Freeform Offner spectrometer

Abstract

The spectrometer is configured to form a spectrally resolved image of the electromagnetic radiation from the electromagnetic radiation source. The spectrometer can include a light guiding device configured to guide the electromagnetic radiation along an optical path. The light guiding device can include a first prism positioned in the optical path. The light guiding device can further include focusing optics. The first prism can have at least one freeform prism face including at least some cylindrical curvature having a freeform polynomial term formed thereon, and the face can be a substantially cylindrical, substantially non-cylindrical, or substantially flat surface having the freeform polynomial term formed thereon.

Description

[Technical Field]

[0001] The present invention relates to a freeform Offner spectrometer. [Background technology]

[0002] In optical applications, groups and arrangements of prisms, gratings, and / or mirrors are often used to direct, separate, and / or combine electromagnetic radiation within optical systems that serve a variety of different purposes. For example, hyperspectral imaging often uses wide-field spectrometers to capture, image, and / or analyze the spectral components of electromagnetic radiation from an object. The spectrometer directs electromagnetic radiation from an object through an optical system with dispersive elements (e.g., prisms, mirrors, gratings) to split the light into different spectral components and directs the light to a capture sensor configured to image / analyze the spectral components contained in the light from the object. Such analysis can be used to determine properties of the object, such as its chemical composition and other properties.

[0003] For a spectrometer to accurately image / analyze the spectral content of an imaging object, it is desirable for it to have high-quality imaging characteristics. To produce a spectrometer with a large, high-resolution, well-corrected image field, it is desirable to optimize not only image quality, resolution, distortion correction, light throughput, and total dispersion, but also the mass and volume of the spectrometer. Improved designs of optical elements and spectrometers continue to be developed to produce spectrometers with improved image quality, distortion correction, throughput, and other imaging characteristics. [Brief explanation of the drawings]

[0004] Features and advantages of the present technology will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, which together illustrate, by way of example, features of the technology.

[0005] [Figure 1] 1 is a schematic diagram of a spectrometer according to an example of the present disclosure. [Figure 2a] FIG. 1 is a front view of a lens configuration according to an example of the present disclosure. [Figure 2b] 1 is a cross-sectional view of an exemplary lens according to an example of the present disclosure. [Figure 2c] 1 is a cross-sectional view of an exemplary lens according to an example of the present disclosure. [Figure 2d] 1 is a cross-sectional view of an exemplary lens according to an example of the present disclosure. [Figure 3a] FIG. 1 is a front view of a lens configuration according to an example of the present disclosure. [Figure 3b] 1 is a cross-sectional view of an exemplary lens according to an example of the present disclosure. [Figure 3c] 1 is a cross-sectional view of an exemplary lens according to an example of the present disclosure. [Figure 3d] 1 is a cross-sectional view of an exemplary lens according to an example of the present disclosure. [Figure 3e] 1 is a cross-sectional view of an exemplary lens having freeform features formed thereon, according to an example of the present disclosure. [Figure 3f] 1 is a cross-sectional view of an exemplary lens having freeform features formed thereon, according to an example of the present disclosure. [Figure 3g] 1 is a cross-sectional view of an exemplary lens having freeform features formed thereon, according to an example of the present disclosure. [Figure 3h] 1 is a cross-sectional view of an exemplary lens having freeform features formed thereon, according to an example of the present disclosure. [Figure 3i] 1 is a cross-sectional view of an exemplary lens having freeform features formed thereon, according to an example of the present disclosure. [Figure 4] 1 is a schematic diagram of a spectrometer according to an example of the present disclosure. [Figure 5] 1 is a schematic diagram of a spectrometer according to an example of the present disclosure. [Figure 6] 1 illustrates a method of configuring a light guide device according to an example of the present disclosure. [Figure 7a] FIG. 1 illustrates a Ferry spectrometer. [Figure 7b] FIG. 1 illustrates a freeform spectrometer. [Figure 8] 10a and 10b show various experimental results comparing the distortion of a Ferry spectrometer with that of a freeform spectrometer. [Figure 9] 10a and 10b show various experimental results comparing the distortion of a Ferry spectrometer with that of a freeform spectrometer. DETAILED DESCRIPTION OF THE INVENTION

[0006] Reference will now be made to exemplary embodiments, and specific language will be used herein to describe the same, it being understood, however, that no limitation on the scope of the technology is thereby intended.

[0007] As used herein, the term "substantially" refers to the complete or nearly complete extent or degree of an action, feature, characteristic, state, structure, item, or result. For example, a "substantially" enclosed object means that the object is completely enclosed or nearly completely enclosed. The precise tolerance for deviation from absolute completeness may vary depending on the specific situation. Generally speaking, however, approximation to completeness will result in the same overall result as if absolute and total completeness were achieved. The use of "substantially" equally applies when used in a negative sense to refer to the complete or nearly complete absence of an action, feature, characteristic, state, structure, item, or result.

[0008] As used herein, "adjacent" refers to the proximity of two structures or elements. In particular, elements identified as "adjacent" may either abut or be connected. Such elements may not necessarily touch each other, but may be near or in close proximity to each other. The exact degree of proximity may depend on the particular context.

[0009] An initial summary of the inventive concepts is provided below, followed by a more detailed description of specific embodiments. This initial summary is intended to aid the reader in understanding the embodiments more quickly, but is not intended to identify key or essential features of the embodiments, nor is it intended to limit the scope of the claimed subject matter.

[0010] Disclosed herein is a light guide device configured to guide electromagnetic radiation along an optical path. The light guide device can include a first prism positioned in the optical path. The light guide device can further include a focusing optic positioned in the optical path. The first prism can include at least one freeform prism surface.

