Incoherent digital holography imaging apparatus and imaging method thereof

Abstract

To provide an incoherent digital holography imaging device and an imaging method of the same, with which it is possible to suppress degradation of high frequency component signal quality of a hologram and realize a reconstructed image having high picture quality, even when the set wavelength width of light entering an imaging element is expanded.SOLUTION: An incoherent digital holography imaging device comprises: light splitting means (103) for splitting incoherent light from an object 101 into first split light and second split light; a concave mirror 104 and a plane mirror 105 for adding distribution of a relatively spherical phase to light split into two; light synthesizing means (103) for synthesizing light from these two mirrors 104, 105; an image sensor 106 for imaging an interference fringe that is formed by the synthesized two pieces of split light interfering with each other; and a wavelength dispersion lens 110 provided in a pre-stage to the image sensor 106, which causes a focal distance of the length corresponding to a wavelength of incident light to be generated, so that sizes of Gabor zone plates formed for each light point of the object 101 are equalized.SELECTED DRAWING: Figure 3

Description

Technical Field

[0001] The present invention relates to an incoherent digital holography imaging apparatus and an imaging method thereof used in a three-dimensional video system or the like, and particularly to an apparatus and a method capable of improving the quality of a reconstructed image obtained by incoherent digital holography.

Background Art

[0002] In recent years, expectations for incoherent digital holography as a method for acquiring three-dimensional information have been increasing. Hereinafter, a configuration example of an optical system of conventional incoherent digital holography will be described with reference to FIG. 6. Light (object light) from an object 201 is branched into two optical paths by a beam splitter 203 via a lens 202. One is reflected by a light splitting surface 203a of the beam splitter 203 and irradiated onto a concave mirror 204, and the other is transmitted through the light splitting surface 203a of the beam splitter 203 and irradiated onto a plane mirror 205. The object light reflected by each of the mirrors 204 and 205 returns to the beam splitter 203. Among the object light from the concave mirror 204, the converging light transmitted through the light splitting surface 203a and, among the object light from the plane mirror 205, the substantially parallel light (which becomes exact parallel light when the object 201 is at the focal length position of the lens 202) reflected by the light splitting surface 203a proceed in the same direction while generating a slight optical path length difference, pass through a wavelength filter 209 that limits the wavelength width to, for example, about 10 nm, and reach an image sensor 206. At this time, the two object lights that have passed through the wavelength filter 209 interfere with each other and form interference fringes (hologram) on the image sensor 206.

[0003] Also, in order to obtain the amplitude-phase information of the hologram, for example, a 4-step phase shift method is used. That is, each time the position of the plane mirror 205 is moved by 1 / 8 of the wavelength in the optical axis direction by a piezo actuator 207 installed on the plane mirror 205 (the optical path length of the light passing through the plane mirror changes by 1 / 4 of the wavelength), a hologram with a different phase pattern is acquired by the image sensor 206.

[0004] By repeating this operation, holograms with four different phase patterns are obtained, and the amplitude phase information of the holograms is calculated from these interference fringes using a conventional phase-shift digital holography technique. Furthermore, by performing backpropagation calculations based on this amplitude phase information of the holograms, a reconstructed image of the object at any depth position can be obtained (Patent Document 1).

[0005] Furthermore, by providing a predetermined convex lens in front of the image sensor 206, the magnification of the object being photographed can be adjusted. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Special Publication No. 2016-533542 [Overview of the project] [Problems that the invention aims to solve]

[0007] In incoherent digital holography, an object is considered as a collection of light points. Since the hologram of these light points takes the shape of a Gabor zone plate (GZP), the hologram of an object is a superposition of GZPs. Furthermore, the resolution of the reconstructed image depends on the size of the GZP. The larger the GZP, that is, the higher the frequency components that can be acquired, the smaller the size of the points in the reconstructed image becomes.

[0008] Figures 7(a) and 7(b) (where (b) is an enlarged portion of (a)) show holograms of light points when the pupil diameter of the optical system is infinitesimally large and the wavelength width of light is infinitesimally small, and Figure 7(c) shows the image reconstructed from these holograms. On the other hand, the hologram of the light point when the pupil diameter of the optical system is set to infinity and the wavelength width of light is 10 nm is shown in Figures 8(a) and (b) ((b) is an enlarged portion of (a)), and the image reconstructed from that hologram is shown in Figure 8(c).

