance pupil or at a similar selected surface or location (e.g., offset NP point 192A or 192B) within the LP smudge volume 188 behind the lens system. Optimization for the lateral component of spherical aberration at an alternative non-paraxial entrance pupil can be achieved using merit function weighting that emphasizes the non-paraxial chief rays.

[0057] Alternatively, in a low-parallax multi-camera panoramic capture device, the fan 170 of chief rays incident on or near the beveled edge of the outer lens element of a camera 120 (see FIG. 2B ) should be parallel to the fan 170 of chief rays incident on or near the beveled edge 132 of the outer lens element of an adjacent camera (see FIG. 1 ). Note that the “edge” of the outer lens element 137 or compressor lens is a three-dimensional structure (see FIG. 2B ), which may have a flat edge across the thickness of the glass and is subject to manufacturing tolerances of that lens element, the entire lens assembly, and the housing 130, as well as adjacent seams 160 and their structure. The location definition of where the beveled edge is cut into the outer lens element depends on factors including material properties, front color, distortion, parallax correction, tolerances, and the extent of any extra extended FOV 215. Outer lens element 137 becomes a faceted outer lens element when beveled edges 132 are cut into the lens, creating a set of polygonal edges that nominally follow a polygonal pattern (eg, a pentagon or hexagon).

[0058] A camera system 120 having an outer lens element with a polygonal shape that captures incident light from a polygonal field of view will therefore form a polygonal image at image plane 150, with the shape of the captured polygonal field nominally matching the shape of the polygonal outer lens element. Cutting these beveled edges for a given pair of adjacent cameras can affect both the imaging and optomechanical structure at or near the intervening seam 160.

[0059] Alternatively, Figure 5E illustrates "front color," which is the difference in nominal ray path due to color versus field of view when directed toward an off-axis or edge field point. Typically, for a given field point, blue rays are offset the farthest. As shown in Figure 5E, blue ray 157, accepted by first lens element 137, is approximately 1 mm further outward (ΔX) than accepted red ray 158, also directed toward the same field point. If lens element 137 is not large enough, this blue light may be clipped or blurred, resulting in color shading artifacts at or near the edge of the field. Front color may appear in the captured image content as a rainbow-like outline of the polygonal FOV or polygon edges of outer compressor lens element 437, which acts as a field stop for the optical system. Local color transmission differences that can cause front color-related color mottle artifacts near the image edges can arise from differential vignetting at the beveled edges of the outer compressor lens elements 137, or from edge truncation in the compressor lens elements (FIGS. 8A, 9A), or through the aperture stop 145. During lens design optimization to provide an improved low parallax camera lens (320), front color can be reduced (e.g., to ΔX≦0.5 mm width) as part of the chromatic correction of the lens design, including through glass selection within the compressor lens group or the entire lens design, or as a trade-off in the correction of lateral color. The effect of front color on the captured image can also be optomechanically reduced by designing the improved camera lens (320) to have an extended FOV 215 (FIG. 7) and by designing the optomechanics to push a straight cut or beveled lens edge 132 at or beyond the edge of the extended FOV 215, so that residual front color occurs outside the core FOV 220. The front color artifact can then be removed during an image cropping step during image processing. The effect of front or lateral color can also be reduced by spatially distinct color correction during image processing.Alternatively, the improved camera lens (920) can have a color-dependent aperture at or near the aperture stop that can provide a larger transmission opening (diameter) for blue light than for red or green light, for example.

[0060] Optical performance at or near a seam can be understood, in part, in relation to distortion ( FIG. 6 ) and a defined set of fields of view ( FIG. 7 ). Specifically, FIG. 7 illustrates the potential set of fields of view for which potential image light can be collected by two adjacent cameras. As an example, a camera with a pentagonal outer lens element, with a seam 160 separating it from the adjacent lens or camera channel, whether associated with a dodecahedron, truncated icosahedron, or other polygonal lens camera assembly, can capture an ideal FOV 200 extending to the apex (60) or polygon edge of the frustum or conical volume within which the lens resides. However, due to various physical constraints that may arise at the seam, including the finite thickness of the lens housing, the physical aspects of the beveled lens element edges, mechanical wedges, and tolerances, a smaller core FOV 205 of transmitted image light can actually be captured. The coated opening for the outer lens element 137 should encompass at least the core FOV 205 with some margin (e.g., 0.5–1.0 mm). Since lenses can be manufactured with AR coatings before beveling, the coating can extend to the seams. The core FOV 205 can be defined as the largest low-parallax field of view that a given practical camera 120 can image. Equivalently, the core FOV 205 can be defined as a sub-FOV of a camera channel whose boundaries are nominally parallel to the boundaries of its polygonal cone (see FIGS. 5A and 5B). Ideally, with small seams 160 and proper control and calibration of FOV pointing, the nominal core FOV 205 approaches or matches the ideal FOV 200 in size.

[0061] During the camera alignment and calibration process, a series of image fiducials 210 can be established along one or more edges of the core FOV 205 to assist in image processing and image tiling or mosaicing. The resulting gap between the core FOV 205 supported by a first camera and that supported by an adjacent camera can result in a blind area 165 (FIGS. 5A and 5B). To compensate for the blind area 165 and the associated loss of image content from the scene, a camera can be designed to support an extended FOV 215, which provides sufficient extra FOV to accommodate the seam width and tolerance, or offset device center 196. As shown in FIG. 7, the extended FOV 215 can be extended sufficiently to provide overlap 127 with the edge of the adjacent camera's core FOV 205, although the extended FOV 215 can be even larger. This limited image overlap may result in a small amount of image resolution loss, parallax error, and some complexity in image processing, as discussed above with respect to FIG. 3, but it may also help reduce the apparent width of the seam and blind areas. However, as provided by the present approach, if the extra overlap FOV is modest (e.g., ≦5%) and the residual parallax error therein is sufficiently small (e.g., perspective error of ≦0.75 pixels), the image processing burden may be very minimal. Image capture up to the extended FOV 215 can also be used to enable intermediate capture steps that support camera calibration and image correction during operation of the improved panoramic multi-camera capture device 300. FIG. 7 shows an inscribed circle within one of the FOV sets, corresponding to a subset of the core FOV 205, that can be captured in all directions from that camera: the common core FOV 220. The angular width of the common core FOV 220 may be useful as a quick reference to the camera's image capacity. An alternative definition of a larger common core FOV 220 to include the entire core FOV 205 may also be useful.The dashed line (225) extending from the common core FOV 220 or core FOV 205 beyond the ideal FOV 200 to nominally include the extended FOV 215 represents the region where the lens design can support careful mapping of the chief or primary rays or control of the spherical aberration of the entrance pupil to enable low parallax error imaging and easy tiling of images captured by adjacent cameras.

[0062] To reduce parallax and improve image tiling across a seam 160 spanning the distance between two adjacent usable apertures between two adjacent cameras, it may be advantageous if image light is captured substantially straight, parallel, and commonly spaced for a finite distance. The amount of FOV overlap required to provide an extended FOV and limit blind areas can be determined by controlling the relative proximity of the entrance pupil (paraxial NP point) or an alternate preferred plane (e.g., to emphasize peripheral light) within the low-parallax amount 188 relative to the device center 196 (e.g., the center of a dodecahedron shape). The amount of extended FOV 215 is preferably 5% or less (e.g., 1.8° or more additional field of view relative to a nominal core FOV of 37.5°), thereby resulting in a camera peripheral field of view of, for example, approximately 0.85-1.05°. If spacing constraints at the device center and manufacturing tolerances are well managed, the extended FOV 215 can be reduced to 1% or less additional field of view. Within the extended FOV 215, parallax should be limited to nominal system levels while both image resolution and relative illumination remain satisfactory. Parallax optimization to reduce parallax error can use either chief ray or pupil aberration constraints, targeting optimization for the high FOV region (e.g., 0.85-1.0 FOV) or beyond to include the extra camera overlap region provided by the extended FOV 215 (e.g., Figure 7, partial FOV range of approximately 0.85-1.05).

[0063] Additionally, in enabling an improved low-parallax multi-camera panoramic capture device (300) with limited parallax error and improved image tiling, it may be important to control image distortion for image light passing at or near the edges of the outer lens element's FOV, e.g., peripheral vision. In geometric optics, distortion is the deviation from the desired state in which straight lines in a scene remain straight in the image (e.g., rectilinear projection). It is a form of optical aberration and describes how light rays from a scene are mapped to an image plane. Generally, for image capture, lens assemblies used for human viewing are advantageous in limiting image distortion to a maximum of + / - 2%. In current applications, it may also be useful to have a moderate distortion of 2% or less for tiling or combining panoramic images from images captured by adjacent cameras. For reference, in cylindrical distortion, image magnification decreases with distance from the optical axis, and the apparent effect is that of an image being mapped around a spherical (or cylindrical) surface. Fisheye lenses, often used to capture hemispherical or panoramic views, typically have this type of distortion as a way of mapping an infinitely wide object plane into a finite image area. Fisheye lens distortion (251) can deviate significantly from f-theta distortion (e.g., 15% or 90° half-width (HW) for the full field of view), but only a few percent for small fields of view (e.g., ≦30° HW). As another example, in laser printing or scanning systems, f-theta imaging lenses are often used to print images with minimal banding artifacts and image processing corrections for pixel placement. Specifically, f-theta lenses are designed with a cylindrical distortion that results in a nearly constant spot or pixel size and pixel positioning that is linear with the field of view θ (h=f*θ).

[0064] Thus, an improved low-parallax camera 320 capturing a half FOV of 35-40° or less may have fisheye distortion 251 because the distortion may be sufficiently low. However, the distortion can be more advantageously optimized for the design of an improved camera lens assembly for use in the improved low-parallax multi-camera panoramic capture device (300). As a first example, it may be advantageous to provide a camera lens assembly with localized nominal f-theta distortion 250A at or near the edge of the image field, as shown in FIG. 6. In one example, image distortion 250 peaks at about 1% at about 0.75 FOV, and the lens design is not optimized to provide f-theta distortion 250A below about 0.85 FOV. However, during the lens design process, the merit function can be constrained to provide nominal f-theta-like distortion 250A or substantially flat distortion 250B for rays imaged at or near the field edge, such as for peripheral fields spanning a partial field range of about 0.9-1.0. This range of high FOV with f-theta type or flat distortion correction includes the fan of chief rays 170 or peripheral rays of FIG. 2B, including rays imaged through corners or vertices 60, such as those of a lens assembly with a hexagonal or pentagonal outer lens element 137. Additionally, due to manufacturing tolerances and dynamic effects (e.g., temperature changes) applicable to the camera 120, including both the lens element 135 and the housing 130, and to the collection of cameras 120 within a panoramic multi-camera capture device, it may be advantageous to extend the region of nominal f-theta or flattened distortion within the peripheral field beyond the nominal full FOV (e.g., 0.85 to 1.05). This is illustrated in FIG. 6, where the region of reduced or flattened distortion extends beyond the full FOV to approximately 1.05 FOV. In such a peripheral FOV range, it may be advantageous to limit the total distortion change to 0.5% or less. Peripheral field distortion is controlled to keep image "edges" straight within the adjacent pentagonal region. This may allow for more efficient use of pixels when tiling an image, and therefore faster image processing.

[0065] The preceding discussion discusses distortion in the classical sense, as image aberration at the image plane. However, in low-parallax cameras, this residual distortion is typically a trade-off or nominal cancellation of the contribution from the compressor lens element (137, or 937 and 938 in FIG. 9A ) for that of the collective inner lens element (140, or 940 in FIG. 9A ). Importantly, the ray redirection caused by the distortion contribution of the outer compressor lens element also affects both the imaging ray path and the projection chief ray path toward the low-parallax volume. This consequently means that for at least some low-parallax lens designs, distortion optimization can affect parallax or field edge NP point or perspective center optimization.

[0066] The definition of the peripheral field or subfield of view range (e.g., approximately 0.85-1.05, or including 5% or less of additional field of view), where parallax, distortion, relative illumination, resolution, and other performance factors are carefully optimized to support image tiling, may depend on the device and camera geometry. As an example, for a hexagonal lens and field of view, the lower limit of the peripheral field of view can be defined as approximately 0.83, and for a pentagonal lens, as approximately 0.8. While Figure 7 illustrates the case with two adjacent pentagonal outer lens elements and FOV sets, the approach of defining peripheral fields and extended FOVs to support small regions of overlapping image capture can be applied to multi-camera capture device designs with adjacent pentagonal and hexagonal cameras, or adjacent pentagonal cameras, or cameras with adjacent edges of other polygonal shapes or any shape or contour in general.

