Imaging device

Abstract

To provide an imaging device which reduces noise that overlaps with interference fringes resulting from a polarizer and a wavelength plate and can acquire interference fringes with different phases at the same time.SOLUTION: The imaging device includes: a plurality of beam splitters for dividing an incoherent optical wave into a plurality of optical waves; a plurality of interferometers for causing the divided optical waves to self-interfere with one another; and a plurality of cameras for acquiring interference fringes generated by the interferometers. Generating interference fringes with different phases by the interferometers allows the simultaneous acquisition of interference fringes with different phases.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to an imaging device, and more particularly to an imaging device for use in incoherent digital holography. [Background technology]

[0002] Incoherent digital holography is a technology that can capture holograms of objects using light sources with low spatial coherence such as sunlight, general indoor lighting, and fluorescent light, making it possible to realize three-dimensional imaging under natural light environments. Incoherent digital holography can reconstruct three-dimensional information of an object by the following procedure. First, the light from the object (object light) is separated into two lights, and a concave mirror or lens is placed in one of the light paths to create a slight optical path difference. These lights are then combined again and interfered with to capture the object as interference fringes. By acquiring multiple interference fringe images with different phases and calculating them using a phase shift method, etc., the complex amplitude distribution of the object on the image sensor surface can be obtained. By performing a backpropagation calculation of light from this complex amplitude distribution, a two-dimensional image (i.e., three-dimensional information) at any depth distance can be obtained.

[0003] FIG. 5 is an example of the optical system of an imaging device using a Michelson interferometer. After passing through lens 1 and wavelength filter 2, object light from a subject is split into two beams by beam splitter (BS) 3. One object light is reflected by plane mirror 4 and the other by concave mirror 5, and the two beams are combined again by beam splitter 3 to generate interference fringes. An interference fringe image can be obtained by photographing this with camera 20. To obtain interference fringes of different phases, the position of plane mirror 4 is changed in the direction of the optical axis by piezoelectric actuator 6. By performing this operation multiple times, multiple interference fringe images of different phases can be obtained.

[0004] However, in this optical system, it is difficult to capture moving images because the position of the plane mirror 4 in the optical axis direction changes over time. To capture moving images, it is necessary to simultaneously capture these interference fringe images of different phases. As a method for this, a method has been proposed in which polarization is used to simultaneously generate and capture interference fringes of different phases.

[0005] FIG. 6 shows a first example of an imaging device that simultaneously acquires multiple interference fringe images. The optical system of this imaging device includes a lens 1, a wavelength filter 2, a polarizing plate 7, a birefringent lens 8, a quarter-wave plate 9, and a polarization camera 21. Object light from a subject passes through the lens 1 and the wavelength filter 2, and is converted into only linearly polarized components in an arbitrary direction by the polarizing plate 7. The linearly polarized object light is incident on a birefringent lens 8 that has different focal lengths for polarization directions that are ±45° from the polarization direction. The object light that passes through the birefringent lens 8 passes through a quarter-wave plate 9 and becomes two circularly polarized lights, clockwise and counterclockwise. This is captured by a polarization camera 21 (having pixels with four types of polarization directions) in which a polarizing plate is provided at every 45° in each pixel. By collecting signals from pixels with the same polarization direction, images for each polarization direction (each phase) can be generated, and interference fringe images with different phases can be acquired simultaneously. By performing a back propagation calculation of light from these interference fringe images, a reconstructed image at any depth distance can be obtained. It has been reported that this optical system can capture video at a frame rate of 25 fps (Non-Patent Document 1).

[0006] FIG. 7 shows a second example of an imaging device that simultaneously captures multiple interference fringe images. In the optical system of this imaging device, a beam splitter (BS) 3 and a reflective liquid crystal panel 10 are used instead of the birefringent lens 8 in FIG. 6, and a diffraction grating 11, a polarizing plate array 12, and a camera 20 are used instead of the polarizing camera 21. After passing through a lens 1, a wavelength filter 2, and a polarizing plate 7, the object light from the subject is incident on the reflective liquid crystal panel 10 via the beam splitter 3 with a polarization direction of, for example, 45°. The reflective liquid crystal panel 10 imparts a planar phase distribution to one polarization direction and a spherical phase distribution to the other polarization direction to the horizontal and vertical polarization components of the incident object light. The object light reflected by the reflective liquid crystal panel 10 and the beam splitter 3 passes through a quarter-wave plate 9. The object light is then split into four directions by a diffraction grating 11, and is photographed by a camera 20 through a polarizing plate array 12 provided with four polarizing plates with polarization directions at 45° intervals, thereby simultaneously acquiring interference fringe images of different phases. This optical system enables continuous photographing at a frame rate of 1 fps (Patent Document 1).