[0009] Comparing Figure 7(b) and Figure 8(b), the high-frequency components of the hologram in Figure 8(b) are degraded compared to the hologram in Figure 7(b) (the stripe pattern becomes less clear midway). This is because, even within a wavelength range of 10 nm, the size of the formed GZP changes slightly with wavelength, and when these are added together, the high-frequency components in particular deteriorate. Therefore, as shown in Figure 8(c), the points in the image reconstructed from the hologram are larger than the points in the image in Figure 7(c), clearly indicating a degradation in resolution. Accordingly, to improve the resolution of the reconstructed image, it is conceivable to use optical elements with a large pupil diameter and wavelength filters with a narrow wavelength range. However, as the wavelength range of the wavelength filter narrows, the efficiency of light utilization decreases, requiring longer exposure times for hologram photography. This makes it particularly difficult to capture video, and therefore unsuitable for use as a video system.

[0010] This invention has been made in view of these circumstances, and aims to provide an incoherent digital holography imaging apparatus and imaging method that can suppress the degradation of the signal quality of high-frequency components of a hologram, even when the wavelength range of light incident on the image sensor is set to be wide, and can realize a high-quality reconstructed image. [Means for solving the problem]

[0011] The incoherent digital holography imaging apparatus of the present invention is A light splitting means for splitting an incoherent light wave from an object into two light waves consisting of a first split light and a second split light, A phase distribution imparting means that modulates the radius of curvature of at least one of the wavefronts of the first divided light and the second divided light to impart a relative spherical phase distribution to the wavefronts of the two divided light waves, A photosynthesis means for synthesizing two light waves consisting of the first divided light and the second divided light that have passed through the phase distribution imparting means, Interference fringe imaging means for imaging interference fringes formed by the interference of the first and second split light produced by this photosynthesis means, In the optical path between the photosynthesis means and the interference fringe imaging means, a focal length corresponding to the wavelength of the incident light is generated for each light point of the object, Regardless of the wavelength of the incident light, the size of the Gabor zone plate formed at each light point of the object is uniform. Set fixed Consists of wavelength-dispersive lenses An optical element is provided. 、 It is characterized by the following:

[0012] The aforementioned It is preferable that the glass material of the wavelength-dispersive lens is made of flint glass. Furthermore, it is preferable to include a calculation unit that performs calculations to form a reconstructed image of the entire object based on the obtained interference fringes.

[0013] Furthermore, it is preferable that the light splitting means and the photosynthesis means are configured by the same beam splitter. In this case, the phase distribution means preferably comprises a concave mirror that reflects either the first splitting light or the second splitting light to the beam splitter, and a plane mirror that reflects the other to the beam splitter.

[0014] Furthermore, the incoherent digital holography imaging method of the present invention is In an incoherent digital holography imaging method, incoherent light from an object is split into two beams of light, consisting of a first-segmented beam and a second-segmented beam; the radius of curvature of at least one of the wavefronts of the first-segmented beam and the second-segmented beam is modulated to impart a relative spherical phase distribution to the wavefronts of the two split beams of light; the two beams of light consisting of the first-segmented beam and the second-segmented beam are then combined, and the interference fringes formed by the interference of the combined first-segmented beam and the second-segmented beam are imaged, The aforementioned Combine two lights means of photosynthesis and The aforementioned interference fringes are imaged.By using an optical element consisting of a wavelength dispersion lens provided in the optical path between the interference fringe imaging means, for each light point of the object, a focal length of a length corresponding to the wavelength of the incident light is generated for the incident light, so that the size of the Gabor zone plate formed for each light point of the object is adjusted to be uniform regardless of the wavelength of the incident light.

Effect of the Invention

[0015] In the incoherent digital holographic imaging apparatus and imaging method of the present invention, a predetermined optical element is inserted into the optical path of the object light reaching the imaging element, and the focal length of a length corresponding to the wavelength of the incident light is adjusted so that the size of the Gabor zone plate formed for each light point of the object is uniform. If the size of the Gabor zone plate formed for each light point of the object is the same, it is possible to suppress the deterioration of the high-frequency components even when the Gabor zone plates for each light point on the image sensor are added together. Thereby, even if the wavelength width of the light used is expanded more than before, the deterioration of the signal quality of the hologram can be suppressed, and an image with improved resolution can be reconstructed.