[0067] For the extended FOV 215 to be functionally useful, the nominal image formed on the image sensor corresponding to the core FOV 205 must underfill the used image area of ​​the image sensor at least sufficiently to allow the extended FOV 215 to also be imaged. This may be done to help accommodate actual variations in the manufactured lens assemblies from ideal, or to accommodate manufacturing variations in designs with offset device centers 196 and improved low-parallax multi-camera panoramic capture devices (300). However, as will be explained later, judicious mechanical design of the lens assemblies can help to limit mechanical displacement or wedging, affecting both the image fields of a given camera and the seams between cameras, reducing parallax error and FOV overlap or underlap. Similarly, compensators or fiducial adjustments of image FOV (core FOV 205) size and position, as well as image centroid tracking and shape tracking, can be helpful. Some combination of optimization of distortion and low- or zero-parallax imaging for the extended peripheral field of view, careful mechanical design to limit and correct for component and assembly variations, and the use of correction standards or compensators can provide a superior overall system solution. As a result, images captured from the cameras can be easily cropped to the expected nominal size and shape relative to the nominal core FOV 205, and images from multiple cameras can then be mosaiced or tiled together to form a panoramic image with reduced image post-processing effort. However, the extended FOV 215 should provide sufficient additional angular width (e.g., θ1≦5% of FOV) to match or exceed any expected wedge or tilt angle θ2 that may occur at seams (θ1≧θ2), if necessary.

[0068] In designing an improved imaging lens (320, 920) of the type that can be used in a low-parallax panoramic multi-camera capture device (100 or 300), several primary parameters can be calculated to inform the design effort. The key parameter is the target size of the truncated cone or cone volume based on the selected polygonal configuration (lens size (FOV) and lens shape (e.g., pentagonal)) and sensor package size. Other key parameters that can be estimated include the nominal position of the paraxial entrance pupil, the focal lengths of the compressor lens group and the wide-angle lens group, and the FOV seen by the wide-angle group.

[0069] However, design optimization for an improved camera lens (320, 920) for use in an improved low-parallax panoramic multi-camera capture device (300) also depends on how numerous other lens attributes and performance metrics are prioritized. Specifically, relevant system parameters may include control of parallax or center of perspective (COP) error at the edge or inner field locations of the image field, or both, as optimized using spherical aberration of the chief ray fan or entrance pupil. These parameters are closely related to other important parameters, including the width and location of the "LP smudge" or volume 188, the size of any center offset distance between the entrance pupil or LP smudge and the device center 196, the target width of the gap or seam, the extent of the blind region 165, and the size of any peripheral or extended FOV to provide overlap. Relevant performance metrics may include image resolution or MTF, distortion (especially in the peripheral field and of the first compressor lens element and compressor lens group), lateral color, relative illumination, front color and color vignetting, telecentricity, and ghosting. Other relevant design variables may include mechanical and material parameters such as the number of compressor lens elements, the configuration of the compressor lens group, the wide-angle lens group, and the fisheye lens group, glass selection, the maximum allowable size of the first compressor or outer lens element, sensor package size, track length, the nominal distance from the image plane to the nearest front lens element (e.g., working distance), the nominal distance from the image plane to the entrance pupil, the nominal distance from the image plane or entrance pupil to the polygon center or device center, manufacturing tolerances and limitations, and the use of compensators.

[0070] While improving the optical design of the camera lens system is important for enabling an improved low-parallax panoramic multi-camera capture device (300), improving the optomechanical design may be equally important. As suggested above, the actual performance of the camera 120 may differ from the designed performance due to material and manufacturing variations and interactions between the individual lens elements 135 and the housing 130 and their component parts. Such variations may result in changes in image quality (e.g., aberrations, including distortion), focal length (EFL) and magnification, working distance or track length, beam pointing or imaging position, and other attributes of the camera 120. These variations may also mean that the assembly and performance of a given camera may differ from that of another camera with a nominally identical optomechanical design. For example, the focal length of a set of nominally identical cameras may vary by ±2%, which in turn causes similar variations in lens magnification and FOV. This variation can be reduced or eliminated by designing improved camera lenses to be variable-focus, including focal length compensators, such as by using lens elements whose axial position can be adjusted. Alternatively, camera 120 can be designed so that a nominal image from a nominal camera underfills the image sensor, so that an image from a camera with a larger (e.g., +2%) focal length lens also fully underfills the sensor, albeit with less margin. During calibration to determine the FOV, the EFL or magnification of the lens can be measured, and the sensor can also be aligned to be in focus with respect to that lens. Image processing software can then be used to correct the image for lens differences, including correcting image size for magnification and distortion variations between lenses.

[0071] As previously mentioned, improved low field of view camera lenses can be optimized using edge-field chief ray groups (170) or by limiting entrance pupil spherical aberration to a low parallax amount. However, analyzing or optimizing perspective error can be another useful method for optimizing parallax error for cameras used in improved low-parallax panoramic multi-camera capture devices. Essentially, parallax (image perspective error (in pixels)) is related to the spatial change in the center of perspective, ΔCOP. Analysis using perspective error or COP has the advantage of being more directly related to visible image artifacts compared to pupil spherical aberration or LP smudge measurements. Therefore, considering perspective error in captured images can be useful in defining target values ​​for less intuitive parameters, such as pupil spherical aberration, that can be used directly in optimization. Similarly, during optimization, it can also be useful to evaluate perspective error for a single lens channel or camera objective, with an emphasis on improving performance within the periphery of the capture FOV, or to provide average performance with reduced error over a larger capture FOV.

[0072] In the following discussion, several alternative or improved imaging lens 920 embodiments (FIGS. 8A, 9A, 10A, 11, and 12) are described in some detail and compared with the previous design of FIGS. 2A and 2B. As a first alternative lens, FIG. 8A shows a camera lens 920 with lens element 935, which is an improved version of lens 120 of FIG. 2A that may be used in the improved low-parallax multi-camera panoramic capture device (300). Although the lens systems of FIGS. 2A and 8A differ in many design and performance details, they share a general lens format and some similar structural features. As before, image light 975 from object space 905 is refracted through first lens element 937 and transmitted to the further inner lens elements and aperture stop to converge to a focused image at F / 2.0 at or near image plane 950, where an image sensor (not shown) is typically located. Because this lens is designed for a dodecahedron system (12 lenses, excluding supports), the first lens element 937 nominally images the FOV with chief rays at 37.377 degrees at the apex (vertex ray 970) and 31.717 degrees at the mid-chord (mid-edge ray 972). The chief ray projections converge or point toward the LP smudge 992, which contains the paraxial entrance pupil. The lens elements can be defined as consisting of a compressor lens group (e.g., first lens element 937), a wide-angle lens group 960 in front of the aperture with two lens elements, including a deep concave lens element 967, and an eyepiece lens group 965 after the aperture with four lens elements. This improved lens 920 is similar to the camera lens 120 of FIG. 2A, but it is designed for further improvement or reduction in parallax error (or center of perspective) and front color.

[0073] The prescription for this camera lens is given in Figure 8C-1, along with the glass or material type, axial thickness, and surface radius identified by lens or lens surface. The lens has seven lens elements, three of which have aspheric surfaces, one of which (deep concave lens element 967) is made of an optical polymer (Zeonex E48R) with a conic surface. The lens assembly fits within a 50.0 mm track length 980 and images to a full width of 3.9 mm at the sensor plane. The aspheric coefficients are also provided in Figure 8C-2. This lens was designed for color imaging at nominal wavelengths of 450, 587, and 656 nm for an image sensor with approximately 2.2 μm wide pixels. This camera lens 920 has a nominal focal length of 2.57 mm, an aperture stop diameter of 1.42 mm, and a low parallax point 992 with a paraxial entrance pupil and a nonparaxial chief ray point NP located closely within it. 8A, the offset distance 925 may be equal to the entrance pupil to image plane distance (EPID), which in this case is ≈16.8 mm behind or beyond the image plane 950. In this example, the ratio of EPID to focal length, i.e., EPID / EFL, is −16.8 mm / 2.57 mm ≈ −6.5:1. The negative value of this ratio, and its magnitude, indicates extreme positioning of the entrance pupil behind the image plane 950.

[0074] Figures 8B and 8D-G show some relevant performance parameters for the example lens design described in Figures 8A and 8C. As an alternative to interpreting the low-parallax amount graphically (e.g., Figure 5C), Figure 8B shows the variation in the perspective center 900 as the error or difference in image pixels versus field angle and color (R, G, B) for the optimized lens of Figure 8A. In this example, imaging of two objects was analyzed: one at a distance of 3 feet from an improved low-parallax multi-camera panoramic capture device (300) with a camera lens 320 of the Figure 8A design, and the other at a distance of "infinity" (∞) from the device. Figure 8B shows a parallax error of <1 pixel for all colors from on-axis to approximately the edge of the field (e.g., up to about 34 degrees). The parallax error for blue light exceeds 1.5 pixels at the extreme field points (e.g., the vertex). However, the human visual system and most visible imaging systems, including cameras using Bayer-type color filter arrays, are less sensitive to resolution-type errors when imaging with blue light compared to imaging with red and green light. Generally, providing a parallax error of ≦2 pixels from a camera within its core FOV 205 and particularly within its peripheral field of view, and preferably even within a modestly sized extended FOV 215, can limit residual image artifacts to an acceptable, difficult-to-detect level. It is preferable to further limit perspective or parallax errors to a sub-pixel level (e.g., ≦0.5 pixels) for imaging within these FOVs, and particularly within the peripheral field of view, at least for green light. If the residual parallax error between adjacent cameras is sufficiently small, captured images obtained from the core FOV can be easily and quickly cropped and tiled together. Similarly, if the residual parallax error within the extended FOV capturing content within or near the seam is similarly small enough and the two adjacent cameras are properly aligned with each other, the overlapping image content captured by the two cameras can be quickly cropped or averaged and included in the output panoramic image.

[0075] Specifically, Figure 8D shows the resolution for this lens as a modulation transfer function (MTF), which generally exceeds 0.75 for most field locations up to 150 cy / mm. In general, for many optical applications where MTF versus field is more difficult to obtain than in this example design, an MTF of ≥ 0.4 at the field edge is a reasonable goal. Figure 8E shows the astigmatism and field curvature focal shift for this lens, which is < ±0.025 mm across the field. The in-focus astigmatism for the tangential and sagittal meridians in this design causes a modest degradation in MTF at or near the polygon apex. Figure 8E also shows the modeled distortion performance for this lens, which is ≤ 1.0% across approximately the entire field, excluding the apex. In this example, the distortion follows a non-monotonic S-shaped curve due to the combined contributions of multiple orders of distortion (e.g., third-, fifth-, and seventh-order). The lens is also nominally telecentric to the image plane to ≦3.9° for all field points and colors.

[0076] Because these camera lens systems (920) are adjacent within the improved low-parallax multi-camera panoramic capture device (300) and therefore provide panoramic images, other parameters may be more important in the design of these lenses than in the design of typical lens systems. Figure 8F shows relative illuminance, which is >70% across the entire field of view, including at or near the field edge (approximately 37 degrees). Relative illuminance (RI) is the combined effect of vignetting and radiometric roll-off in an imaging lens and is generally given as the percentage of illuminance at any point on the image plane normalized to the position in the field of view with maximum illuminance (typically an on-axis position). The graph in Figure 8F is partly due to the fact that the Code V lens design program assumes that the lens element surfaces have simple MgF antireflection (AR) coatings, which produce different curves for RGB relative illuminance. However, the use of typical multilayer AR coatings can enhance these curves and reduce color differences. Chromatic differences in vignetting through the lens system can also cause RI differences for color. Increasing RI versus field of view helps reduce exposure and obtain differences within a camera and between adjacent cameras, thus making image tiling easier. Preferably, the target level for RI is ≧0.50 at the extremes of the imaging FOV. On the other hand, extreme angles of incidence (AOI) on lens element surfaces can increase Fresnel reflection losses and reduce RI, even with AR coatings.