[0011] Further disclosed herein is a spectrometer configured to form a spectrally resolved image of electromagnetic radiation from a source. The spectrometer may include a slit configured to receive the electromagnetic radiation. The spectrometer may further include a light guide device positioned in the optical path downstream from the slit. The light guide device may be configured to guide the electromagnetic radiation along the optical path. The light guide device may include a first prism positioned in the optical path. The light guide device may further include focusing optics positioned in the optical path. The first prism may have at least one freeform prism surface including at least some cylindrical curvature with a freeform polynomial term formed thereon. The freeform prism surface may include at least one of a cylindrical surface, a non-cylindrical surface, or a substantially planar surface.

[0012] Further disclosed herein is a method of configuring a light guide device. The method can include configuring the light guide device to include a first prism in the optical path. The method can further include configuring the light guide device to include a focusing optic positioned in the optical path. The method can further include configuring the first prism to include at least one freeform prism surface having at least some cylindrical curvature with a freeform polynomial term formed thereon. The freeform prism surface can include at least one of a cylindrical surface, a non-cylindrical surface, or a substantially planar surface.

[0013] Further disclosed herein is an optical system. The optical system may include an electromagnetic radiation inlet configured to receive electromagnetic radiation from a source. The optical system may further include a light guide device configured to guide the electromagnetic radiation along an optical path from the electromagnetic radiation inlet to a focal plane. The light guide device may include a first prism positioned in the optical path. The light guide device may further include a focusing optic positioned in the optical path. The first prism may include at least one freeform prism surface including at least some cylindrical curvature having a freeform polynomial term formed thereon. The freeform prism surface may include at least one of a cylindrical surface, a non-cylindrical surface, or a substantially planar surface.

[0014] To further illustrate the present technology, examples are now provided with reference to the drawings. Referring to Figure 1, an optical system is shown, according to an example of the present disclosure. The optical system may be a spectrometer 100, operable to collect light from an object 101 for imaging / analysis by the spectrometer 100. The spectrometer 100 includes a light guiding device 102, which may be configured to direct electromagnetic radiation and separate the electromagnetic radiation into various spectral components.

[0015] The light guide device 102 can include collection optics 104, which can include one or more reflective surfaces. For example, the collection optics 104 can be configured as an Offner relay including a first reflective surface 108, a second reflective surface 110, and a third reflective surface 112, each configured to receive electromagnetic radiation and reflect it in a desired direction. As shown in FIG. 1 , the first reflective surface 108 and the third reflective surface 112 can be formed on a single mirror 114. The first reflective surface 108 and the third reflective surface 112 are not separate, discrete surfaces, but instead can be different sections of a single, continuous reflective surface 115 formed on the mirror 114. In other words, in the spectrometer 100, the first reflective surface 108 and the third reflective surface 112 are not separate from each other, meaning that the first reflective surface 108 and the third reflective surface 112 are different portions of a single, continuous reflective surface. The first reflective surface 108 and the third reflective surface 112 are not separate reflective surfaces, even if such separate reflective surfaces are in contact or formed on the same substrate. Instead, the first reflective surface 108 and the third reflective surface 112 are formed such that there is no discontinuity in the reflective surface 115 and no discontinuity between the first reflective surface 108 and the third reflective surface 112. The second reflective surface 110 can be formed on a second mirror 116 spaced apart from the mirror 114 in an Offner relay configuration. Alternatively, the second mirror 116 can be a diffraction grating instead of a mirror. As described below with respect to FIG. 5, the collection optics can be a collection optics other than an Offner relay, such as the Dyson spectrometer shown in FIG. 5.

[0016] The light guide device 102 may further include a first prism 118 including a first surface 118a and a second surface 118b. The light guide device 102 may further include a second prism 120 including a first surface 120a and a second surface 120b. As shown in FIG. 1 , the first prism 118 and the second prism 120 may be prisms having one or more surfaces (e.g., 118a, 118b, 120a, and 120b) configured to collimate, reflect, refract, and / or direct incident light. It should be understood that either the first prism 118 or the second prism 120 may be a Ferry prism, a standard prism, or other prism, and the other of the first prism 118 or the second prism 120 may be the same, similar, or a different type of prism. A standard prism, such as a Ferry prism, may have a flat, spherical, cylindrical, aspherical, non-cylindrical, or freeform surface. Any number of additional standard or Ferry prisms with planar, spherical, cylindrical, aspherical, non-cylindrical, and / or freeform surfaces may be added.

[0017] The light guide device 102, including its various components (e.g., prism 118, mirror 114, mirror 116, and prism 120), can define an optical path along which electromagnetic radiation is directed. As shown in FIG. 1 , the optical path can be as follows: Light incident on a first surface 118a of the first prism 118 can pass through the first prism 118 and exit from a second surface 118b of the first prism 118. The light from the first prism 118 can be directed toward a first reflective surface 108 of the collection optics 104. The first reflective surface 108 can be positioned in the optical path downstream from the first prism 118 and can reflect the light from the first prism 118 toward a second reflective surface 110 of the collection optics 104. The second reflective surface 110 of the collection optics 104 can direct the light toward a third reflective surface 112 of the collection optics 104, which is positioned in the optical path downstream from the second reflective surface 110. Light incident on the third reflective surface 112 may be reflected toward a second prism 120 positioned in the optical path downstream from the third reflective surface 112. The light may be incident on a first surface 120a of the second prism 120, pass through the second prism 120, and exit from a second surface 120b of the second prism 120.

[0018] The light guide device 102 can be used as a light guide device in a spectrometer 100. The spectrometer 100 can further include a slit 103 that functions as an electromagnetic radiation entrance, through which light 122a from the source or object 101 enters the spectrometer 100. Alternatively, the slit can be a beam slicer. Light 122b from the slit 103 can travel along an optical path to the light guide device 102, specifically to a first prism 118. Light incident on a first surface 118a of the first prism 118 can pass through the first prism 118 and exit from a second surface 118b of the first prism 118 as light 122c. The light 122c can be directed toward a first reflecting surface 108 of the collection optical system 104, which can reflect the light 122c from the first prism 118 toward a second reflecting surface 110 of the collection optical system 104 as light 122d. The second reflective surface 110 of the light collection optical system 104 may direct the light 122d as light 122e toward a third reflective surface 112 positioned in the optical path downstream from the second reflective surface 110. The light 122e incident on the third reflective surface 112 may be reflected as light 122f toward a second prism 120 positioned in the optical path downstream from the third reflective surface 112. The light 122f may be incident on a first surface 120a of the second prism 120, pass through the second prism 120, and exit from a second surface 120b of the second prism 120 as light 122g.