[0007] FIG. 8 shows a third example of an imaging device that simultaneously acquires multiple interference fringe images. The optical system of this imaging device uses a Michelson interferometer using a polarizing beam splitter (PBS) 13, a quarter-wave plate 9, a plane mirror 4, and a concave mirror 5, instead of the birefringent lens 8 in FIG. 6. The object light from the subject passes through a lens 1, a wavelength filter 2, and a polarizing plate 7, and then enters the PBS 13. The p-polarized component of the object light passes through the PBS 13, becomes circularly polarized light at the quarter-wave plate 9, and is reflected by the mirror 4. The reflected light passes through the quarter-wave plate 9 again to become s-polarized light, and is reflected by the PBS 13. The s-polarized component of the object light is reflected by the PBS 13, becomes circularly polarized light at the quarter-wave plate 9, and is reflected by the concave mirror 5. The reflected light passes through the quarter-wave plate 9 again to become p-polarized light, and passes through the PBS 13. The object light that leaves the PBS 13 is further circularly polarized clockwise and counterclockwise by the quarter-wave plate 9. This is photographed by a polarization camera 21, in which a polarizing plate is provided at every 45° for each pixel. By collecting signals from pixels with the same polarization direction, images can be generated in each polarization direction, and interference fringe images with different phases can be obtained simultaneously. This optical system can capture video at a frame rate of 60 fps (Patent Document 2).

[0008] FIG. 9 shows a fourth example of an imaging device that simultaneously captures multiple interference fringe images. In the optical system of this imaging device, a diffraction grating 11, a polarizing plate array 12, and a camera 20 are used instead of the polarizing camera 21 in FIG. 8. After passing through a lens 1, a wavelength filter 2, and a polarizing plate 7, the object light from the subject is incident on a polarizing beam splitter (PBS) 13. The p-polarized component of the object light passes through the PBS 13, becomes circularly polarized light at the quarter-wave plate 9, and is reflected by the mirror 4. The reflected light passes through the quarter-wave plate 9 again to become s-polarized light, and is reflected by the PBS 13. The s-polarized component of the object light is reflected by the PBS 13, becomes circularly polarized light at the quarter-wave plate 9, and is reflected by the concave mirror 5. The reflected light passes through the quarter-wave plate 9 again to become p-polarized light, and passes through the PBS 13. These object lights are further circularly polarized clockwise and counterclockwise by the quarter-wave plate 9. The light is then split into four directions by a diffraction grating 11 and photographed by a camera 20 through a polarizing plate array 12, which has polarizing plates arranged every 45°, making it possible to simultaneously obtain interference fringe images of different phases (Patent Document 2). [Prior art documents] [Patent documents]

[0009] [Patent Document 1] Patent Publication No. 2021-131457 [Patent Document 2] JP 2023-77325 A [Non-patent literature]

[0010] [Non-Patent Document 1] KiHong Choi, et al., “Compact self-interference incoherent digital holographic camera system with real-time operation,” Optics Express, (2019), Vol. 27, No. 4, pp. 4818-4833 Summary of the Invention [Problem to be solved by the invention]

[0011] The optical systems of Figures 6 and 8 can simultaneously capture multiple interference fringe images with a single polarization camera 21. However, because the polarization camera 21 has pixels with polarizers in the same direction arranged on every other pixel, the resolution when capturing each interference fringe image is half that of the original pixel density of the camera.

[0012] On the other hand, the optical system of Figures 7 and 9 does not use the polarization camera 21, and therefore the resolution of the acquired interference fringe image is twice as high as that of the optical system of Figures 6 and 8. However, in the optical system of Figures 7 and 9, the light from the four directions emitted from the diffraction grating 11 is obliquely incident on the camera 20 at different angles, and therefore different distortions are generated in the interference fringe image.

[0013] Furthermore, both optical systems use a polarizing plate 7 and a wave plate 9. The spatial non-uniformity of these optical elements causes the amount of phase change to be non-uniform. This non-uniformity of the phase causes noise in the reconstructed image.