[0077] FIG. 8G illustrates "front color," which is the difference in nominal ray path due to color versus field when directed toward an off-axis or edge field point. As shown in FIG. 8G, the blue light beam received on the outer surface of the first lens element 937 is approximately 1 mm further out than the red light beam received directed toward the same image field point. In other words, in this example, the width of the front color near the edge of the outer lens element is approximately 1.0 mm. If that lens element is not large enough, this blue light may be clipped or blurred, and color shading artifacts may occur at or near the image field edge. More broadly, local color transmission differences that can cause front color-related color shading artifacts near local image edges may arise from differential vignetting at the beveled edges of the outer compressor lens element 937, from edge truncation in the other compressor lens element 938 (FIGS. 9A and 10A), or through the aperture stop 945. As mentioned above, with respect to Figure 5E, front color can appear in the captured image content as a narrow, rainbow-like outline of the polygonal edges of the outer compressor lens element 937, which acts as a field stop for the optical system. As also mentioned above, front color can be reduced or corrected by various means, including as part of the chromatic correction of the lens design, with appropriate glass selection, or as a trade-off in correcting for lateral color. Front color can also be thought of as an artifact of axial chromatic aberration at or near the entrance pupil, so reducing the latter can aid the former.

[0078] While the example imaging lenses of Figures 8A-G are reasonable lenses for use in the improved low-parallax multi-camera panoramic capture device (300), different design tradeoffs can be made. For example, some MTF performance, relative illuminance (RI) performance, lateral color performance, or telecentricity performance can be sacrificed to further improve front color or parallax error performance. It can be useful to add a second compressor lens element between the outer compressor lens element 937 and the deep concave lens element 967. Alternatively, similar lenses can be designed utilizing optical polymers, although color correction can be difficult to achieve as a result.

[0079] As illustrated by FIG. 8A, this type of camera lens, or lens format, with a compressor lens group or element (937), a wide-angle lens group 960 in front of the aperture, and an eyepiece lens group 965 behind the aperture, in whole or in part (lens groups 960 and 965), visually resembles a fisheye lens, but it is quite different. Unlike present lens designs (e.g., FIG. 8A), a fisheye lens is an ultra-wide-angle lens with severely overcorrected spherical aberration of the pupil, such that its entrance pupil is located close to the first lens element and near the front of the lens. This pupil aberration also causes a substantial shift and rotation of the non-paraxial entrance pupil relative to the paraxial entrance pupil. Such lenses also offer a long back focal length, reversing the telephoto, and a positive value for the ratio of the entrance pupil to the image plane (EPID) divided by the lens focal length (EPID / EFL). Fisheye lenses also offer strong visual distortion, typically following a monotonic curve (e.g., H = fθ (f-theta)) and capturing images with a characteristic convex, nonlinear appearance. While a typical fisheye lens nominally captures a wide-angle full FOV of 180°, fisheye lenses capturing images with even larger FOVs (270–310°) have also been described in the literature. In contrast, the wide-angle cameras of the present approach, used within the improved low-parallax multi-camera panoramic capture device (300), are intentionally designed with low distortion, particularly at or near the edges of the imaging FOV, to facilitate image cropping and tiling. Furthermore, while the present cameras are wide-angle, they typically capture image light from a significantly smaller FOV than fisheye lenses. For example, cameras for dodecahedron devices nominally capture images from a full-width FOV of ≈63–75°. On the other hand, an octahedral device may have a camera that captures image light from a nominally full-width FOV of ≈71-110°, and an icosahedral device may have a camera that captures image light from a nominally full-width FOV of ≈42-75°.

[0080] The compressor lens or lens group (937) sharply redirects incoming light toward the wide-angle lens group 960. The first compressor lens refracts incoming light so that rays traveling toward the paraxial entrance pupil (190 (FIG. 5)) or chief ray offset NP point (192) are positioned at an appropriate location behind the image plane, allowing both the lens elements and the sensor to be packaged within a planned conical volume or frustum. The steep ray refraction provided by the first compressor lens or compressor lens group dramatically moves the inner lens elements and the image plane closer to the compressor element, thus shortening the total lens track length overall. The use of multiple compressor lens elements in a lens group, rather than a single lens element, provides more optimization variables and opportunities for improving imaging performance. For the example lens of FIG. 8A, the incident extreme chief ray at 37.377 degrees may be directed at an angle of approximately 65 degrees relative to the optical axis 985. The wide-angle lens group 960 accepts this light and bends it sharply at an angle of approximately 33 degrees relative to the optical axis through a small aperture stop 945. The eyepiece lens group (965) then presents the passing image light across the imaging FOV and nominally telecentric to the image plane 950.

[0081] The wide-angle lens group 960 before the aperture and the eyepiece lens group 965 after the aperture are not used as standalone systems for this application if the compressor lens group 937 is removed, but these two inner groups can also work together to form an image at or near the image plane or sensor. In the optical design of the camera lens (920), these lens groups, and particularly the wide-angle lens group 960, visually resemble a door peeper lens design. However, although this combination of two groups of lens elements may again appear to resemble a fisheye or door peeper-type lens, they again do not image with fisheye-type f-theta lens distortion (e.g., H=fθ).

[0082] Specifically, with respect to the example lens of FIG. 8A , the two inner groups of lens elements are not designed or optimized to operate in isolation without the compressor lens element, and parameters such as resolution or chromatic correction may be suboptimal. However, when analyzed in isolation without the compressor lens, the wide-angle lens group 960 before the aperture and the eyepiece group 965 after the aperture of the lens of FIG. 8A require only minor defocus correction. Notably, in this example, the wide-angle lens group 960 before the aperture and the eyepiece group 965 after the aperture do not contribute F-theta distortion, but instead contribute a nearly constant, approximately 0.0% distortion to about a 45° FOV, after which the distortion grows rapidly with negative or barrel distortion (e.g., to −20% at a 65° FOV). The compressor lens contributes a nearly opposite or canceling distortion, such that the net distortion for the entire lens system ( FIG. 8E ) is a few percent or less.

[0083] In contrast, the optical structure of the rear lens group (965), or sub-system, resembles an eyepiece structure similar to that used as the eyepiece of a microscope or telescope, but used in reverse without the presence of an eye. An eyepiece is an optical system whose entrance pupil is always located outside the system. The entrance pupil of the eyepiece, where the eye may be located in visual applications, nominally overlaps with the plane where the aperture stop 945 is located. Similarly, the nominal input image plane for visual applications corresponds to the sensor plane (950) in this application. The eyepiece group (965) is not designed to interface with the eye and therefore does not meet the requirements of an actual eyepiece for pupil distance, accommodation, FOV, and pupil size. However, this eyepiece-like lens group solves similar problems and therefore has a similar shape to that of an eyepiece. Depending on the application, the optical design can provide nominal optical performance more or less similar to that of a more typical eyepiece.

[0084] In contrast, camera lens 920 includes a compressor lens group (first lens element 937), a wide-angle lens group, and an eyepiece lens group 965 that have similar features or aspects to other recognizable lens types. Understanding these comparisons is useful for informing the lens design, tolerances, mechanical design, alignment, and assembly of these lens systems. However, camera lens 920 as a whole is unique in its correction or control of parallax error, which involves providing extreme positioning of the entrance pupil (or LP smudge 992) well behind or beyond the image plane 950.

[0085] FIG. 9A shows an alternative improved camera lens 920 with lens element 935, an enhanced version of the lens 120 of FIG. 2A that can be used in the improved low-parallax multi-camera panoramic capture device (300). FIG. 9A shows the entire lens on the left, with a partial enlargement showing inner lens element 940 in more detail. This lens, also designed for a dodecahedron system, has lens element 935, which includes both compressor lens group 955, which includes outer lens element 937 and compressor lens element 938, and inner lens element 940. In this design, compressor element 938 is not fully bonded, either cemented or as an air-spaced doublet. Also shown in FIG. 9A, inner lens element 940 is composed of a front wide-angle lens group 960 and a rear eyepiece-like lens group 965. Image light 975 from object space 905 is refracted and transmitted through compressor lens element group 955, which has three lens elements, so that the 37.377 degree chief ray at the vertex (vertex ray 970) is redirected at a steep angle of approximately 80 degrees toward optical axis 985. Front wide-angle lens group 960 includes two lens elements that direct the passing image light through aperture stop 945. Eyepiece lens group 965, which has five lens elements, redirects the passing image light coming from aperture stop 945 to provide the image light telecentrically at F / 2.8 to the image sensor at image plane 950. Because this lens is designed for a dodecahedron system, the first lens element 937 nominally accepts image light for an FOV width of 31.717 degrees at the middle chord (middle edge ray 972). The chief ray projections converge or point towards the LP smudge 992, which includes a paraxially defined entrance pupil.

[0086] This improved lens 920 is similar to the improved camera lens 920 of FIG. 8A but has been designed for a more demanding set of conditions related to parallax correction, a larger image size (4.3 mm wide), and an entrance pupil that has been moved farther away to provide more room for the use of a larger sensor substrate. The last requirement increases the distance between the image sensor plane and the entrance pupil or low-parallax volume 992. Specifically, the focal length is larger (5.64 mm) to project the image onto the large sensor. As shown in FIG. 9B, which shows a portion of the eyepiece group, there are several potentially useful reference planes or locations within the LP smudge 992, including the paraxial entrance pupil 990, the position of the perspective center 902, the position relative to the nonparaxial chief ray NP point, or the position of the circle of least confusion where the LP smudge or parallax volume has a minimum size in the plane tangent to the optical axis. The entrance pupil 990 is a good reference because it is easily calculated from general first-order optical equations. The axial location of the perspective center 902 is also a good reference, as it is directly related to perceived image quality. The distance from the image plane 950 to either of these locations can be used as a reference, but an offset distance 925 relative to the entrance pupil 990 may be preferred. In this example (FIG. 9B), the entrance pupil 990 is positioned approximately 30 mm behind the image plane 950, for a negative ratio of entrance pupil distance to focal length, EPID / EFL=−5.3:1. Depending on how it is measured, the LP smudge 992 may have an axial width 997 of ≦2 mm.

[0087] 10A and 10B show a second alternative camera lens 920 having lens element 935, along with a close-up view of inner lens element 940. This camera lens 920 is an improved version of lens 120 of FIG. 2A that can be used in the improved low-parallax multi-camera panoramic capture device (300). This lens, also designed for a dodecahedron system, includes lens element 935, which is composed of compressor lens group 955 and inner lens element 940. Compressor lens group 955 is composed of outer lens element 937 and compressor element 938, which are provided combined as a cemented doublet. Also shown in FIG. 10A, inner lens element 940 is composed of wide-angle lens group 960 and eyepiece-like lens group 965. Image light 975 from object space 905 is refracted and transmitted through compressor lens element group 955, which has three lens elements, so that the 37.377-degree chief ray at the vertex (vertex ray 970) is redirected at a steep angle of approximately 75 degrees toward optical axis 985. Wide-angle group 960 includes three lens elements that direct the passing image light through aperture stop 945. Eyepiece lens group 965, which has five lens elements, redirects the passing image light coming from aperture stop 945 to provide the image light telecentrically at F / 2.8 to the image sensor at image plane 950. Because this lens is designed for a dodecahedron system, the first lens element 937 nominally accepts image light for a 31.717-degree FOV width at the middle chord (middle edge ray 972). The chief ray projection converges or points toward LP smudge 992, which contains the paraxial entrance pupil. As shown in FIG. 10B, the lens has a track length 980, from the axial apex to the image plane 950, of 78.5 mm.

[0088] This improved lens 920 is similar to the improved camera lens 920 of FIG. 9A, but has been designed for an improved set of conditions. The image size is slightly smaller (3.7 mm wide), but parallax and front color are better corrected, and the entrance pupil is moved farther away to provide more room for the sensor board, wiring, and cooling hardware. Specifically, the lens offers a smaller focal length (4.9 mm) and a large offset distance 925 (approximately 35.5 mm) to the entrance pupil or LP smudge 992 behind or beyond the image plane 950, with a relative ratio of -7.25:1.

[0089] Although the camera lens of FIGS. 10A and 10B has a similar structure to that of FIG. 8A , with a large compressor optic, wide-angle group 960, and eyepiece group 965, the details of the lens structure, related to the number and shape of lens elements, are visually quite different. The actual lens performance is also similar, but not identical. As an example, FIG. 10C shows the MTF performance of this second alternative camera lens 920, which is approximately 0.75 averaged over the entire field of view at 100 cy / mm. As another example, FIG. 10D shows the relative illuminance, which is approximately 0.57 at the full field of view (approximately 37 degrees). While this performance is smaller than that of the FIG. 8F example, it is still high enough to significantly reduce the amount of exposure correction required between one camera lens system and another adjacent camera lens system (see, for example, FIG. 9 ). In general, it is preferred that camera lenses 920 for use within the improved low parallax multi-camera panoramic capture device (300) be designed to have an RI≧0.5 at the edges of the imaging FOV, and even more preferably an RI≧0.75.