Brief Description of the Drawings

[0016] [Figure 1] It is a schematic diagram for explaining the configuration of the optical system of the incoherent digital holographic imaging apparatus according to the present embodiment. [Figure 2] It shows the overall hologram image (a) of the light point, a partially enlarged view (b) of this (a), and the image (c) of the light point reconstructed from the hologram when the wavelength width of the light of the optical system of the incoherent digital holographic imaging apparatus according to the present embodiment is 10 nm. [Figure 3] It is a schematic diagram showing the configuration and specific parameters of the optical system when a wavelength dispersion lens is used as the wavelength dispersion optical element in the incoherent digital holographic imaging apparatus shown in FIG. 1. [Figure 4]It is a graph showing the relationship between the output light wavelength and the focal length, which forms a GZP with a high correlation value with the GZP formed by a light spot having an output light wavelength of 535.0 nm and a focal length of 80.0 mm. [Figure 5] It is a graph showing the wavelength characteristics of the refractive index of SF-15 (flint glass). [Figure 6] It shows the optical system of an incoherent digital holographic imaging device according to the prior art. [Figure 7] It shows the overall hologram image (a) of a light spot when the wavelength width of the light of the optical system of an incoherent digital holographic imaging device according to the prior art is infinitesimal, a partially enlarged view (b) of this (a), and the image (c) of the light spot reconstructed from the hologram. [Figure 8] It shows the overall hologram image (a) of a light spot when the wavelength width of the light of the optical system of an incoherent digital holographic imaging device according to the prior art is 10 nm, a partially enlarged view (b) of this (a), and the image (c) of the light spot reconstructed from the hologram.

Embodiments for Carrying Out the Invention

[0017] Hereinafter, an incoherent digital holographic imaging device and an imaging method according to an embodiment of the present invention will be described while referring to the drawings.

[0018] FIG. 1 shows the principle of an incoherent digital holographic imaging device 50 according to an embodiment of the present invention. In other words, the incoherent digital holography imaging device 50 shown in Figure 1 passes spatially incoherent light waves propagating from object 1 through lens 2, making them parallel light with respect to the light from the focal position of lens 2, and then splitting them by beam splitter 3 to obtain first and second split beams. The first split beam is made into a spherical wave by concave mirror 4, reflected in a way that slightly converges it, and returns to beam splitter 3, while the second split beam is specularly reflected by plane mirror 5 and returns to beam splitter 3. The converged light from the first split beam from concave mirror 4 that has passed through the light splitting surface 3a and the parallel light from the second split beam from plane mirror 5 that has been reflected by the light splitting surface 3a travel in the same direction, and these two split beams superimpose with a slight difference in optical path length to form interference fringes (holograms) on the imaging surface of image sensor 6.

[0019] Furthermore, to obtain amplitude phase information of the hologram, a phase shift method is used, for example. In the phase shift method, the position of the plane mirror 5 is moved by a piezo actuator 7 installed on the plane mirror 5 in predetermined intervals in the optical axis direction. For example, in the most common 4-step phase shift method, a hologram with a different phase pattern is acquired by the image sensor 6 each time the position is moved by 1 / 8 of the wavelength (moved so that the optical path length of the light passing through the plane mirror changes by 1 / 4 of the wavelength).

[0020] Through this operation, holograms with four different phase patterns are acquired, and the amplitude phase information of the holograms is calculated from these interference fringes using the usual phase-shift digital holography technique. Furthermore, by performing backpropagation calculations based on this amplitude phase information of the holograms, a reconstructed image of the object at any depth position can be obtained. This calculation for acquiring the reconstructed image is performed in the object image reconstruction means 11. The reconstructed image of object 1 output from the object image reconstruction means 11 is displayed on the monitor 12.