[0019] Light 122g may then travel to and be incident on reflective surface 124 of reflective field corrector 126 (e.g., a field flattener) positioned downstream from second prism 120 and configured to provide correction for incident light 122g to ensure a high-quality, high-resolution, well-corrected field for imaging / analysis by spectrometer 100. Light 122h reflected by field corrector 126 may then travel to focal plane 128 configured to receive the spectrally separated light in a well-corrected field or image. Focal plane 128 may include a capture sensor 130, such as a CCD imaging sensor, a CMOS imaging sensor, or any other sensor or capture device, configured to analyze the light or capture and image it for later analysis. The devices, sensors, or other objects located within focal plane 128 are not intended to be limited in any way by this disclosure.

[0020] As shown in FIG. 1 , spectrometer 100 is a prism-based spectrometer that includes prisms 118 and 120 and reflective surfaces 108, 110, and 112. Spectrometers can also be grating-based, including gratings for directing and splitting light. However, grating-based spectrometers have low optical efficiency and require complex order-selection filters that are difficult to manufacture. Therefore, grating-based spectrometers tend to adversely affect several desirable image quality metrics, such as image quality, modulation transfer function (MTF), optical transfer function (OTF), spot size, wavefront error, light throughput, total dispersion, and image distortion. In contrast, prism-based spectrometers, such as those described herein, offer significant performance advantages over grating-based spectrometers, including improvements in each of the image quality metrics listed above. Modern optical design, including computer-aided design of lenses, prisms, mirrors, and their surfaces, allows for the fabrication of highly specialized and unique surfaces on lenses, mirrors, and prisms to provide precise image correction and high resolution images from the spectrometer. In other words, the spectrometers described herein provide hyperspectral imaging with a desirable large and well-corrected imaging field.

[0021] To provide high-quality, large-scale, and well-corrected imaging in spectrometers such as spectrometer 100, optical elements and their surfaces can be designed and manufactured to have unique and highly specialized surfaces. For example, prisms 118 and 120 can be designed and manufactured to include one or more freeform surfaces, as described in more detail below.

[0022] The surfaces of lenses, mirrors, and prisms can be of several types and designs. In one example, optical elements with spherical surfaces can be fabricated. FIG. 2a shows a front view of an exemplary optical element 200 (e.g., a surface, lens, mirror, or prism) having a circular profile. An example of the spherical surface of the optical element 200 is shown in the cross-sectional view of the example optical element 200a shown in FIG. 2b. The cross-section can be taken along any diameter of the optical element 200 (e.g., lines AA, BB, CC, or DD, or any other diameter). The spherical surface 202a can be characterized by a consistent radius of curvature across the entire surface, such that the surface of the optical element 200a is a partial cross-section of a geometric sphere. The spherical surface 202a can exhibit rotational symmetry across the entire surface and can be symmetrical about any diameter of the optical element 200a (e.g., the diameters identified and shown in FIG. 2a, as well as any other diameter that bisects the optical element).

[0023] The optical element 200 may alternatively include an aspheric surface. An example of an aspheric surface of the optical element 200 is shown in the cross-sectional view of the example optical element 200b shown in FIG. 2c. The cross-section can be taken along any diameter of the optical element 200b (e.g., along lines AA, BB, CC, or DD, or any other diameter). An aspheric surface is characterized by an aspheric and non-cylindrical shape. Unlike a spherical surface, an aspheric surface has a radius of curvature that varies from the center of the optical element 200b to the edge of the optical element 200b. However, like a spherical surface, an aspheric surface also exhibits rotational symmetry. As shown in FIG. 2c, the spherical surface 202b can exhibit rotational symmetry across its entire surface and can be symmetric about any diameter of the optical element 200b (e.g., the diameter identified and shown in FIG. 2a, as well as any other diameter that bisects the optical element).

[0024] Furthermore, those skilled in the art will understand that a cylindrical surface 202d can be formed on the optical element 200a, having the radius of curvature shown in FIG. 2b but lacking rotational symmetry across the entire surface. In other words, the cylindrical surface of an optical element is characterized by having curvature along one axis (e.g., line AA) and being flat along an orthogonal axis (e.g., line CC). This gives cylindrical optical elements the unique property of functioning as lenses or focusing elements only along a single axis. Cylindrical lenses form line foci rather than points. For example, a cylindrical surface 202d can be formed on the optical element 200. However, the cylindrical surface may be symmetric only about a particular diameter, such as line CC and line AA perpendicular to line CC, and may be asymmetric about lines BB and DD. Similarly, a non-cylindrical surface 202e can be formed on the optical element 200 and have the same symmetry relationship as a cylindrical surface with respect to line CC and line AA perpendicular to line CC, but may be asymmetric about lines BB and DD of the optical element 200.

[0025] Optical element 200 can be configured as an example of optical element 200c. As shown in FIG. 2d, optical element 200c can have a freeform surface 202c. In contrast to aspheric, spherical, cylindrical, and non-cylindrical surfaces 202a, 202b, 202d, and 202e, freeform surface 202c can be specifically designed and highly customized to include curvatures with peaks P and valleys V, and / or other features formed on its surface that are not symmetrical across the surface. In other words, freeform surfaces can lack rotational symmetry and can even lack symmetry across any diameter of optical element 200c. It should be understood that freeform polynomial terms can be formed on substantially spherical, cylindrical, or flat surfaces that are different from freeform terms formed on surfaces.