[0090] FIG. 11 shows a third alternative camera lens 920 with a lens element 935 comprising four optical polymer lens elements, which is an improved version of the lens 120 of FIG. 2A that can be used in the improved low-parallax multi-camera panoramic capture device (300). Designed for a system shaped as an icosahedron (20 lenses, excluding supports) with triangular sides or lens faces, this lens includes lens element 935 composed of a compressor lens group 955 and an inner lens element 940. As also shown in FIG. 11, inner lens element 940 is composed of a front wide-angle lens group 960 and a rear eyepiece-like lens group 965, both of which have several aspheric or conic surfaces. Image light 975 from object space 905 is refracted and transmitted through compressor lens group 955, which has a single lens element, so that the chief ray at 37.4 degrees at the vertex (vertex ray 970) is redirected at a steep angle of approximately 60 degrees toward the optical axis 985. Front wide-angle group 960 includes two lens elements that direct the passing image light through aperture stop 945. Eyepiece group 965, having four lens elements, redirects the passing image light coming from aperture stop 945 to provide the image light to an image sensor at image plane 950 at F / 2. Chief ray projections converge or point toward LP smudge 992, which includes a paraxial entrance pupil.

[0091] FIG. 12 shows a fourth alternative camera lens 920 with lens element 935, which is an improved version of lens 120 of FIG. 2A capable of capturing light within a 110° FOV for use in an improved low-parallax multi-camera panoramic capture device (300) with an octahedral (eight lenses, excluding supports) structure. This lens includes lens element 935, shaped as an octahedron with triangular sides or lens faces, designed for a system composed of compressor lens group 955 and inner lens element 940. As also shown in FIG. 12, inner lens element 940 is composed of front wide-angle lens group 960 and rear eyepiece-like lens group 965, both of which have several aspheric surfaces. Image light from object space 905 is refracted and transmitted through compressor lens group 955, which has a single lens element, so that the 55-degree chief ray at the vertex (vertex ray 970) is redirected at a steep angle of approximately 80 degrees toward optical axis 985. Wide-angle group 960 includes two lens elements that direct the passing image light through aperture stop 945. Eyepiece group 965, with four lens elements, redirects the passing image light coming from aperture stop 945 to provide the image light to an image sensor at image plane 950 at F / 2. Chief ray projections converge or point toward LP smudge 992, which includes a paraxial entrance pupil. Due to the larger FOV, the offset distance 925 between LP smudge 992 and image plane 950 tends to be smaller than in other systems.

[0092] The example alternative camera lens 920 of FIG. 12 is designed for potential use in the low-cost surveillance or smartphone market. It can use several lens elements, potentially manufactured using optical polymers, such as the last element in the eyepiece group 965, located just before the cover glass of the image sensor 950. This lens element has a size and aspheric shape similar to elements used in cell phone cameras. The lens has a focal length of 1.98 mm and provides image light to the image plane 950 at F / 2.

[0093] Considered as a set, the improved camera lens systems 920 of Figures 8A, 9A, 10A, 11, and 12 provide various examples of lens formats with a common set of features: an initial compressor lens group that bends light sharply toward the optical axis, a physically much smaller wide-angle lens group that redirects light into the aperture stop, and an eyepiece-like lens group that focuses the passing image light toward the image plane. The requirement to reduce parallax or perspective error while allowing multiple polygonal cameras to be adjacent to form an even larger improved low-parallax multi-camera panoramic capture device (300) results in extreme lens formats where the lens elements in the compressor lens group can be somewhat large (e.g., 80-120 mm in diameter), and more typically, at least some of the lens elements in the wide-angle and eyepiece lens groups are simultaneously somewhat small (e.g., 5-10 mm in diameter). In these types of lens designs, the first compressor lens element or outermost lens element 937, and adjacent outer lens elements of adjacent lens systems, may alternatively be portions of a continuous faceted dome or shell. It is also typical for some (e.g., 2-4) of the lens element surfaces to have aspheric or conic contours to bend or direct light rays passing near the edge of the lens element differently than light rays passing near the center or optical axis. Typically, the wide-angle lens group 960 also has a lens element (967) with a deep concave surface. In some cases, during optimization, the surface may want to be overly hemispherical to improve element manufacturability, but such contours are preferably avoided. Another measure of the ultimate performance of this lens type is the offset distance of the paraxial entrance pupil (or, equivalently, LP smudge) behind or beyond the image plane. Unlike a typical lens, the entrance pupil is not in front of the image plane, but instead may be pushed far behind or beyond it. This is emphasized by the negative ratio of entrance pupil to image plane distance / focal length, EPID / EFL, which can range from -2:1 to -10:1, but is typically ≥ -4:1.

[0094] As shown in the exemplary details of FIG. 5C , optimizing the size, location, and characteristics of the LP smudge or low-parallax volume 922 affects the performance and design of the improved camera lens system 920, as illustrated in FIGS. 8A , 9A , 10A , 11 , and 12 . Low-parallax volume optimization is heavily influenced by merit function parameters and chief ray weighting for both entrance pupil spherical aberration and entrance pupil axial or longitudinal chromatic aberration. Lens element and lens barrel manufacturing tolerances can also affect the size and positioning of this volume, or equivalently, the amount of residual parallax error provided by the lens. Thus, even though these lenses may be considered to have extreme shapes, optimization can help reduce sensitivity to manufacturing errors and provide insight into how and where to provide corrective adjustments or compensators.

[0095] As previously mentioned, the distortion of the compressor lens or compressor lens group (955) tends to cancel the combined distortion of the inner lens group (940). High-magnitude distortion from the compressor lens group is useful for controlling parallax and providing ray bending to position paraxial and non-paraxial “NP” points or low-parallax amounts beyond or behind the image plane. As a result, however, the inner group of lens elements must provide large, nearly canceling distortions to yield small residuals across the imaging FOV. Because the shape and magnitude of the individual distortions of the compressor lens group, as well as those of the combined inner group of lens elements, can vary dramatically between lens designs of this type, the overall shape of the distortion can also vary dramatically, even if the overall distortion has a magnitude of <2%. Thus, distortions in the intermediate or edge fields can be locally optimized to have a curved shape (e.g., as in FIG. 8E ) or a flatter shape (e.g., as in 250B in FIG. 6 ). As another factor, note that the distortion optimization trade-off between the compressor lens element and the inner lens element may also affect the chief ray pointing from the first compressor lens element 937, and therefore the parallax correction, as represented by the location of the off-axis or non-paraxial chief ray NP point within the low parallax amount.

[0096] In designing this type of lens system for visible applications, it can be quite useful to use high-index, low-to-medium dispersion optical materials such as Ohara S-LAH53 or SLAL-18, especially for the compressor lens element. As an alternative, the optical ceramic Alon from Surmet Corporation of Burlington, Massachusetts, USA, has a refractive index comparable to these materials but even less dispersion, which can make it very useful in these lens designs. Using optical polymers or plastics in these lens designs can be particularly useful to reduce cost and weight, but also for other reasons. Compressor lens elements, and especially the first or outermost compressor lens element 937, can be very large and subject to complex edge beveling, making them good candidates for glass-to-polymer replacement. High-index optical polymers, such as OKP4 from Osaka Gas Chemicals or EP5000 from Mitsubishi Gas Chemical, can be particularly useful for such purposes. Similarly, it can be beneficial to use optical polymers for deep concave lens elements 967 (such as Zeonex E48R) compared to fabricating surfaces with extreme hemispherical or conical contours. Unfortunately, optical polymers have a much more limited range of optical properties than optical glasses, and high refractive index polymers have both lower refractive indices and higher dispersions than glasses, which can constrain optical design or performance.

[0097] Depending on priorities, these lens systems can be further optimized, and various variations in lens formats may be better suited for different markets or applications. Typically, the outermost lens element, or first compressor lens element, uses Ohara SLAH52, SLAH53, and SLAH63 glasses (or equivalent glasses from Schott Glass, e.g., N-LAF36 and N-LASF43), which are high-index, low-dispersion flint glasses with a visible spectrum refractive index n of approximately 1.80 and an Abbe number Vd of approximately 41.5. It should be understood that other optical materials can be used for the lens elements in camera lens 520 in general, including the compressor lens element. For example, the use of a high-index, low-dispersion, mid-crown glass such as Ohara SLAL-18 can be useful for color correction. As another example, lens elements can also be made from optical ceramics such as Alon (n ~ 1.79, Vd ~ 57-72) or spinel, which are extremely durable materials similar to sapphire but with excellent optical transparency, low dispersion, and a controllably tunable isotropic crystal structure. Lens designs using optical polymers for at least a portion of the large compressor lens element and high-index, low-dispersion flint glasses, such as Alon or spinel, for at least a portion of the smaller inner lens elements can also be advantageous. It should also be understood that camera lenses of this approach can be designed with optical elements composed of or including refractive, gradient index, glass or optical polymer, reflective, aspherical or freeform, kinoform, Fresnel, diffractive or holographic, subwavelength, or metasurface optical properties. These lens systems can also be designed with achromatic or apochromatic color correction or thermal defocus desensitization.

[0098] Another aspect of the design of these improved camera lenses 920 for use within the improved low-parallax multi-camera panoramic capture device (300) relates to their fit within larger polygonal (e.g., dodecahedron, icosahedron, Goldberg polyhedron) devices. As shown in FIG. 10B , the camera 920 can be positioned an offset distance 925 in front of the image plane 950. The exact distance 925, relative to its location within the width 997 of the LP smudge 992, depends on the location at which it is calculated or measured (e.g., the paraxial entrance pupil-to-image plane distance (EPID) or the distance from the surface with the non-paraxial chief ray NP point to the image plane 950) and the color or wavelength (e.g., green) used. The device center 910 (e.g., polygon center) can also be located within the width 997 of the LP smudge 992, or it can be located a center offset distance 915 outside the smudge width or volume. In some device designs, it can be particularly useful to provide an axial center offset distance 915 so that the LP smudge 992 or entrance pupil 990 is positioned at an axially greater distance from the device center 910. Designing in a center offset distance 915 (e.g., 1-4 mm) can provide extra space for power, communication, or cooling connections, cables, and mechanical support for the sensor package 952 or other structural components. In the example camera system 920 shown in FIG. 10B , the improved low-parallax multi-camera panoramic capture device (300) has a dodecahedron shape, and the device center 910 is the nominal center of the dodecahedron polygonal structure. This offset distance needs to be determined during the design process of the camera 920 and the overall device 300, as it interacts with the optimization of lenses near the edges of the FOV. This optimization is therefore dependent on or interacts with the budget for the extended FOV 215, distortion correction, front color control, optimization for the range of reduced parallax or LP smudge 992, and lens element sizing (particularly for the compressor lens group 955).

[0099] The following discussion describes issues surrounding the installation or assembly of improved low-parallax camera lenses 920 for use within the improved low-parallax multi-camera panoramic capture device 300. These improved camera lenses 320 may be based on any of the various designs for the improved imaging or camera lenses 920 described in connection with Figures 8A, 9A, 10A, 11, and 12, or the lens of Figure 2A, or variations thereof.

[0100] FIG. 13 shows a cross-sectional view of an improved optomechanical structure for a low-parallax multi-camera panoramic image capture device 300 and a 3D view of its camera channels. Manufacturing variations for individual cameras and the optomechanical interactions between them can significantly affect the design and performance of such a multi-camera capture device 300. During initial assembly of the multi-camera capture device 300, tolerances and mechanical wedge interactions between the housing 430 of a first camera 320 and the housing 430 of a second adjacent camera 320 can affect the seam 400, the aiming of the core FOV 205 or extended FOV 215 of each individual camera (causing FOV overlap or underlap), and consequently the FOV captured by each camera. Furthermore, mutual alignment mounting stresses imposed on adjacent cameras by close mounting can physically deform one or more of the camera housings, potentially distorting or distorting the optical imaging function of the camera lenses. As a result, the camera lens system may provide an image to the sensor that is shifted or rotated or tilted out of plane. These potential problems, or the risk of their occurrence, may be exacerbated by environmental effects, such as asymmetric thermal loads or substantially asymmetric optical loads.