[0021] Incidentally, between the beam splitter 3 and the image sensor 6, in addition to the wavelength filter 9, a wavelength-dispersive optical element 10 is also placed. The wavelength filter 9 is configured to limit the wavelength width to, for example, about 10 nm in order to interfere the two split beams of incoherent light. On the other hand, the wavelength-dispersive optical element 10 is set to produce a focal length corresponding to the wavelength of the incident light, so that the size of the Gabor zone plate is uniform regardless of the wavelength of the light incident on the optical element 10.

[0022] Here, the term "zone plate" in Gabor zone plates refers to an annular band in which white (transparent) and black (blocking) areas are arranged alternately. In the case of Fresnel zone plates, the change in the intensity of the black and white areas of this annular band is binary (rectangular), whereas in the case of Gabor zone plates, it changes sinusoidally. In an incoherent hologram, an object can be considered as a collection of light points, and a Gabor zone plate corresponding to each light point is formed on the image sensor 6.

[0023] A key feature of the incoherent digital holography imaging apparatus according to this embodiment is that, unlike conventional incoherent digital holography imaging apparatuses, a wavelength-dispersive optical element 10 is provided in the optical path. In other words, in order to effectively interfere with incoherent light, it is common to narrow the wavelength range of the light (for example, to less than 10 nm), but doing so reduces the amount of light obtained by the image sensor 6 and significantly lowers the signal-to-noise ratio.

[0024] Therefore, in this embodiment, the image sensor 6 is configured to capture a good hologram when the wavelength range of light is widened (for example, 10 nm or more). In other words, to avoid degrading the signal quality of the high-frequency components of the hologram, the size of the GZP formed by the light points of each wavelength on the image sensor 6 should be matched. Matching the size of the GZP greatly improves the signal quality of the high-frequency components of the hologram, and at the same time, the signal quality of the low-frequency components is also improved.

[0025] Figure 3 shows the specific optical system of the incoherent digital holography imaging apparatus according to this embodiment. In this embodiment, the wavelength-dispersive optical element 10 in Figure 1 is a wavelength-dispersive lens 110, and specific numerical values ​​are provided for the distance between each optical component, the range of wavelengths of light from the object 101, and the focal lengths of the concave mirror 104 and the wavelength-dispersive lens 110. However, since the other optical components have the same configuration and function as the optical components of the optical system shown in Figure 1, they are represented by the same reference numerals as those given to the optical components in Figure 1 plus 100, and detailed explanations of them are omitted.

[0026] This wavelength-dispersive lens 110 is considered a sub-concept of the wavelength-dispersive optical element 10 shown in Figure 1, and therefore performs the basic functions of this wavelength-dispersive lens 110, having the function of changing the focal length according to the wavelength. For example, it has a wavelength-dispersive function that changes the focal length according to the wavelength of light so that the size of the GZP formed by a light point with a wavelength of 535.0 nm matches the size of the GZP formed by a light point with a wavelength of 530.0 nm. Furthermore, by employing a wavelength-dispersive lens 110 with lens functionality, it also has the advantage of being able to adjust the magnification of the object being photographed.

[0027] Figure 2 shows the overall hologram of a light point with a wavelength width of 10 nm (a), a magnified view of a part of (a) (b), and an image reconstructed from that hologram (c). In other words, when the wavelength range is from 530.0 nm to 540.0 nm and the wavelength width is 10 nm, the size of each GZP formed on the image sensor 6 by the light spot of each wavelength is matched to the size of the GZP formed by the light spot of wavelength 535.0 nm, and the resulting hologram and reconstructed image are obtained by adding them together.

[0028] Comparing the image in Figure 2(b) of this embodiment with the image in Figure 8(b) of the prior art, it is clear that the signal quality of the high-frequency components of the hologram is particularly improved in the image in Figure 2(b) of this embodiment. Furthermore, in the reconstructed image in Figure 2(c) of this embodiment, it is clear that the size of the points in the reconstructed image is smaller compared to the reconstructed image in Figure 8(c) of the prior art, indicating an improvement in resolution.

[0029] Figure 4 shows the GZP formed by a light point emitting light at a wavelength of 535.0 nm and a focal length of 80.0 mm, after having previously determined the GZP by changing the focal length by 0.1 mm increments for each wavelength. For each wavelength, the focal length with the highest correlation value was selected and represented as a point.