[0026] While exemplary surface curvatures are shown in FIG. 2d, it should be understood that any cross-section of optical element 200c taken along any diameter (e.g., diameters AA, BB, CC, and DD shown in FIG. 2a) can have a different contour than surface 202c shown in FIG. 2d. With the aid of computer design and manufacturing, freeform surfaces can be formed or imparted onto lenses, mirrors, and prisms to provide highly localized, customized features and precise image and optical corrections with specific and predictable amounts. Freeform surfaces enable the fabrication of mirrors, lenses, and / or prisms that are nearly flat yet have highly specific and unusual geometries that can provide desired image corrections. Freeform surfaces can have geometries defined by highly customized, custom-tuned polynomials (e.g., defining the shape and contour in the x, y, and z axes), enabling image corrections that are specifically tailored within very specific regions of an image. Furthermore, the highly specialized surface geometry of freeform surfaces allows very specific amounts of correction to be applied to an image without over or undershooting the desired amount of correction. For example, by allowing odd polynomial terms, such as cubic or quintic terms, and special shapes on the surfaces of prisms, lenses, or mirrors, image problems such as coma can be corrected without introducing astigmatism or anamorphic distortion. In other words, nearly flat freeform surfaces allow for the correction of multiple imaging aberrations and distortions, even by small amounts, without introducing additional undesirable image distortion.

[0027] Similar to the example of the circular optical element in FIG. 2a, other optical elements with a variety of different surfaces can be fabricated. For example, a front view of an optical element 300 with a rectangular or square profile is shown in FIG. 3a. Like optical element 200, optical element 300 can be formed with any of a spherical surface 302a, an aspherical surface 302b, a cylindrical surface 302d, and a non-cylindrical surface 302e symmetrical about lines EE and FF. The cylindrical surface of an optical element is characterized by having curvature along one axis (e.g., line EE) and being flat along an orthogonal axis (e.g., line FF). This gives cylindrical optical elements the unique property of functioning as lenses or focusing elements only along a single axis. Cylindrical lenses form line foci rather than points.

[0028] Similar to optical element 200, optical element 300 can also be formed with a freeform surface 302c. In contrast to the aspherical, spherical, cylindrical, and non-cylindrical surfaces 302a, 302b, 302d, and 302e shown in FIGS. 3B and 3C, respectively, freeform surface 302c can be specifically designed and highly customized on optical element 300 to include curvatures with peaks P and valleys V, and / or other features formed on its surface that are not symmetrical on the surface relative to lines EE or FF. In other words, freeform surface 302c can lack rotational symmetry and can even lack symmetry relative to a line bisecting optical element 300. While exemplary surface curvatures are shown in FIG. 3d, it should be understood that any cross-section of optical element 300 taken along any cut line through optical element 300 can have a different profile than surface 302c shown in FIG. 3d.

[0029] With the aid of computer design and manufacturing, freeform surfaces can be formed or imparted onto lenses, mirrors, and prisms to provide highly localized, customized features and precise image and optical correction in specific and predictable amounts. Freeform surfaces allow for the fabrication of mirrors, lenses, and / or prisms that are nearly flat yet have specific geometric shapes that can provide the desired image correction. For example, odd polynomial terms and special shapes on the surfaces of prisms, lenses, or mirrors can correct image problems such as coma without introducing astigmatism or anamorphic distortion. In other words, nearly flat freeform surfaces allow for the correction of multiple imaging aberrations and distortions without introducing additional undesirable image distortion.

[0030] It should be understood that the freeform polynomial terms and shapes can be formed on a substantially spherical, cylindrical, or flat surface, distinct from the freeform terms formed on the surface. For example, Figures 3e-3i illustrate freeform portions on a lens having shapes corresponding to substantially spherical, cylindrical, or flat shapes. For example, Figure 3e illustrates a cross section of a substantially spherical lens 310a taken along an axis of the lens (e.g., line EE in Figure 3a). Figure 3f illustrates a cross section of a substantially spherical lens 310a taken along an orthogonal axis of the lens (e.g., line FF in Figure 3a). It should be understood that for a substantially spherical lens 310a, the lens exhibits substantially the same radius of curvature anywhere along the spherical contour of the lens 310a. As illustrated, the lens 310a can have a substantially spherical shape 312a on the lens 310a, with the freeform portion 314a or freeform polynomial formed on the substantially spherical shape 312a of the lens 310a.

[0031] FIG. 3g shows a cross section of a substantially cylindrical lens 310b taken along an axis of the lens (e.g., line E-E in FIG. 3a). FIG. 3h shows a cross section of a substantially spherical lens 310b taken along an orthogonal axis of the lens (e.g., line F-F in FIG. 3a). As shown in FIG. 3g, the substantially cylindrical lens 310b can exhibit a radius of curvature only along one axis of the lens 310b (e.g., line E-E in FIG. 3a) as shown in FIG. 3g, while the orthogonal axis (e.g., line F-F in FIG. 3a) has substantially no curvature as shown in FIG. 3h. As shown, the lens 310a can have a substantially cylindrical shape 312b on the lens 310b, and a freeform portion 314b or a freeform polynomial can be formed on the substantially cylindrical shape 312b of the lens 310b.

[0032] Additionally, FIG. 3i illustrates a cross section of a substantially flat lens 310c. As shown in FIG. 3i, the substantially flat lens 310c can exhibit substantially no curvature along the flat shape 312c of the lens 310c, as shown in FIG. 3i. As shown, the lens 310c can have a substantially flat shape 312c on the lens 310b, and a freeform portion 314c or freeform polynomial can be formed on the substantially flat shape 312c of the lens 310c. While the flat shape 312c, cylindrical shape 312b, and spherical shape 312a are shown in the figures as discrete layers or surfaces, it should be understood that shapes 312a-c are only meant to represent the substantial general shape of the lens and are not meant to represent separate surfaces or layers of the lens. Instead, it should be understood that the freeform portions 314a-c can be formed on lenses of any shape.