[0014] Therefore, in consideration of the above-mentioned problems, an object of the present invention is to provide an imaging device that can reduce the noise superimposed on interference fringes caused by polarizing plates and wavelength plates in incoherent digital holography, and can simultaneously acquire interference fringes of different phases. [Means for solving the problem]

[0015] In order to solve the above problems, an imaging device according to the present invention comprises: (1) An imaging device comprising a plurality of beam splitters that split incoherent light waves into a plurality of beams, a plurality of interferometers that cause each of the split light waves to self-interfere, and a plurality of cameras that acquire the interference fringes generated by the interferometers, characterized in that each interferometer generates interference fringes with different phases, thereby simultaneously acquiring a plurality of interference fringes with different phases.

[0016] (2) In the imaging device of (1) above, it is preferable that the interferometer further comprises a beam splitter and two mirrors having different curvatures.

[0017] (3) In the imaging device of (1) or (2) above, it is further preferable that the interferometer is a Michelson interferometer.

[0018] (4) It is preferable that any of the imaging devices described above in (1) to (3) further splits the object light into four using three beam splitters that split the incident light 1:1, and simultaneously acquires four interference fringes with different phases.

[0019] (5) It is preferable that any of the imaging devices described above in (1) to (3) further splits the object light into three using a beam splitter that splits the incident light in a 2:1 ratio and a beam splitter that splits the incident light in a 1:1 ratio, thereby simultaneously acquiring three interference fringes with different phases.

[0020] (6) It is preferable that the imaging device according to any one of (1) to (5) above further obtains a complex amplitude distribution from the obtained plurality of interference fringes, and obtains three-dimensional information of the subject. Effect of the Invention

[0021] The imaging device of the present invention can reduce noise caused by polarizing plates and wave plates in incoherent digital holography and simultaneously obtain interference fringes with different phases. In addition, a high-quality moving image can be reconstructed from the interference fringes. [Brief description of the drawings]

[0022] [Figure 1] 1 is a diagram illustrating the principle of an incoherent digital holography optical system according to the present invention. [Diagram 2] 2 is an example of an optical system of the imaging device according to the first embodiment. [Diagram 3] 4 shows four interference fringes with different phases acquired by the imaging device of the first embodiment. [Figure 4] 13 is an example of an optical system of an imaging device according to a second embodiment. [Diagram 5] 2 is an example of an optical system of an imaging device using a Michelson interferometer. [Figure 6] This is a first example of an imaging device that simultaneously acquires multiple interference fringe images. [Figure 7] 13 is a second example of an imaging device that simultaneously acquires multiple interference fringe images. [Figure 8] 13 is a third example of an imaging device that simultaneously acquires multiple interference fringe images. [Figure 9] 13 is a fourth example of an imaging device that simultaneously acquires multiple interference fringe images. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0023] 1 is a diagram illustrating the principle of an incoherent digital holography optical system according to the present invention. The optical system includes n interferometers for self-interference of light waves.

[0024] The object light from the subject may be reflected light, transmitted light, or light emitted from the subject, and is an incoherent light wave. After passing through a lens 1 and a wavelength filter 2, the object light is branched into multiple beams (n-branch, n≧3).

[0025] Each of the split object beams is incident on an interferometer (for example, a Michelson interferometer). The Michelson interferometer is similar to that shown in FIG. 5, in that it has a beam splitter (BS) 3 (3 1 ~3 n ), Plane mirror 4(4 1 ~4 n ), and concave mirror 5(5 1 ~5 n The object light emitted from the interferometer generates interference fringes, which are then photographed by a camera 20 (20 1 ~20 n ) is used to capture the images. At this time, the position of the plane mirror 4 or concave mirror 5 of each interferometer in the optical axis direction is adjusted so that the phases of the interference fringes captured by each camera 20 differ by 2π / n. This makes it possible to simultaneously capture n interference fringes with a phase difference of 2π / n using n cameras 20.

[0026] The complex amplitude distribution of the interference fringes is obtained from the n obtained interference fringe images using the phase shift method. Furthermore, by performing the back propagation calculation of light from this complex amplitude distribution as in the conventional method, a reconstructed image at any depth distance can be obtained. By performing this for each frame, a moving image can be obtained.

[0027] In the above description of the optical system, a Michelson interferometer is used as the interferometer in consideration of the efficiency in implementing the optical system, but the interferometer may have another structure. Also, each interferometer is configured with a plane mirror 4 and a concave mirror 5, but it may be configured with mirrors with different curvatures. The reason why the phases of the interference fringes are made to differ by 2π / n is to efficiently perform the calculation for obtaining the complex amplitude distribution thereafter, and it is sufficient to obtain multiple interference fringes with different phases from each other.