[0101] To combat such problems, an improved multi-camera capture device 300, as shown in FIG. 13, may include features for providing kinematic mounting of the individual cameras 320 or objective lenses. Specifically, FIG. 13 shows two views of a dodecahedral multi-camera capture device 300, including a partial cross-sectional view, in which eleven pentagonal cameras 320 are mounted on a central support 325 that occupies the nominal position of a twelfth potential camera channel. Each camera 320 has a separate base lens assembly or housing 430, consisting of a lens mount that mounts a compressor lens (437), as well as an inner lens element 440 that together comprise the base lens assembly. For each camera 320, the lens elements and housing 430 fit within a nominal conical space or volume, although they need not nominally fill that space. In practice, the sharp ray bending introduced by the compressor lens elements may mean that the inner lens elements 440 and their housings or barrels underfill the available space, and the entire lens housing 430 may taper further inward, potentially leaving an open interior volume 390 between adjacent lens assemblies.

[0102] The housing 430, or base lens assembly, of FIG. 13 also includes a turned section that can be machined on a CNC multi-axis (5-axis) machine and mates with the tripod-like channel centering hub 330. The channel centering hub 330 can be machined entirely on a lathe except for the pentagonal flange, which is completed in a finishing operation after lathe turning. Machining on a lathe means exceptional concentricity and runout can be achieved, aiding in ultimate channel alignment. The housing 430 mates with the inner diameter of the channel centering hub 330, a key part of the center-mounted mechanical assembly, which is designed to fit with it, ranging from a slip fit to a slight interference fit to ensure axial alignment without significant variation due to clearance tolerances. This same fit reduces perpendicularity error with respect to the channel axis.

[0103] The tripod or channel centering hub 330 also includes a swivel or ball pivot that engages with a socket 345 of a spherical socket array 346 provided on the central support 325. In this system, the camera 320, located at the polar position opposite the central support 325, is a precisely positioned reference channel. The central support 325 consists of a cylindrical post with a ball on top. The geometry of the central support 325 and tripod or centering hub 330 can be designed to provide more space for power and communication cables, cooling tubes, and mechanisms for securing the lens housing 430 or cables. In this example, the ball includes sockets 345, each of which can accept a ball pivot 340. The ball pivot 340 is at the end of an extension pin or ball pivot arm 342. This ball and socket portion of the central support 325 can be expensive to machine, but given the expected precision regarding socket location and depth, the advantage is that centerline pointing is controlled, while there is only one part per device 300 that requires exceptional precision. On the other hand, each of the camera channels 320 can be machined with less precision, which reduces both manufacturing and replacement costs.

[0104] Each camera lens housing 430 in FIG. 13 features an external, or outer, inter-channel datum 335 located midway along a pentagonal side. These inter-channel datums 335 can include two parallel, convex, slightly curved, protruding rods separated by a groove between them. Both datums are designed to provide single-point or localized kinematic contact or interaction between the lens housings, whereby the datum features intertwine in such a way that only one part or housing dominates from a tolerance perspective. Because they intertwine, only one part variation affects the distance between each camera channel and, therefore, the angle between the channels. Specifically, if one datum 335 is large, the other, which does not contact, dominates. Thus, only one tolerance contributes to the two parts. The intertwining of the inter-channel datums 335 from one camera 320 to another allows limited angular movement of the lens housing 430 while also limiting lateral movement between the mating (pentagonal) faces or sides.

[0105] Individually and collectively, the interaction between the camera lens housings 430 or base lens assemblies limits mechanical wedging or channel pointing errors (roll, pitch, and yaw) between the cameras due to both the ball and socket arrangement and the datum features (335). Each camera channel assembly works with its neighbors to limit channel pointing errors. The outer lens element 437 or the portion of the base lens assembly (430) that holds the compressor lens also has an internal functional datum that allows the compressor lens to be positioned perpendicular to the optical or mechanical channel axis, and it has additional internal datum features that limit axial misalignment. As described in more detail below with respect to FIG. 16, an additional set of alignment features provided on the polished edge of the compressor lens can also function as a datum and interact with these internal datum features to limit rotation about the pentagonal channel axis.

[0106] The use of alignment features shown in FIG. 13, specifically the ball pivot and socket datum (350 and 356) and inter-channel datum feature 335, reduces the risk of rotation, pivoting, or spreading from one camera channel (320) to another. These features therefore also help allow the seams 400 between cameras 320 to have a more consistent thickness relative to their nominal design values ​​than might otherwise occur. The use of internal features within the lens housings (e.g., compensators, adjustment screws, and shims) and external features between the lens housings (e.g., inter-channel datum, ball and socket datum, and channel loading support) helps control core or extended FOV aiming so that one camera channel can align with another adjacent channel. The combined use of inter-channel datum, ball and socket datum (FIG. 14), and channel loading support (FIG. 17) can also help reduce the device's sensitivity to mechanical or thermal loads.

[0107] 14 illustrates further design options for improving the optomechanical design of the outer lens elements 437, or compressor lens elements, proximate the seam 400 in the improved multi-camera panoramic image capture device 300. Specifically, the edges of the outer lens elements 437 can protrude above or beyond the outer edge of the lens housing 430 so that two adjacent outer lens elements 437 of adjacent cameras 320 can be in near contact at the seam 400. If these outer lens elements 437 are fabricated, at least in part, from a somewhat compliant material, some degree of actual physical contact can be possible. However, if these outer lens elements 437 are fabricated from a brittle material, such as glass, even greater care is required.

[0108] In the configuration of Figure 14, two adjacent outer lens elements 437 with protruding edges are in close contact at seam 400. In a preferred design approach, the opposing edges have a stepped edge angle 365 or structure. At the outermost portions, the two lenses and housing may result in a parallel seam 400 that is 1.0 mm or less in width.

[0109] The innermost portion, where the inner beveled edges 370 of adjacent outer lens elements 437 approach each other, can provide the lens housing 430, the inter-channel datum 335, and the flat surface datum 670. The edge of each outer lens element 437 then has a stepped edge groove 380 that can be filled with a compliant adhesive. Additionally, along the edge 432 or seam 400, the outermost edges of adjacent outer lens elements 437 extend, and these lens elements can be nearly adjacent, separated by a gap that can be only 0.5 mm wide or less. In practice, optimizing the seam width can depend on how brittle or compliant the lens material is (glass or polymer), the flexibility of the adhesive filling the seam, the use of other protective measures, and the application.

[0110] FIG. 17 provides further details regarding how these and other features can be used during alignment and assembly. Specifically, FIG. 17 shows a cross-sectional view of a portion of the improved multi-camera panoramic image capture device 300 of the present approach, in which eleven camera channels 320 are mounted on a column or vertical central support 325 through which both wiring and cooling can be provided (see FIG. 15 for further details). FIG. 17 specifically shows the design for the optomechanical hardware, although assembled without lens elements (see FIGS. 13 and 15 for illustrations of optomechanical hardware including lens elements). FIG. 18A shows in greater detail the major elements or components used in this example alignment and assembly approach.

[0111] As shown in FIG. 17 , the top camera 320 can be identified as the primary alignment channel 610, while all other camera channels 320 shown are identified as secondary channels 615. Specifically, with respect to the primary channel, ball pivot arm 342 with ball pivot 340 of channel centering hub 330 engages within ball socket datum 356 of ball socket array 346, where it interacts with locking axis retaining pin 348 that slides into a recess between ball pivot arm 342 and ball pivot 340 to prevent Z-axis (vertical) translation. Anti-rotation keying pin 349 is press-fit into a localized hole provided in the ball pivot arm to prevent rotation of the primary camera channel 610 about the z-axis. The ball pivot arm 342 of the primary camera channel 610 is lightly press-fit into its ball socket datum 356 to prevent rotation about the X and Y axes.

[0112] Alternatively, ball socket 345 may have a latching mechanism (not shown) for loading ball pivot 340 against ball socket datum 356 as a means to provide both a mechanism with reduced sensitivity to load forces and mechanical load forces being applied to the camera or device. For example, the latching mechanism may include a latch that is actuated with a linkage assembly. Compared to the use of a retaining pin, the latching mechanism is more compliant, sturdy, and reliable when camera channel 320 or device 300 are subjected to load forces. It should also be noted that lens housing 430 may also have identification markings to facilitate alignment of the housing with adjacent lens housings.

[0113] As previously shown in FIG. 13 and shown in more detail in FIG. 18A, each camera channel 320 has a pair of peripheral inter-channel datums 335 on each face, which are small (local) and nominally centered along the face or side of the lens housing. For pentagonal lenses, each of the five side faces may have a pair of datums (335). These datums have curved surfaces 373 (shown in bold and elongated for emphasis) to allow for point-to-point type contact (375). Specifically, each pair of inter-channel datums 335 has a side surface that serves as a lateral datum between the camera channels. They are curved to accommodate relative angular movement between the channels. Only each side datum has an effective single tangent point 375 where each of their mating datum radial surfaces meets. Because the radii are so large, the datum surfaces approximate a straight line near the intended single tangent point. Therefore, any shift of the camera channels relative to one another, which may be an angular or centerline offset, will consequently have only a small effect on the relative lateral offset of a given camera channel.

[0114] 17, the improved multi-camera panoramic image capture device 300 may also have channel loading supports 630 that bias all of the secondary channels 615 relative to the primary channels 610. The channel loading supports 630 employ peripheral datum pairs that are nominally identical to the inter-channel datums 335 used on each channel. The channel loading supports 630 also have spring elements 635 that facilitate loading of the secondary channels 615 relative to the primary channels 610. While the use of keyed supports may impose excessive constraints, the channel loading supports 630 may also utilize a key feature to prevent rotation.

[0115] The primary alignment channel 610 (see FIG. 17) is aligned and locked in place when its ball pivot arm 342 and ball pivot 340 engage with pins (348, 349) on the channel centering hub 330. The secondary camera channels 615 are then added and loosely aligned by fitting their ball pivots 340 against datums in sockets on the channel centering hub 330. When the channel loading support 330 is added at seam 400, it gently presses the secondary channels 615 against the primary channel 610 and against each other. The relative size of the ball socket 345 to the secondary channel's ball pivot 340 is provided such that the secondary channels 615 are constrained only in the Z direction by the ball socket 345.

[0116] Then, at seam 400, for any two faces of adjacent camera channels (610, 615) to be parallel to one another, the configuration requires that at least three camera channels have their peripheral inter-channel datums 335 contact the opposing inter-channel datum 335 at their tangent points 375. This effectively constrains the secondary channel 615 to three degrees of freedom (DOF). It has been recognized that applying traditional kinematic machine design principles, in which motion and constraints in six DOFs can be tightly limited with little crosstalk, can be inherently challenging in devices capable of arranging numerous complex polygonal faces (e.g., pentagons or hexagons) in close proximity. In cases where four or more camera channels are in close proximity, the camera channels may become overly constrained if they are not in contact with one another. The system may then be pseudo-kinematic, and mechanical stresses and strains may then cause component misalignment or damage. These potential problems can be overcome in various ways. As one example, the relatively small size and centering location of inter-channel datum 335 can limit any angular or spatial misalignment to a relatively small amount. As another example, a compliant material such as RTV can be applied to seam 400 to absorb any stress or strain, while lens housing 430 can be designed and manufactured to be sufficiently rigid to resist deformation, misalignment, or damage. Seam width variations, including dynamic ones caused by thermal or mechanical loads, can also be corrected by providing an improved camera 320 that captures image light 415 with an extended FOV 215.

[0117] As shown in FIG. 18A , the contact points can occur along a line across the radial surface of the datum. Thus, each inter-channel datum 335 makes “line” contact with the adjacent set of inter-channel datums 335 if the mating surfaces are preferably parallel; otherwise, only point contact occurs. Typically, peripheral inter-channel datums 335 make contact with the opposing surfaces at only a single point. For each inter-channel datum pair, only a single datum may make contact due to dimensional differences. This is an advantage of this design approach, since only a single datum can affect the gap or seam 400 between adjacent camera channels. This approach can be used in various designs for improved multi-camera panoramic image capture devices 300, including devices with “soccer ball” or truncated icosahedron ( FIG. 4 ) geometries, where the primary alignment (camera) channel can be an outer hexagonal lens. This approach can also be used in “hemispherical” device configurations.

[0118] FIG. 18B-1 shows a cross-sectional view across the seam of an alternative or improved version of the lens housing 430 structure and their interface near seam 400 shown in FIGS. 14 and 18A. Specifically, FIG. 18B-1 shows portions of two adjacent lens housings 430 arranged around seam 400, each housing supporting an outer compressor lens element 437 and at least one second compressor lens element 438. The lens housings 430 include inner light traps 457 and side surfaces that extend into grooves 433 cut into the edges 432 of the outer lens elements 437. The outer walls of the lens housings 430 are tapered, diverging toward the center of the device to provide greater mechanical rigidity and sturdiness. The interaction of two pairs of adjacent inter-channel datums 335 can be seen in the cross-sectional view.