[0033] In the spectrometers 100 / 400, light guide devices 102 / 402, and collection optics 104 / 404 described herein, freeform surfaces can be formed to be very nearly flat, with only slight variations in the peaks or valleys of the surface due to the polynomial terms in the equations defining the freeform surfaces. In such freeform surfaces of the present disclosure, most or all of the correction to the image can be attributed to the freeform polynomial terms rather than to any radius of curvature of any element. For example, prisms 118 and 120 can be Ferry prisms having a radius of curvature typical of Ferry prisms. However, it will be understood that one or more freeform surfaces, instead of including a spherical surface—in other words, rather than a spherical surface—the prisms described herein, and in some cases including Ferry prisms, used in spectrometers designed according to the principles of the present disclosure can have substantially cylindrical or non-cylindrical surfaces, characterized in that their surfaces have at least some cylindrical curvature in one axis (e.g., line EE), are flat in an orthogonal axis (e.g., line FF), and form a line focus rather than a point.

[0034] Because the image correction is due to freeform terms rather than the radius of curvature of prisms 118 and 120 (as is common in Ferry prisms with spherical surfaces), the freeform surfaces of prisms 118 and 120 can be formed on one or more surfaces, such as substantially flat (i.e., the surface is not flat or planar, but contains a small degree of cylindrical or non-cylindrical curvature (nearly zero)), cylindrical, or non-cylindrical surfaces, of prisms 118 and 120. Similarly, the surfaces of mirror 114, second mirror 116, field corrector 126, surface 408, and surface 412 can also have freeform surfaces formed thereon to ensure that most or all of the image correction is performed by freeform polynomial terms rather than by any curvature or radius of the respective elements.

[0035] In accordance with the principles described herein, to improve image correction in an Offner spectrometer, one or more of the prisms 118 and 120 can have one or more freeform surfaces that are substantially cylindrical, substantially non-cylindrical, or substantially flat with freeform polynomial terms formed thereon, as opposed to having a spherical surface with freeform polynomial terms formed thereon. More than one surface of a prism can be configured as substantially cylindrical, substantially non-cylindrical, or substantially flat with freeform polynomial terms formed thereon, although not all prisms need include one or more such surfaces. The shape and configuration of the surfaces of the prisms that do not have freeform polynomial terms formed thereon is not intended to be particularly limited by this disclosure and can alternatively be of any shape or configuration.

[0036] Therefore, to improve imaging and light directionality within spectrometer 100, freeform surfaces (e.g., substantially cylindrical, substantially non-cylindrical, or substantially flat surfaces with freeform polynomial terms formed thereon) can be formed on prisms 118 and 120 to improve image quality, correct image distortion and aberrations, and provide highly corrected images from the spectrometer. An example is provided to quantify experimental data collected regarding the improved image quality and corrected image distortion of the spectrometers disclosed herein. In this example, an Offner spectrometer including a Ferry prism with a spherical surface ("Ferry-Offner") is compared to an Offner spectrometer of the type shown in FIG. 1 in which the Ferry prism has a freeform surface formed thereon ("freeform Offner"). An example of a Ferry-Offner is shown in FIG. 7a, showing Ferry prisms 718 and 720 with spherical surfaces 718a, 718b, 720a, and 720b. An example of a freeform Offner is shown in FIG. 7b, which shows freeform prisms 728 and 730 according to the principles of the present disclosure, each having one face of the XY polynomial type and one flat (plano) face.

[0037] Both the Ferry-Offner and freeform Offner spectrometers use a combination of a first reflective surface 108 and a third reflective surface 112 formed on a single mirror 114 as a continuous freeform surface as described herein. Both designs were made to cover a 40 mm field of view with an f / 7.7 f-number and to cover the spectrum from 400 nm to 700 nm. In the freeform Offner spectrometer, the spectrometer was designed so that each Ferry prism (e.g., 118 and 120) has one surface (e.g., 118a or 118b, and 120a or 120b) that is an XY polynomial-type freeform surface, and one surface that is flat (plano) (e.g., one of 118a or 118b, and one of 120a or 120b).

[0038] Experimental data for both the Ferry-Offner and the free-form Offner revealed the following results: Compared to the Ferry-Offner, the free-form Offner provided 3.5 times better wavefront error correction, 33% better smile distortion correction, 30% better keystone distortion correction, and less than 1% anamorphic distortion compared to 8% for the Ferry-Offner. Figures 8A-9B show various distortion and error results comparing the Ferry-Offner with the free-form Offner. Figure 8a shows the smile distortion observed when comparing the free-form Offner with the Ferry-Offner. Figure 8b shows the keystone distortion observed when comparing the free-form Offner with the Ferry-Offner. Figure 9a shows the wavefront error observed within the red, green, yellow, and blue wavelengths for the Ferry-Offner. Figure 9b shows the wavefront error observed within the red, green, yellow, and blue wavelengths for the free-form Offner.

[0039] It should be understood that the spectrometers and optical devices of the present disclosure are not limited to the freeform Offner configuration described in the experimental results above. Other configurations are possible and contemplated within the principles of the present disclosure. One or more of surfaces 118a and 118b of first prism 118 can be formed as freeform surfaces (e.g., substantially cylindrical, substantially non-cylindrical, or substantially planar surfaces with freeform polynomial terms formed thereon) to provide correction to light traveling within the spectrometer. One or more of surfaces 120a and 120b of second prism 120 can be formed as freeform surfaces (e.g., substantially cylindrical, substantially non-cylindrical, or substantially planar surfaces with freeform polynomial terms formed thereon) to provide further correction to light traveling within the spectrometer. In accordance with the present disclosure above, first prism 118 and second prism 120 can be Ferry prisms having one or more freeform surfaces (e.g., substantially cylindrical, substantially non-cylindrical, or substantially planar surfaces with freeform polynomial terms formed thereon) formed thereon. Furthermore, only one of the Ferry prisms of first prism 118 and second prism 120 may have a single freeform surface, and the other prism may not have a freeform surface. It will be appreciated that to achieve the benefits of the present disclosure of limiting manufacturing complexity, only one freeform surface may be used on one of the prisms. However, any number of additional freeform surfaces may be formed on any number of the prism surfaces, if desired to achieve the benefits of the present disclosure.