[0028] In the optical system of the present invention, since the polarization camera 21 used in Figs. 6 and 8 is not used, the resolution when acquiring the interference fringes is twice as high in both the vertical and horizontal directions as when the polarization camera 21 is used. In addition, the diffraction grating 11 used in Figs. 7 and 9 is not used, and the optical axis of the object light does not enter the camera 20 at an angle. Therefore, there is no distortion of the interference fringes due to each phase, and the degradation of the resolution of the reconstructed image is small. Furthermore, since this optical system does not use a polarizing plate and a wave plate, noise caused by these optical elements is not generated, and it is possible to acquire interference fringes with low noise compared to the conventional method. And it is possible to reconstruct a high-quality moving image.

[0029] Hereinafter, an embodiment of the present invention will be described with reference to the drawings.

[0030] (Embodiment 1) As a first embodiment, an incoherent digital holography imaging device will be described when the number of branches, n, is set to 4. FIG. 2 shows an example of an optical system of the imaging device of the first embodiment. The optical system of this imaging device includes a lens 1, a wavelength filter 2, and three beam splitters (BS) 3. 10 ~3 12 , four Michelson interferometers, and camera 20 1 ~20 4 It consists of:

[0031] Lens 1 collects the object light and makes it enter wavelength filter 2. Like a normal camera lens, lens 1 has an adjustment function for magnifying the image, widening the angle, etc. Note that the subject can be placed at any position regardless of the focal position of lens 1.

[0032] The wavelength filter 2 is a bandpass filter that transmits light waves of a predetermined wavelength width (for example, 3 nm to 20 nm) and improves the temporal coherence of the light waves. Note that narrowing the wavelength width improves image quality but reduces the amount of light in the image, so an appropriate wavelength filter 2 is selected depending on the desired image. The light waves that have passed through the wavelength filter 2 are then sent to the beam splitter 3. 10 is incident on

[0033] The lens 1 and the wavelength filter 2 are not essential components of the imaging device, and can be provided as necessary.

[0034] Beam splitter 3 10 ~3 12 is a beam splitter that splits incident light into transmitted light and reflected light in a 1:1 ratio. The object light from the subject passes through lens 1 and wavelength filter 2, then enters beam splitter 3. 10 Each object beam is further split into two by beam splitter 3. 11 ,3 12 The object light is equally split into four beams.

[0035] The four split object beams are fed through beam splitters (BS) 3 (3 1 ~3 4 ), Plane mirror 4(4 1 ~4 4 ), and concave mirror 5(5 1 ~5 4 The object light is incident on four Michelson interferometers consisting of four cameras 20 (20 1 ~20 4) captures the interference fringes generated by each interferometer. At this time, it is desirable to adjust the position of each plane mirror 4 or concave mirror 5 in the optical axis direction so that the phases of the interference fringes captured by the four cameras 20 are shifted by π / 2.

[0036] FIG. 3 shows four interference fringes with different phases obtained by simulation using the imaging device of this embodiment (FIG. 2) when a point light source is used as the object light. In this way, the imaging device of this embodiment can simultaneously obtain interference fringes with four phases that differ in phase by π / 2. Based on the obtained interference fringes, a reconstructed image at any depth distance can be obtained by performing a phase shift method and a back propagation calculation of light in the same manner as in the conventional method. Furthermore, a moving image can be obtained by performing an interference fringe acquisition and a reconstruction calculation for each frame.

[0037] (Embodiment 2) As a second embodiment, an incoherent digital holography imaging device in which the number of branches n is set to 3 will be described. FIG. 4 shows an example of an optical system of the imaging device of the second embodiment. The optical system of this imaging device includes a lens 1, a wavelength filter 2, and two beam splitters (BS) 3. 13 ,3 14 , three Michelson interferometers, and camera 20 1 ~20 3 The description of the configuration that overlaps with the first embodiment will be omitted.

[0038] Beam splitter 3 13 is a beam splitter that splits the incident light into transmitted light and reflected light in a ratio of 2:1. Beam splitter 3 14 is a beam splitter that splits incident light into transmitted light and reflected light in a 1:1 ratio. The object light from the subject passes through lens 1 and wavelength filter 2, then enters beam splitter 3. 13 The beam is split into two parts with a ratio of 2:1. 13 The object light that passes through the beam splitter 3 14 This ensures that the light intensity of the three branched object beams is the same.