[0119] FIG. 18B-2 shows an alternative cross-sectional view of these same components but cut along or within seam 400 to illustrate the interaction of inter-channel datum 335. As in FIG. 18A, adjacent inter-channel datum pairs 335a and 335b each have curved surfaces 373 that interact locally to help align or line up adjacent camera channels together. In this example, to further reduce the potential for over-constraint, the inter-channel datum pairs are asymmetric and partially offset, thus more likely to provide local point contact and less likely to cause over-constraint. Circular hole 530 provides access through the sidewall of lens housing 430 to allow for the application of adhesive or RTV used in attaching outer lens element 437 to the housing.

[0120] As an alternative to the camera channel 320 (e.g., FIGS. 13 and 17 ) having a ball pivot 340 at the end of the tripod or channel centering hub 330, the tripod could instead have a concave socket. Specifically, the socket could have a concave surface that contacts a nominally matching convex surface of a spherical center hub (not shown). The center hub, which can be attached to a support post with its center nominally positioned at the device center, has a series of mounting areas, each of which interacts with a concave socket in the camera channel. A cable with a ball on one end can be used to pull or tension the socket against the center hub. The primary and secondary channels can each be held together by a similar tensioned cable that descends into the support post, where they are fastened and locked. Alternatively, the primary channel can be held in place with a tightened bolt. Compared to the previous approach of FIGS. 13 and 17 , this approach trades multiple balls and sockets for an inverted configuration with multiple sockets (one per camera channel) contacting one main ball or hub. The tensioned cable replaces previous approaches that used retaining pins, latches, or springs. This approach can allow multiple camera channels to be simultaneously and reliably tensioned around the center of the device in alignment with a ball hub. The ball hub can be machined from a precision ball bearing.

[0121] During assembly and alignment of the camera channel, it is also important to properly position the lens elements within the housing. For example, the compressor lens (437) may include datum features along its edge 432 that interact with a set of mating datum features on the inside of the lens housing 430. Datum features provided on the lens elements may be intelligently machined or polished features intended to limit perpendicularity and concentricity errors, or may be designed to be adhesively attached to the beveled surface of a polished glass lens. As shown in FIG. 16, these features may include flat datum features 650 fabricated at corners on the bottom surface of the compressor lens, outside the FOV. Other datums, including adjacent flat edges 660 that can be polished onto the round lens element before it is formed into a pentagon with truncated or beveled edges 370, can mate with flat surface datums 670 on the lens mount. Because glass is typically adhesively attached to metal, these flat edge datums can provide guided alignment without the risk of overconstraint. The outer lens element 437 may also include an anti-clocking datum feature 680, possibly bonded to one of the beveled surfaces.

[0122] Because of these datum features, chamfers or beveled finishes on the pentagonal lens surfaces cannot be manufactured with great precision, thus helping to reduce lens cost. Although Figure 13 shows a pentagonal-shaped compressor lens or outer lens element 437 and housing 430, these mechanical approaches to reducing misalignment errors can also be applied to hexagonal-shaped lenses, or lens elements having other polygonal shapes.

[0123] In providing an improved multi-camera panoramic image capture device 300 with outer lens elements 437 having inner beveled edges 370 (FIG. 13), it is recognized that centering tolerances may be poorer than typical lens elements manufactured with conventional cylindrical edges. Such decentering can consequently affect the centering of the FOV captured by the entire camera lens system 320. As one approach to correcting such errors, the positioning of the effective image centroid or central pixel can be determined either optically or electronically. As another approach, the improved lens housing 430 can be designed to provide corrective lateral adjustment for one or more internal lens elements. For example, means can be provided to adjust the positioning or tilt of intermediate internal lens elements, such as for lens elements located between the aperture and the image sensor. The adjustment means can include or use micrometers, pins, shims, flexures, or springs. Z-axis compensators for adjusting focus or magnification differences can also be provided within each camera channel using similar mechanisms. The mechanisms for enabling correction may be built into the device or camera, or may be internal, external, or a combination thereof.

[0124] For example, the axial alignment or focal position of the image sensor relative to the imaging plane provided by the camera lens assembly (320) can be improved by an appropriate mechanism. For example, three adjustment screws can be used to control X-translation in conjunction with Z-axis rotation, another set of three screws can be used to control Z-axis translation in conjunction with XY-axis rotation, and an additional screw is used to control Y-axis translation. A pair of springs can be used to hold the gimbal plate, simultaneously allowing X- and Z-axis translation, respectively. Other adjustment designs or devices can be used within the tight spatial constraints allowed by the camera 320 and the entire panoramic multi-camera capture device 300, such as using pins and micrometers or pins and shims. Optical fiducials, using known and controlled light sources, can also be used within the camera assembly to allow image centroid pixels on the image sensor to be identified and tracked. The use of optical fiducials can also assist in identifying, tracking, and correcting polygon edges of polygonal FOVs captured by the camera channels. Similarly, camera channels may also have baffles incorporated into their lens housings that provide sharp shadows at the image plane or sensor. For example, a camera channel may have a baffle positioned between the outer lens element 437, or compressor lens, and the subsequent inner lens element. The baffle may also provide a sharp polygonal-edged opening (e.g., pentagonal or hexagonal) that follows the nominally polygonal shape of the outer lens element 437 and core FOV 205, as well as a blackened surface to block and absorb light outside the intended FOV. Alternatively, light-absorbing baffles may be printed or coated onto the inner lens element surfaces. Thus, the baffle or mask may also define the edges of the passing image light, thus casting an edge shadow onto the image sensor 270. As such, the baffle may help clearly define the illuminated polygonal edges of light incident on the image sensor, thus aiding in the determination and spatial and temporal tracking of the imaged polygonal FOV, relative to either the core FOV 205 or the extended FOV 215.

[0125] 13-14 and 16-18, the lens housing 430 may include one or more tab- or post-like structures (not shown) that protrude from the lens housing 430 outside the nominal conical space or volume and can interact with similar protruding structures on adjacent lens housings, or alternatively, with recessed structures on adjacent lens housings. For example, these protruding structures may be provided generally adjacent to the sensor 270 and sensor package 265, be several millimeters long, and have datum features to help kinematically control DOF, such as radial position from the device center or tilt or rotation from one camera channel to an adjacent camera channel. A camera assembly may have two such protruding structures, symmetrically or asymmetrically positioned around the lens housing and oriented orthogonally to each other, to control different degrees of freedom. Alternatively, or in addition, the camera channel may have the lens housing 430 including one or more protruding tab or post structures positioned within the seam 400. For example, such a datum feature (not shown) could be provided in the joint at vertex 60 on the polygonal outer surface of the camera channel, protruding outside the nominal conical space or volume into the joint between two adjacent channels. Depending on the device design and intended application, the protruding tabs or structures, whether located within the outer joint 400 or recessed deeper, such as near the image sensor, can be fabricated from either a compliant or rigid material, or a combination thereof. As another alternative, one or more tabs of a given lens housing 430 need not protrude outside the nominal conical volume or truncated cone, but rather a clamp bridging one tab to the tab of an adjacent lens housing could provide an interface or control to limit the degrees of freedom.

[0126] 13 and 17, in which the primary and secondary channels 610, 615 have tripods 330 that interact with a central ball-socket array or hub, and the variations and alternatives described above, it should be noted that the available space within the center of the device can be tight, which can make it difficult to design in power and communication cables, cooling pipes, and associated support mechanisms.

[0127] As another option for providing additional access for cabling, support, and thermal management hardware, the ball-and-socket approach of FIG. 17 can be replaced with a polygonal-sided internal frame ( FIG. 19 ) with an access hole in the hollow center. For an improved multi-camera panoramic image capture device 300 constructed with a dodecahedron pattern, the internal frame can also be a dodecahedron with pentagonal faces and oriented with the internal pentagonal faces nominally aligned with the external pentagonal geometry. The internal frame can be machined separately and assembled from two or more pieces, or it can be made as a single-piece structure by casting or 3D printing. While manufacturing a single-piece frame is more complex, the resulting structure is stiffer, stronger, and can support tighter mechanical tolerances. For example, a dodecahedron frame with a hollow center can be cast from stainless steel and then selectively post-cast machined to provide precision datum features. This internal frame may then be provided with flexures or adjusters on all or most of the pentagonal faces to provide kinematic-type adjustments to reduce or avoid over-constraints during device assembly and use. As before, the adjusters available on these internal faces may be different for the secondary channels as compared to the primary channels. Alternatively, the internal frame may be made at least in part from a more compliant material, such as brass or invar. The central volume of this internal frame may be at least partially hollow, so that space can be provided for electrical cabling, thermal management hardware, and other support structures.

[0128] FIG. 19 provides an example of such an internal frame 800 with multiple pentagonal faces 810 arranged in a dodecahedron pattern with a hollow center. The internal frame 800 can be designed as a mounting mechanical assembly for an 11-camera system, with a support post attached at the 12th position (similar to FIGS. 13 and 15). A polygonal internal frame, or a half or partial internal frame, can also be used in a partial or hemispherical system, in which case a camera assembly, including an image sensor, is attached to the frame. Alternatively, a hemispherical system with internal frame 800 can use a central hollow space (e.g., a linkage) to allow image light to pass through, including through, an intervening relay lens system to reach the image sensor on the other side. As shown in FIG. 19, one pentagonal face (810A) can have three adjusters 820, such as a set of screws or flexures, oriented nominally 120° apart, that interact with features on the camera housing and thus can be used to assist in aligning a given camera channel. As previously mentioned, the mounting and adjustments for the secondary channels may have a different design or configuration than those for the primary channels. As another alternative (not shown), one or more of the pentagonal faces 810A, 810B, or 810C may each include one or more adjusters that can be used to gently press the camera channel against precision V-groove structures (also not shown). These V-groove structures may be fabricated within or protrude from the inner edges of the pentagonal vertices 60 of the pentagonal faces. The internal frame approach can be used with other polyhedral device structures, such as for an icosahedron.

[0129] Note that in some systems, one or more of the respective camera objective lens systems (e.g., 320 or 920) may be paired with image relay optics to reimage the original image plane to a more distant image plane. The original image plane provided by the objective or camera lens system is essentially an actual intermediate image plane within a larger optical system. It may be reimaged at a magnification (e.g., 1:1 or 2.75:1) to a subsequent image plane (not shown) where the image sensor is located. Thus, advantageously, the image sensor may be larger and provide a higher pixel count, and the relay lens system 725 may reimage the image provided by the objective lens (920) at the appropriate magnification to nominally fill the more distant sensor with the projected image. The relay lens system may include a field lens behind the image plane of the camera lens to help contain the space or volume necessary for the image light to pass through. The optics of the relay lens system may also be designed consistently with the camera 920 to correct or compensate for its aberrations.

[0130] FIG. 20 illustrates an alternative optical path in which low-parallax imaging 320 is paired with relay optics 725, which reimages an intermediate or first image plane 755A to a second image plane 755B. While relay optics 725 is shown as having all refractive elements (e.g., lens elements), the relay can also be reflective (having all reflective elements) or catadioptric (having a combination of refractive and reflective elements). The relay can also be designed to collect image light from the intermediate image plane (image plane 755A) either telecentrically or non-telecentrically and similarly present image light to a distant image sensor telecentrically or non-telecentrically. The relay can also be double-telecentric, meaning that it is simultaneously telecentric with respect to both the intermediate image plane and the distant sensor plane. Either aperture stop 945A or second aperture stop 945B can be the limiting stop for the system. As an example, the first aperture stop can be a limiting stop, and the second aperture stop can be slightly oversized but still contribute to vignetting of stray light. Image light propagating through the second aperture stop may not be as well controlled or may deviate more than image light through the first aperture.

[0131] However, it may be difficult to construct an improved low-parallax panoramic multi-camera capture device (300) in which the imaging channels further include optical relays mechanically within a nominally hemispherical volume. The use of prisms or mirrors to fold or redirect the mechanical volume may reduce the overall volume. Alternatively, image light entering the relay optics may cross each other during passage from the imaging lens 320 to enter each relay optic 725. The internal frame 800 of FIG. 19 may enable this approach, with a pentagonal surface 810 on one side of the frame interacting with the low-parallax imaging lens 310 and a pentagonal surface on the opposite side interacting with the optomechanics of the relay optics. The relay optics may have a working distance 760 nominally matching the diameter of the internal frame 800.