[0040] Freeform surfaces can also be formed on the mirror 114. As shown in FIG. 1, the mirror 114 can be a single mirror 114 including a single, continuous freeform surface 115. A mirror 114 having a single, continuous freeform surface 115 can be surface-treated to include two distinct portions of the single, continuous freeform surface 115 (e.g., the first reflective surface 108, which is one portion of the continuous freeform surface 115, and the third reflective surface 112, which is another portion of the continuous freeform surface 115). This is in contrast to certain Offner relay imaging systems in which the first and third reflective surfaces are separate, discrete surfaces, even if such discrete surfaces are formed on a common substrate or contact each other. The first reflective surface 108 can be a freeform surface configured to reflect light to the surface 110 of the mirror 116, and the third reflective surface 112 can be a freeform surface configured to receive light from the mirror 116 and reflect light to the second prism 120.

[0041] Forming a single, continuous, free-form reflective surface 115 to function as both the first reflective surface 108 and the third reflective surface 112 offers advantages over traditional separate reflective surfaces in an Offner relay. For example, by limiting the amount of adjustment and movement that a single reflective surface can have between the first reflective surface 108 and the third reflective surface 112, the first reflective surface 108 and the third reflective surface 112 can be properly positioned relative to one another, reducing the likelihood of misalignment. This is achieved by fabricating both the first reflective surface 108 and the third reflective surface 112 as a single surface. By being fabricated as a single surface, the first reflective surface 108 and the third reflective surface 112 are specifically aligned and in a fixed position relative to one another by being part of the same, single surface. Because they are fixed relative to one another in this manner, the first reflective surface 108 and the third reflective surface 112 cannot be misaligned because they are formed from a single piece of glass or metal. In other words, the first reflective surface 108 and the third reflective surface 112 can be fabricated as one large mirror during manufacturing, rather than two smaller mirrors. Furthermore, one single mirror can be formed to provide the functions of both the first and third reflecting surfaces, thereby simplifying and minimizing the manufacturing process for producing the mirrors and reflecting surfaces of the spectrometer.

[0042] The surface 110 of the mirror 116 is not intended to be limited in any way by this disclosure. The reflective surface 110 of the mirror 116 can be any type of surface, including, but not limited to, spherical, aspherical, cylindrical, non-cylindrical, freeform, etc.

[0043] As further shown in FIG. 1 , spectrometer 100 can include a reflective field corrector 126 positioned downstream from second prism 120 and configured to provide correction for incident light 122g and reflect the light toward focal plane 128. Reflective surface 124 of reflective field corrector 126 can be any type of surface, including, but not limited to, a spherical, aspherical, cylindrical, non-cylindrical, or freeform surface. Reflective field corrector 126 can be a folding optical component, such as a folding mirror. Field corrector 126 can correct residual field aberrations, also known as aberrations that vary across the field of view, including curvature, field-dependent astigmatism, etc.

[0044] It will be understood from the above disclosure that any combination of surfaces included in spectrometer 100, collection optics 104, and light guide device 102 can be freeform (e.g., a substantially cylindrical surface with freeform polynomial terms formed thereon, a substantially non-cylindrical surface, or a substantially planar surface). For example, any element in the light guide device can include one or more freeform surfaces.

[0045] In a first example, only the first prism 118, the collection optics 104, and the light guiding device 102 of the spectrometer 100 have one or more freeform surfaces (e.g., a substantially cylindrical surface with a freeform polynomial term formed thereon, a substantially non-cylindrical surface, or a substantially planar surface). The first surface 118a can be a freeform prism surface, or the second surface 118b can be a freeform surface, or both surfaces 118a and 118b can be freeform surfaces.

[0046] In a second example, only the second prism 120, the collection optics 104, and the light guiding device 102 of the spectrometer 100 have one or more freeform surfaces. The first surface 120a can be a freeform prism surface, or the second surface 120b can be a freeform surface, or both surfaces 120a and 120b can be freeform surfaces.

[0047] In a third example, only mirror 114, collection optics 104, and light guiding device 102 of spectrometer 100 have one or more freeform surfaces. First reflective surface 108 can be a freeform surface, or third reflective surface 112 can be a freeform surface, or both reflective surface 108 and reflective surface 112 can be freeform surfaces. Furthermore, reflective surfaces 108 and 112 can be formed from a single continuous freeform reflective surface, as described elsewhere herein.

[0048] In another example, surface 110 of second mirror 116 can be a freeform surface. In another example, surface 124 of field corrector 126 can be a freeform surface.

[0049] It will be understood that, without limitation, any and all combinations of surfaces described and illustrated in spectrometer 100, collection optics 104, and light guide device 102 can be freeform surfaces, as long as one or more elements of spectrometer 100, collection optics 104, and / or light guide device 102 include at least one freeform surface in accordance with this disclosure.

[0050] An alternative configuration of spectrometer 400 is shown in FIG. 4. Elements having the same configuration and function as spectrometer 100 are labeled in FIG. 4 with the same numbers used in FIG. 1 for the same respective elements. As shown in FIG. 4, the light guide device 402 and collection optics 404 can be varied relative to light guide device 102 and collection optics 104 by using two discrete reflective surfaces / mirrors: first reflective surface 408 and third reflective surface 412. Although two discrete reflective surfaces 408 and 412 are used instead of a single continuous reflective surface 115, the optical path and function of each spectrometer 400, light guide device 402, and collection optics 404 can remain the same. However, to provide adjustability between the alignment and angle of the first reflective surface 408 and the third reflective surface 412, the first reflective surface 408 and the third reflective surface 412 can also be separate in the exemplary spectrometer 400.

[0051] It will be appreciated from the above disclosure that any combination of surfaces included in spectrometer 400, collection optics 404, and light guide device 402 can be freeform surfaces. For example, any element within the light guide device can include one or more freeform surfaces.