[0039] The three split object beams are fed through beam splitter (BS) 3 (3 1 ~3 3 ), Plane mirror 4(4 1 ~4 3 ), and concave mirror 5 (5 1 ~5 3 The object light is incident on three Michelson interferometers consisting of three cameras 20 (20 1 ~20 3 ) captures the interference fringes generated by each interferometer. At this time, it is desirable to adjust the position of each plane mirror 4 or concave mirror 5 in the optical axis direction so that the phases of the interference fringes captured by the three cameras are shifted by 2π / 3. This makes it possible to simultaneously capture three interference fringes with phase differences of 2π / 3.

[0040] Then, based on the obtained interference fringes, a reconstructed image at any depth can be obtained by performing the phase shift method and backpropagation calculation of light in the same manner as in the conventional method. Furthermore, a moving image can be obtained by acquiring interference fringes and performing reconstruction calculations for each frame.

[0041] In this embodiment, the number of branches is three, so the amount of light of each interference fringe is greater than in the first embodiment, and the signal-to-noise ratio of the interference fringes is improved. 13 However, a 1:1 beam splitter may be used to halve the exposure time of the camera 20 for interference fringes with strong light intensity.

[0042] (Other embodiments) In this embodiment, the number of branches n is 4 or 3, but it may be 5 or more. Although each interferometer is composed of a plane mirror 4 and a concave mirror 5, the same effect can be obtained by using mirrors with different curvatures. Furthermore, a Michelson interferometer is used as the interferometer, but other interferometers capable of acquiring interference fringes, such as a Mach-Zehnder interferometer, may be used. In the case of a Mach-Zehnder interferometer, the object light is split into two by a beam splitter, and then the two object lights are reflected by mirrors with different curvatures, and then passed through another beam splitter again, and the interference fringes are captured by a camera.

[0043] In the above embodiment, the configuration and operation of the imaging device have been described, but the present invention is not limited thereto, and may be configured as an imaging method for simultaneously acquiring multiple interference fringes in incoherent digital holography. That is, the present invention may be understood as an invention of an imaging method for simultaneously acquiring n interference fringes with different phases according to the flow of processing the object light of the imaging device in Figs. 1, 2, and 4.

[0044] Although the above-mentioned embodiment has been described as a representative example, it is clear to those skilled in the art that many modifications and substitutions can be made within the spirit and scope of the present invention. Therefore, the present invention should not be interpreted as being limited by the above-mentioned embodiment, and various modifications or changes can be made without departing from the scope of the claims. For example, the functions included in each block, each step, etc. described in the embodiment can be rearranged so as not to be logically inconsistent, and multiple configuration blocks, steps, etc. can be combined into one or divided. [Explanation of symbols]

[0045] 1 Lens 2 wavelength filter 3 Beam splitter (BS) 4. Plane Mirror 5. Concave Mirror 6 Piezo Actuators 7 Polarizing Plate 8 Birefringent Lenses 9 1 / 4 wave plate 10 Reflective LCD panel 11 Diffraction Grating 12 Polarizer Array 13 Polarizing Beam Splitter (PBS) 20 Camera 21 Polarization Camera

Claims

1. The optical system includes a plurality of beam splitters that split an incoherent light wave into a plurality of beams, a plurality of interferometers that cause each of the split light waves to self-interfere with each other, and a plurality of cameras that capture the interference fringes generated by the interferometers, An imaging device characterized in that each interferometer generates interference fringes having different phases, thereby simultaneously acquiring a plurality of interference fringes having different phases.

2. 2. The imaging device according to claim 1, 1 is a perspective view of an imaging device according to the first embodiment of the present invention;

3. 3. The imaging device according to claim 2, 13. An imaging apparatus, wherein the interferometer is a Michelson interferometer.

4. 2. The imaging device according to claim 1, An imaging device characterized in that an object beam is split into four beams using three beam splitters that split the incident beam 1:1, and four interference fringes with different phases are simultaneously obtained.

5. 2. The imaging device according to claim 1, An imaging device characterized in that an object beam is split into three beams using a beam splitter that splits the incident light 2:1 and a beam splitter that splits the incident light 1:1, thereby simultaneously acquiring three interference fringes with different phases.

6. 6. The imaging device according to claim 1, An imaging apparatus characterized in that a complex amplitude distribution is obtained from a plurality of acquired interference fringes, and three-dimensional information of a subject is obtained.

Images