[0132] As shown in FIG. 20 , the optical relay 725 can also include one or more beamsplitting prisms 735 that can direct light to an image sensor 730 or other type of sensor. The beamsplitter can be positioned in telecentric or non-telecentric space, or either before or after the aperture stop of the relay optical path. The combination of a beamsplitter and a secondary sensor can be used to support additional or secondary optical functions, such as IR imaging, brightfield imaging, or LIDAR depth imaging. The beamsplitting prism can also be used for color splitting to create separate RGB color imaging channels, each with its own image sensor. If a reduction in the front color range is then selected and lateral color optimization is relaxed, the resulting image differences can be corrected before the images are digitally recombined during image processing. One or more folding mirrors can also be included in the optical relay.

[0133] As previously mentioned, seam width can affect parallax error, the size of blind region 165, the size of any complementary image overlap (FIG. 3), and image processing and mosaicing or tiling time, so seam 400 can be an important parameter in the design of an improved multi-camera capture device 300. For example, in instances where an expensive multi-camera capture device 300 is used in a controlled environment, narrow seams with tight tolerances may be possible or acceptable. However, localized impact or stress on expensive, brittle materials (e.g., glass) can result in delamination and other damage and should be avoided. Of course, there are other external damage risks, including from external materials in contact with outer lens element 437. Therefore, because multi-camera capture device 300 can be an expensive unit, optomechanical designs that provide risk mitigation or protection for the cameras or device can be beneficial.

[0134] As one solution, seam 400 can be filled with a compliant material, such as an RTV or silicone adhesive. As another solution, the optical design of the camera lens can include an outer lens element 437, which can be designed as a polymer or plastic material, such as Zeonex, that is less brittle than glass. The use of a polymer outer lens element 437 and a compliant material-filled seam 400 can further reduce the risk.

[0135] However, in optical designs, glass typically offers better and more predictable performance than polymer materials. As one approach, the outer lens element 437 can be designed using a common, more robust, and less expensive optical glass, such as BK7, while higher index materials are likely required for other compressor lens elements. The outer lens element, whether glass or polymer, can also be topcoated with a scratch-resistant AR coating and an oleophobic or hydrophobic coating. Compliant seam-filling adhesives should also resist penetration or contamination by water, oil, or other common materials.

[0136] In the improved multi-camera capture device 300, the entire camera 320 can also be designed to be modular or potentially a field-replaceable unit (FRU). In the latter case, the camera 320, including the housing 430, and associated lens elements, including the outermost compressor lens element 437, can be removed and replaced as a unit. The improved multi-camera capture device 300, and the camera 320 therein, can also be protected by a dome or shell (not shown) with nominally concentric inner and outer spherical surfaces through which the device can image. The addition of an outer dome can be used to enclose within the interior volume the generally spherical device of FIG. 15, or a generally hemispherical device, or a device with an alternative geometry or total FOV. The dome can be composed of a pair of interlocking hemispherical or roughly hemispherical domes joined at a joint, or it can be a single roughly hemispherical shell. The transparent dome or shell material can be glass, plastic or polymer, a hybrid or reinforced polymer material, or a robust optical material such as ceramic, sapphire, or Alon. The optically clean dome or shell helps eliminate environmental contamination and also functions as a FRU and can be replaced if damaged. Replacing a FRU dome can be easier than replacing the entire camera 320 or an FRU-type outer lens element or outer lens element assembly. Depending on the complexity of the design, FRU replacement of a modular camera lens assembly or dome can occur in the field, at a service center, or at the factory, but with relative ease and speed.

[0137] The dome or shell can also be enhanced with an AR, oleophobic, or hydrophobic coating on the exterior surface and an AR coating on the interior surface. The use of a dome or shell can also reduce the need for or burden of using a carrying case or shipping container, although such an enclosure can still be useful. Alternatively, or in addition, the dome or shell can be faceted to provide a series of integrated, adjacent lens elements that serve as the outer lens elements for the associated adjacent cameras. This approach can have the potential advantage of reducing the width of both the intervening seams 160 (e.g., seam width ≦0.5 mm) and their associated blind areas, allowing the device center 196 to coincide with the low parallax amount 188.

[0138] The preference for a narrow width for seam 400 has been emphasized. However, for clarity, in relation to the design and performance of multi-camera capture device 300, it may be unnecessary for chief rays 170 (e.g., rays 970 and 975) passing through adjacent cameras 320, and for a given object distance, to maintain an exact zero millimeter separation between the beams. Rather, the goal may be to have at most one pixel of missing information (or a few pixels, depending on the application) at the object distance of interest. Depending on the allowed seam width and pixel loss, cameras 320, and multi-camera capture device 300, may be more or less robust, or more or less tolerant of both manufacturing tolerances and protective design approaches, including those described above.

[0139] Specifically, some manufacturing tolerances of the cameras 320 (see, e.g., FIG. 13 ), housing 430, outer lens element 437, and seam 400 can be tolerated or accommodated. In part, the use of optical or electronic fiducials, or shadows cast by baffles, allows for greater tolerance or flexibility in collecting low-parallax error images from adjacent cameras 320 with slight FOV overlap, as these correction approaches can allow a “centered” image to be found. However, the multi-camera capture device 300 can also tolerate some image loss. For example, for a device with a 5-foot object distance and whose cameras support a total 320,000 pixel resolution (8k output equirectangular images), a 1-pixel-wide seam 400 corresponds to a 1.2 mm gap. If the improved multi-camera capture device 300 is designed to hold mechanical seams to 3 mm after construction, the cameras can be designed for a 1.5 mm gap while allowing for some FOV overlap (Δθ). The device may have a mechanical design that tolerates a coincidence pointing error of up to Δθ, within a tolerance. After camera assembly, the actual light rays collected along the edge surface shared between the camera lenses may shift, but always within the pointing error. Thus, the gap or seam 400 may be constrained to a maximum width of 3 mm. For some camera designs, markets, or applications, a wider seam may be acceptable, either in absolute size (e.g., 4.5 mm seam width) or in lost image content (e.g., 2–20 pixels per seam). The lost pixels can be corrected by increasing the FOV overlap or extended FOV 215 between adjacent cameras to capture the overlapped content, at the cost of some parallax error and a slight increase in image processing burden. However, for a moderate amount of extended FOV (e.g., ≦5%), the residual parallax error (e.g., Figure 8B) may still be modest. If the seam 400 is smaller, and with better knowledge of the location of the image centroid 480 and image size and shape, the core FOV 205 on the sensor can be larger, and less lens performance and sensor area can be devoted to providing a larger extended FOV 215.

[0140] The seam 400 may be nominally the same width at the juncture between adjacent cameras for all cameras 320 in the improved multi-camera capture system 300. Careful camera and camera housing design, as well as inter-camera mounting (FIGS. 13 and 14), can help this occur. Nevertheless, the seam width may vary either during system assembly or during dynamic environmental conditions. For example, on one side of a first camera, the seam width between that camera and an adjacent camera may be only 0.75 pixels wide, while at the same time, the seam width between the first camera and another adjacent camera may be 3.25 pixels wide. Seam width may also vary non-uniformly. The images generated for the sensor may be shifted or rotated relative to expectations. Such variations may therefore increase parallax error for the images captured by these adjacent cameras, complicating image mosaicing or tiling. However, through the use of electronic or optical fiducials, or shadows cast by internal baffles, the image centroid and image edges can be monitored for each camera, allowing for a quick reference to a nominal calibration or expected state. This reference or correction data can also be compared from one camera to another, with the goal of defining for each camera the effective image centroid and image edges that most effectively reduce parallax error, either individually or collectively (e.g., on average) for all cameras. This information can then be used during image processing to quickly and robustly determine image edges, allowing for efficient image mosaicing or tiling.

[0141] FIG. 15 shows an electronics system diagram for an improved multi-camera capture device 300. In this example, a dodecahedron-type device has eleven cameras 320 and an electromechanical interface at the twelfth camera position. Image data is collected from each of the eleven cameras and directed through an interface input-output module and through a cable or cable bundle to a portable computer that can provide image processing, including live image cropping and mosaicing or tiling, and camera and device control. Output image data is directed to an image display, a VR headset, or a further computer located locally or remotely. Electrical power and cooling may also be provided as needed.

[0142] Also, as previously suggested, the performance of a multi-camera capture device can be affected by both internal and external environmental factors, both in relation to the optomechanics and image quality. Each of the image sensors 270 and the entire sensor package 265, along with the data interface and power support electronics, can be a localized heat source. To reduce the thermal impact on the camera lenses and the images they provide, the mechanical design for the improved multi-camera capture device 300 can thermally isolate the sensors 270 from the lens optomechanics. To further help reduce thermal gradients between the sensors and their electronics and the optical system, micro-heat pipes or Peltier devices can be used to cool the sensors and redirect heat. Heat can be removed from the entire device by either active or passive cooling provided through the electromechanical interface at the twelfth camera position, as shown in FIG. 15. This cooling can be provided by convection or conduction (including liquid cooling), or a combination thereof.

[0143] Similarly, as previously suggested, external ambient or environmental factors can also affect the performance of a multi-camera capture device. These factors can include the effects of illuminating ambient light or thermal extremes or changes in the environment. For example, because sunlight is typically highly directional, certain outdoor image capture scenarios can result in cameras on one side of the device seeing plenoptic illumination from a brightly illuminated scene while other cameras see plenoptic illumination from a shaded scene. In such cases, the captured images may exhibit dynamic exposure changes, which can then be corrected by exposure correction, which can be provided locally (see FIG. 15 ). In an improved multi-camera capture device 300, light from an optical reference can also be used for exposure correction of the captured images. Light or pixel signals from a portion of the peripheral image region between the edge of the core FOV 205 and the extended FOV 215 can also be used for exposure correction before the image is cropped to the size of the actual current core FOV 205. It should also be noted that the extended FOV 215 of a first camera may overlap, at least in part, with the extended FOV 215 of an adjacent camera, so that light level and color comparisons can be made on content or signals being captured simultaneously by both cameras. The signals or pixel data from these overlapping regions can be used to determine exposure changes between the two cameras by having a common reference point (e.g., matched feature points—using SIFT, SURF, or similar algorithms to find common feature points within the overlapping region).

[0144] Note that peripheral image or exposure data may also be retained for later use in image post-processing. Additionally, exposure correction may also be enabled by embedding photodetectors within the seams 400 or at the vertices, between the outer lens elements 437. These abrupt exposure differences may cause spatial and temporal differences in the thermal load of some image sensors 270 compared to others within the multi-camera capture device 300. The aforementioned sensor cooling, whether enabled by heat pipes, heat sinks, liquid cooling, or other means, may be designed to absorb such differences. Performance may be verified by finite element analysis (FEA).

[0145] Alternatively, one or more camera systems can be protected by the attachment of a shield or mask to cover the polygonal shape of its outer lens elements, seam-to-seam and vertex-to-vertex. Such shields can be provided to cover a single camera lens system or multiple lens systems. These shields can be shaped to generally fit the outer surface shape of the outer lens elements, and they can be used to prevent saturation or overexposure from bright directional light (e.g., sunlight) or to prevent contamination from local directional environmental factors. While these caps are nominally removable for some user applications, they can remain in use for extended periods. Excessively bright exposure from the sun or other light sources can also be controlled with an image sensor having an electronic shutter or drain, or a physical shutter or electro-optical neutral density filter, photochromic or electrochromic filter, which can be designed into, for example, the camera 320, within the grouping of inner lens elements 440. A signal to initiate or control an electronic or secondary shutter can be obtained from the image sensor or from other internal or external photodetectors. As another robustness improvement, one or more camera channels can use a dichroic color filter array integrated into the image sensor package instead of the standard dye-based CFA.

[0146] Environmental influences may also heat or cool the multi-camera capture device asymmetrically. The aforementioned kinematic mounting or coupling of adjacent camera housings 430 to the improved multi-camera capture device 300 can help reduce this impact by attempting to deflect or average mechanical stresses and limit mechanical movement. However, it may be even more advantageous to provide channels or materials to transfer or shift asymmetric thermal loads so that they are more evenly shared between or by the cameras 320 and their housings 430. With respect to FIG. 13 , this means that the space around the lens housing 430 and channel centering hub 330, such as inner volume 390, may be filled, at least in part, with a compliant, yet highly thermally contacting, thermally conductive material (e.g., Sil-Pad (from Henkel Corporation) or CoolTherm (Lord Corporation, Cary, North Carolina, USA)) to help spatially average out asymmetrical heat loads or differences. Alternatively, or additionally, thermally conductive straps or tapes, such as the 88xx series adhesive tapes from 3M (Saint Paul, Minnesota, USA), can be used. At the same time, however, some of the effects of thermal variations on the imaging performance of the camera lens 320 can be mitigated by both judicious selection of optical glass and athermal mounting of the optical elements within the lens housing 430. In combination, an effective design approach may be to allow heat transfer or crosstalk between the lenses 320 and their housings 430 to environmental effects, while simultaneously isolating the lenses and housings from the sensors 270 and their electronics.