[0052] In a first example, only the first prism 118, the collection optics 404, and the light guiding device 402 of the spectrometer 400 have one or more freeform surfaces. The first surface 118a can be a freeform prism surface, or the second surface 118b can be a freeform surface, or both surfaces 118a and 118b can be freeform surfaces.

[0053] In a second example, only the second prism 120, the collection optics 404, and the light guiding device 402 of the spectrometer 400 have one or more freeform surfaces. The first surface 120a can be a freeform prism surface, or the second surface 120b can be a freeform surface, or both surfaces 120a and 120b can be freeform surfaces.

[0054] In a third example, only the first reflective surface 408, the collection optics 404, and the light guide device 402 of the spectrometer 400 have one or more freeform surfaces. The first reflective surface 408 can be a freeform surface. Alternatively, only the third reflective surface 412 can be a freeform surface. Alternatively, both the reflective surface 408 and the reflective surface 412 can be freeform surfaces.

[0055] In another example, surface 110 of second mirror 116 can be a freeform surface. In another example, surface 124 of field corrector 126 can be a freeform surface. It will be understood that this disclosure allows any and all combinations of surfaces described and illustrated in spectrometer 400, collection optics 404, and light guiding device 402 to be freeform surfaces, without limitation, so long as one or more elements of spectrometer 400, collection optics 404, and / or light guiding device 402 include at least one freeform surface.

[0056] While the above examples are directed specifically to spectrometers, it should be understood that the collection optics (e.g., collection optics 104 and collection optics 404) and light-guiding devices (e.g., devices 102 and 402) described herein can be utilized with any other suitable optical system, without limitation. Any optical system used to receive and direct electromagnetic radiation can incorporate collection optics (e.g., collection optics 104 and collection optics 404) and light-guiding devices (e.g., devices 102 and 402) in accordance with the principles described herein.

[0057] The example spectrometers, focusing optics, Offner relays, and light-guiding devices described herein offer many advantages and benefits. For example, computer-aided design of prism optical surfaces allows for specific and specialized shapes to be imparted to freeform surfaces of optical elements. Such specific and specialized freeform surfaces can be used in one or more of the optical components (e.g., prism 118, prism 120, reflective surface 108, reflective surface 112, reflective surface 115, reflective surface 110, field corrector 126). Because they are precisely designed using computer-aided design and manufacturing, it becomes possible to manufacture specialized and specifically designed surfaces on optical elements to correct specific aberrations and distortions of images and electromagnetic radiation that were previously difficult to correct. The use of specialized surfaces designed by computer-aided design tools allows for precise correction of specific aberrations. Thus, the principles described herein can result in relays, light-guiding devices, optical systems, and spectrometers with well-corrected imaging fields and high resolution.

[0058] The above description is primarily directed to an Offner spectrometer that includes an Offner relay as the collection optic. However, other collection optics designed according to the principles described herein can be used in a spectrometer while still achieving the benefits and advantages described above. For example, FIG. 5 illustrates another optical system according to an example of the present disclosure. The optical system can be a spectrometer 500 and is operable to collect light from an object 501 for imaging / analysis by the spectrometer 500. The spectrometer 500 includes a light guiding device 502 that can be configured to direct electromagnetic radiation and separate the electromagnetic radiation into various spectral components.

[0059] Light guide device 502 can include collection optics 504, which can include one or more reflective surfaces. In this example, collection optics 504 can be configured as a relay, making spectrometer 500 a Dyson spectrometer. Collection optics 504 in such an example can include a first reflective surface 508, which can be configured to receive and reflect electromagnetic radiation in a desired direction.

[0060] The light guiding device 502 can further include a prism 518, which can include a first surface 518a and a second surface 518b configured to collimate, reflect, refract, and / or direct incident light. The light guiding device 502, including its various components (e.g., the prism 518 and the reflective surface 508), can define an optical path for directing electromagnetic radiation. As shown in FIG. 5 , the optical path can be as follows: Light incident on the first surface 518a of the prism 518 can pass through the prism 518 and exit the second surface 518b of the prism 518. The light from the first prism 518 can be directed toward a first reflective surface 508 of the collection optics 504. The first reflective surface 508 can be positioned in the optical path downstream from the prism 518 and can reflect the light from the first prism 518 back toward the first prism 518 of the collection optics 504. Light from reflective surface 508 can be directed toward second surface 518b of prism 518. Light incident on second surface 518b can pass through prism 518 and exit prism 518 through first surface 518a.

[0061] The light guide device 502 can be used as a light guide device in a spectrometer 500. The spectrometer 500 can further include a slit 503 that functions as an electromagnetic radiation entrance, through which light 522a from the source or object 501 enters the spectrometer 500. Alternatively, the slit can alternatively be a beam slicer. Light 522b from the slit 503 can travel along an optical path to the light guide device 502, specifically to a first prism 518. Light incident on a first surface 518a of the prism 518 can pass through the first prism 518 and exit from a second surface 518b of the prism 518 as light 522c. The light 522c can be directed toward a reflective surface 508 of the collection optics 504, which can reflect the light 522c from the prism 518 back toward the second surface 518b of the prism 518 as light 522d. Light 522d may pass through prism 518 and exit from a first surface 518a of prism 518 as light 522e. Light 522e may then either enter or pass through a field corrector 526 (e.g., a field flattener) positioned downstream from prism 518 and configured to provide correction for incident light 522e to ensure a high-quality, high-resolution, well-corrected field for imaging / analysis by spectrometer 500. Thereafter, or alternatively, light 522e may proceed to a focal plane 528 configured to receive the spectrally separated light within a well-corrected field or image. Focal plane 528 may include a capture sensor, such as a CCD imaging sensor, a CMOS imaging sensor, or any other sensor or capture device, configured to analyze the light or capture and image it for later analysis. The devices, sensors, or other objects disposed within focal plane 528 are not intended to be limited in any way by this disclosure.

[0062] As shown in FIG. 5 , spectrometer 100 is a prism-based spectrometer that includes prism 518 and reflective surface 508. Modern optical design, including computer-aided design of lenses, prisms, mirrors, and their surfaces, allows for the fabrication of specialized and unique surfaces on lenses, mirrors, and prisms to provide precise image correction and high-resolution images from the spectrometer. Like the prisms described in spectrometer 100, prism 518 can be formed by freeform surfaces. One or more of surfaces 518a and 518b can be freeform surfaces that provide correction of incident light, as described elsewhere in this disclosure. One surface (518a or 518b) of prism 518 can be freeform, and the other surface (the non-freeform surface 518a or 518b) can be flat. Alternatively, both surfaces 518a and 518b can be freeform, if desired. The freeform surface(s) of prism 518 can be specifically designed to provide a specific amount of correction for a specific aberration in the incident light, similar to that described above with respect to spectrometer 100.

[0063] Further described herein is a method 600 of configuring the light guide device shown in FIG. 6. Method 600 may include step 602 of configuring the light guide device to include a first prism in the optical path. Method 600 may further include step 604 of configuring the light guide device to include collection optics in the optical path from the first prism. Method 600 may further include step 606 of configuring the first prism to include at least one freeform prism surface. These methods may be performed by the spectrometers described herein.

[0064] Reference has been made to the examples illustrated in the drawings, and specific language has been used herein to describe these. It should be understood, however, that no limitation of the scope of the technology is intended thereby. Modifications and further variations of the features illustrated herein, as well as additional uses of the examples illustrated herein, should be considered within the scope of the description.

[0065] Although the present disclosure may not explicitly disclose that some embodiments or features described herein may be combined with other embodiments or features described herein, the present disclosure should be read to describe any such combinations feasible by one of ordinary skill in the art. The use of "or" in this disclosure should be understood to mean a non-exclusive or, i.e., "and / or," unless otherwise stated herein.

[0066] Furthermore, the described features, structures, or characteristics may be combined in any suitable manner in one or more embodiments. In the foregoing description, numerous specific details, such as example configurations, are provided to provide a thorough understanding of examples of the described technology. However, it will be understood that the technology may be practiced without one or more of the specific details, or with other methods, components, devices, etc. In other instances, well-known structures or operations have not been shown or described in detail to avoid obscuring aspects of the technology.

[0067] Although the subject matter has been described in language specific to structural features and / or operations, it should be understood that the subject matter defined in the appended claims is not necessarily limited to the particular features and operations described. Rather, the specific features and operations described above are disclosed as example forms of implementing the claims. Numerous modifications and alternative arrangements may be devised without departing from the spirit and scope of the described technology.

Claims

1. 1. A light guiding device configured to guide electromagnetic radiation along an optical path, comprising: a first prism positioned in the optical path; a collection optical system positioned in the optical path; A light guide device, wherein the first prism includes at least one freeform prism surface, the at least one freeform prism surface having at least some cylindrical curvature and having a freeform polynomial term formed thereon.

2. The light guide device of claim 1 further comprising a second prism positioned downstream in the light path from the first prism.

3. The light guide device of claim 2 , wherein the collection optic is positioned downstream in the light path from the first prism and upstream in the light path from the second prism.

4. The light guide device of claim 2 , wherein one or more of the first prism and the second prism is a Ferry prism.

5. The light guide device of claim 2 , wherein the second prism includes at least one freeform prism surface.

6. The light guide device of claim 2 , wherein one or more of the first prism and the second prism comprises at least two freeform prism surfaces.

7. The light guide device of claim 1 , wherein the collection optic is positioned downstream or upstream in the light path from the first prism.

8. The focusing optical system includes: a first reflective surface positioned in the optical path; a second reflective surface positioned in the optical path downstream from the first reflective surface; and a third reflective surface positioned in the light path downstream from the second reflective surface.

9. 9. The light guide device of claim 8, wherein the first reflective surface, the second reflective surface, and the third reflective surface of the collection optic are arranged to form an Offner relay.

10. 9. The light guide device of claim 8, wherein the first reflective surface is formed on a first mirror and the third reflective surface is formed on a third mirror separate from the first mirror.

11. the first reflecting surface and the third reflecting surface are disposed on a common substrate; The light guide device of claim 8 , wherein one or more of the first reflective surface and the third reflective surface are freeform surfaces.

12. 9. The light guide device of claim 8, wherein the first reflective surface and the third reflective surface are both part of a single continuous free-form reflective surface formed on a first mirror.

13. 13. The light guide device of claim 12, wherein the second reflective surface is formed on a second mirror separate from the first mirror.

14. 14. The light guide device of claim 13, wherein one or more of the first reflective surface and the third reflective surface are freeform surfaces.

15. The light guide device of claim 1 , further comprising a field corrector disposed in the optical path downstream from the first prism and the collection optics.

16. 16. The light guide device of claim 15, wherein the field compensator is a reflective field compensator.

17. The light guide device of claim 16 , wherein the field corrector comprises a freeform surface.

18. The light guide device of claim 1 , wherein the freeform prism surfaces comprise at least one of a cylindrical surface, a non-cylindrical surface, or a substantially planar surface.

19. 1. A spectrometer configured to form a spectrally resolved image of electromagnetic radiation from a source, comprising: a slit configured to receive the electromagnetic radiation; and and the light guide device of claim 1 positioned in the light path downstream from the slit.

20. 1. A method of constructing a light guide device, comprising: configuring the light guide device to include a first prism in an optical path; configuring the light guide device to include a collection optic positioned in the light path; configuring the first prism to include at least one freeform prism surface, the at least one freeform prism surface having at least some cylindrical curvature and having freeform polynomial terms formed thereon.

21. an electromagnetic radiation inlet configured to receive electromagnetic radiation from a source; a light guiding device configured to guide electromagnetic radiation along an optical path from the electromagnetic radiation entrance to a focal plane, the light guiding device comprising: a first prism positioned in the optical path; a collection optical system positioned in the optical path; a light guiding device, wherein the first prism includes at least one freeform prism surface, the at least one freeform prism surface having at least some cylindrical curvature and having a freeform polynomial term formed thereon.

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