[0147] The improved camera 320 for use in the improved multi-camera image capture device 300 may also use an adjustable lens element to correct for thermally or mechanically induced focus changes (e.g., defocus). This adjustable lens may preferentially be located between the inner lens elements 440 and may be a liquid crystal or elastic polymer type device, such as an electrically actuated adjustable focus lens from Optotune (Dietikon, SW).

[0148] Emphasis has been placed on developing an improved camera 320 with polygonal outer lens elements to capture and image light from a polygonal FOV for use in an improved multi-camera image capture device 300. A large number of such adjacent cameras can be used in a nominally spherical or hemispherical device. However, devices 300 with fewer cameras covering a smaller total FOV can be developed. For example, a system with only four or six low-parallax adjacent polygonal cameras may be suitable for some market applications. Additionally, a single camera with polygonal outer lens elements capturing image light from a nominally matching polygonal FOV can be used alone, such as for security or surveillance applications. For example, a single camera optomechanically designed to fit within the shape of one-eighth of an octahedron could be mounted in a corner of a room ceiling and capture image content of the room environment with little or no blind areas. For cameras for such applications, low-parallax optimization may be different because there are no concerns about adjacent low-parallax lens systems. Similarly, while emphasis has been placed on developing an improved camera lens system 320 in which parallax error can be reduced at least within the core FOV 205, a moderate extended FOV 215 (e.g., ≦5% excess) and image capture overlap with adjacent cameras can also be provided. Similarly, the present approach can be extended to support possible applications with even larger overlap FOVs of image capture between adjacent cameras (e.g., 10-25% excess for a dodecahedral system with a nominal core FOV of 37.45°, or ≈4-10° excess FOV), where simultaneous parallax or perspective error control at least within the core FOV 205 is a priority. The camera design can be extended even further to provide an even larger overlap FOV (e.g., 10-20° excess), but without the benefit of reduced parallax for angles much beyond the designed core FOV.

[0149] As mentioned above, this approach can be used to design low parallax lens systems for a variety of applications. The method for device development can be outlined, for example, as follows: 1. Define in terms of customer or application space, polygonal device configuration, resolution, expected extent of seams and blind areas, and minimum object distance. The minimum object distance is the shortest distance at which instantaneous image stitching or image merging of low-parallax overlapping images can be applied, provided image processing is acceptable. At this distance, estimate the maximum total gap or seam for which image loss or difference is generally imperceptible to the human eye. For example, for an 8k, etc. rectangular projection image, an expected image loss (along the equator) of 1 / 40% of the output image corresponds to a seam of 2 pixels wide each. Alternative minimum object distances can be defined that are even further away, at which even modest image processing is not acceptable. 2. Design an imaging lens while controlling both parallax and front color. Depending on the results, the design can be further modified to provide an additional or extended FOV where parallax can be controlled. This additional FOV can also provide room for rainbow tinting of the remaining front color, placed some distance outside the core FOV. 3. Design the lens and device optomechanics. Determine the expected seam width and expected thickness and wedge variations between and within seams from manufacturing and assembly tolerance variations. Determine the expected extended FOV overlap to cover the expected mechanical and optical seam width to be at least as high as or larger than the expected maximum optomechanical wedge and thickness variations so that parallel chief rays between adjacent lens systems can be selected without underlap (widened field of view). 4. Updated lens design to provide field of view overlap along the seams between outer compressor lens elements, controlling parallax therein while exceeding both maximum expected mechanical variation and residual front color.

[0150] In conclusion, the purpose of this application is to provide a means for designing a low-parallax lens system that may have outer (polygonal) shaped lens elements, which, when advantageously used with an appropriate mechanism to facilitate mounting of adjacent camera assemblies, can then provide reduced optical seam widths and blind areas. To push the chief rays to the edges of the polygonal faces, the aberrations of the entrance pupil, and in particular the spherical aberration and axial chromatic aberration of the pupil, should be optimized or reduced. For context, the entrance pupil and exit pupil, which are the projected images of the aperture stop in object space and image space, respectively, are typically academic curiosities, representing the virtual image of the aperture stop that one can see when looking through the lens.

[0151] In typical optical systems, aberrations are significant at the image plane, even if the values ​​at individual internal surfaces, whether positive or negative, are on a larger scale, with the typical goal being to provide a small net individual surface contribution to provide good image quality. On the other hand, aberrations at the aperture stop are often less of a concern, other than properly positioning the stop to define the lens system's F-stop while minimizing clipping or vignetting of the ray bundle relative to the field of view. It should be noted that when an object is placed at the aperture stop, pupil aberrations can affect the apparent image quality of the object's image as seen by a viewer at the entrance or exit pupil, although these pupil aberrations do not necessarily represent the image quality at the image plane.

[0152] In low-parallax lenses, pupil aberrations, and especially entrance pupil aberrations, are important. First, to begin properly designing a lens to control parallax, the entrance pupil must be positioned behind the image plane. Second, the redirection of peripheral chief rays from the outer compressor lens element toward the low-parallax area must be controlled. As mentioned above, optimizing the spherical aberration of the entrance pupil can be an effective way to limit parallax error. In this context, slight, moderately corrected, or insufficiently corrected optimization of the spherical aberration of the entrance pupil can occur, meaning that the nonparaxial chief ray NP point can lead to or follow the paraxial NP point, respectively. Additionally, axial chromatic aberration of the entrance pupil causes chromatic differences that can affect the optimization of the spherical aberration of the entrance pupil. Front color, which can be considered an artifact of this axial chromatic aberration, and the axial chromatic aberration itself can be reduced through the judicious selection and use of high- and low-dispersion optical materials within the lens design. While optimization of spherical aberration in the entrance pupil or chief ray pointing provides fine tuning for non-paraxial chief ray NP pointing and parallax reduction, distortion from the compressor lens group also has an increasing effect on the projected chief ray pointing for increasing fields of view, towards positions further back in the lens, and towards lower parallax amounts. The magnitude and characteristics of the distortion, which also defines the chief ray height on the outer surfaces of the outer compressor lens elements, can be significantly determined by the use of aspheric surfaces within the compressor lens group.

[0153] While this description has emphasized the design of improved multi-camera image capture devices 300 for use in broadband visible, or human-sensible, applications, these devices can also be designed for narrowband visible applications (modified using spectral filters, ultraviolet (UV), or infrared (IR) optical imaging applications). Polarizers or polarizer arrays can also be used. Additionally, while the imaging cameras 320 have been described as using an all-refractive design, the optical design can also be reflective, or catadioptric, and can use a combination of refractive and reflective optical elements.

Claims

1. An imaging device, a first imaging lens configured to image a portion of incident light from within a first field of view; a second imaging lens configured to image a portion of incident light from within a second field of view adjacent to the first field of view; An imaging device comprising: The first imaging lens comprises: a first lens element group proximate an outer surface of the imaging device, the first lens element group including outer lens elements that direct incident light including paraxial chief rays and non-paraxial chief rays within the first field of view as image light; a second group of lens elements configured to receive the image light from the first group of lens elements; an aperture stop configured to accept the image light from the second group of lens elements; a third group of lens elements configured to redirect image light from the aperture stop to form an image at an image plane; Including, The outer lens element comprises: a first projection of a paraxial chief ray entering the outer lens element converges onto an entrance pupil located behind the image plane, the entrance pupil including a first no-parallax (NP) point; and A second projection of the non-paraxial chief ray entering the outer lens element converges to a low parallax point spaced from the entrance pupil, the low parallax point including a second NP point. The imaging device is configured to reduce parallax by being shaped as follows.

2. 2. The imaging device of claim 1, wherein the first lens element group includes a compressor lens group that refracts the image light toward an optical axis, the second lens element group is a wide-angle lens group including a first plurality of lens elements, one of the plurality of lens elements including a deep concave lens element, and the third lens element group is an eyepiece lens group including a second plurality of lens elements.

3. The imaging device of claim 1 , wherein the outer lens element is further shaped to reduce perspective centroid error within at least a portion of the first field of view.

4. 4. The imaging device of claim 3, wherein the first lens element group, the second lens element group, and the third lens element group reduce perspective center errors in the outer lens elements by at least one of limiting horizontal components of spherical aberration at surfaces at or near the entrance pupil, limiting vertical width of low parallax amounts, and positioning a perspective center at or near the entrance pupil.

5. the first field of view includes a first nominal field of view and a first extended field of view that is larger than the first nominal field of view; the second field of view includes a second nominal field of view and a second extended field of view that is larger than the second nominal field of view; the first extended field of view and the second extended field of view overlap at a seam between the first imaging lens and the second imaging lens; a residual parallax error within the overlap of the first extended field of view and the second extended field of view is 2 pixels or less; The imaging device according to claim 1 .

6. The imaging device is a first lens housing coupled to the first lens element; a second lens housing coupled to the second lens element; Furthermore, the first lens housing and the second lens housing are configured to separate the outer lens element from a second outer lens element of the second imaging device by a seam; The imaging device according to claim 1 .

7. a mechanical assembly proximate a center of the imaging device, the first lens housing and the second lens housing being attached to the mechanical assembly; The imaging device of claim 6 further comprising:

8. The imaging device of claim 1 , wherein the outer lens element has a polygonal perimeter and the first field of view is a polygonal field of view.

9. The imaging device of claim 1 , wherein the distance between the entrance pupil and the low parallax point is about 2 millimeters or less.

10. 10. The imaging device of claim 1, wherein the ratio of entrance pupil to image plane distance (EPID) to focal length (EFL) is from about negative two (-2) to about negative ten (-10).

11. The imaging device of claim 1 , wherein the first imaging lens reduces front color for non-paraxial chief rays near an edge of the outer lens element.

12. The imaging device of claim 1 , wherein the first imaging lens has a relative illuminance of 0.5 or greater.

13. 1. An imaging lens for use in a low parallax multi-camera imaging system, said imaging lens comprising: a compressor lens group including an outer lens element configured to refract at least a portion of incident light within a field of view as image light, the image light including paraxial and non-paraxial chief rays; a wide-angle lens group configured to receive the image light and direct the image light toward an aperture stop; an eyepiece group configured to receive image light passing through the aperture stop and direct the image light to an image plane; Equipped with The outer lens is a first projection of a paraxial chief ray entering the outer lens element converges onto an entrance pupil located behind the image plane, the entrance pupil including a first no-parallax (NP) point; A second projection of the non-paraxial chief ray entering the outer lens element converges to a low parallax point spaced from the entrance pupil, the low parallax point including a second NP point. and configured to reduce parallax associated with non-paraxial chief rays entering proximate an edge of the outer lens element by being shaped such that An imaging lens, wherein the distance between the entrance pupil and the low parallax point defines a range of low parallax (LP) amounts.

14. 14. The imaging lens of claim 13, wherein the compressor lens group, the wide-angle lens group, and the eyepiece lens group reduce at least one of perspective error for accepted rays of non-paraxial chief rays accepted at positions proximate to edges of the outer lens element, and parallax error between the imaging lens and adjacent imaging lenses.

15. 15. The imaging lens of claim 14, wherein the compressor lens group, the wide-angle lens group, and the eyepiece lens group reduce at least one of the perspective error and the parallax error by at least one of limiting a horizontal component of spherical aberration and limiting a vertical width of a low parallax amount.

16. 14. The imaging lens of claim 13, wherein the imaging lens reduces distortion as an impact on image quality at the image plane and / or perspective or parallax error for accepted rays of non-paraxial chief rays accepted at positions proximate to an edge of the outer lens element.

17. 17. The imaging lens of claim 16, wherein the perspective error is reduced by optimization for at least some non-paraxial chief rays located within a field between the paraxial chief ray and a non-paraxial chief ray near an edge of the outer lens element.

18. 14. The imaging lens of claim 13, wherein the imaging lens reduces front color experienced by non-paraxial chief rays received proximate an edge of the outer lens element.

19. 14. The imaging lens of claim 13, wherein the outer lens element has a polygonal periphery and is configured to collect incident light within a polygonal field of view.

20. 14. The imaging lens of claim 13, further designed to operate in an imaging relay system, wherein an image plane provided by the imaging lens is re-imaged at a second image plane with a magnification.

Images