[0030] Here, the reciprocal of SAD (Sum of Absolute Difference), expressed in equation (1) below, is used as the correlation value.

number

[0031] The points in Figure 4 show stepped values ​​because the focal length was determined in 0.1 mm increments. By forming a wavelength-dispersive lens 110 having the wavelength characteristics of the focal lengths shown at each of these points, it is possible to suppress the degradation of the hologram signal quality even when the wavelength width is 10 nm.

[0032] The specific method for forming this lens will be explained below. Figure 5 shows the wavelength characteristics of the refractive index of flint glass (SF-15) (Optical Data - Glass Materials, Sigma Koki Co., Ltd., Internet).<URL:https: / / www.global-optosigma.com / jp / category / opt_d / opt_d01.html> (See reference).

[0033] The wavelength characteristics of the focal length calculated using the lens formula (2) below, based on this refractive index distribution, are shown as a straight line in Figure 4.

number

[0034] While there are multiple options for glass materials and radii of curvature, it is preferable to select options that minimize monochromatic aberration (aberrations other than chromatic aberration, such as distortion and field curvature) in the optical system. The straight line shown in Figure 4 is set to pass through the distribution region of each point. Therefore, by fabricating a lens with high refractive index, high dispersion flint glass (SF-15), the high-frequency components of the hologram can be improved, and the resolution of the reconstructed image can be improved, even when the wavelength width is set to 10 nm.

[0035] The incoherent digital holography imaging apparatus and manufacturing method of the present invention are not limited to those of the above embodiments, and can be modified to various other forms. For example, in the optical system shown in Figure 3, the wavelength-dispersive lens 110 is inserted between the beam splitter 103 and the image sensor 106, but it may also be placed between object 101 and the beam splitter 103, or it may be placed in place of lens 102, for example. It may also be placed on an optical path through which only one of the split beams passes, such as between the concave mirror 104 and the beam splitter 103, or between the plane mirror 105 and the beam splitter 103. Furthermore, the incoherent digital holography imaging apparatus of the above embodiment includes an object reconstruction means 111 and a hologram reproduction unit such as a monitor 112, but this hologram reproduction unit may be configured as a separate device.

[0036] Alternatively, instead of the concave mirror 104 or the flat mirror 105, a reflective spatial light modulation element or the like may be used to provide the function of a wavelength-dispersive lens. Furthermore, in Figure 3, the light splitting means and the photosynthesis means are configured by a single beam splitter, but of course, the light splitting means and the photosynthesis means may be configured by other optical components, such as a combination of a polarizer and a spatial light modulation element (liquid crystal panel or liquid crystal lens). Alternatively, instead of wavelength-dispersive lenses, diffractive lenses or metasurfaces may be used. Furthermore, in the embodiments described above, flint glass is used as the glass material for the wavelength-dispersive lens, but of course, other glass materials may be used instead. However, it is preferable to use a glass material with a high refractive index and high dispersion as much as possible.

[0037] Furthermore, although a Michelson interferometer type optical system is used in this embodiment, other equi-path-length optical systems such as a Mach-Zehnder interferometer type or a bypass-circuit Fizeau interferometer type may also be used. [Explanation of Symbols]

[0038] 1, 101, 201 object 2, 102, 202 lenses 3, 103, 203 Beam Splitter 3a, 103a, 203a Light splitting plane 4, 104, 204 Concave mirrors 5, 105, 205 Flat Mirror 6, 106, 206 Image Sensors 7, 107, 207 Piezo actuators 9, 109, 209 wavelength filters 10 Wavelength Dispersive Optical Elements 110 wavelength dispersion lens 11, 111, 211 Object image reconstruction means 12, 112, 212 monitors 50, 150, 250 Incoherent Digital Holography Imaging Systems

Claims

1. A light splitting means for splitting an incoherent light wave from an object into two light waves consisting of a first split light and a second split light, A phase distribution imparting means that modulates the radius of curvature of at least one of the wavefronts of the first divided light and the second divided light to impart a relative spherical phase distribution to the wavefronts of the two divided light waves, A photosynthesis means for synthesizing two light waves consisting of the first divided light and the second divided light that have passed through the phase distribution imparting means, Interference fringe imaging means for imaging interference fringes formed by the interference of the first and second split light produced by this photosynthesis means, An incoherent digital holography imaging apparatus is characterized by having an optical element, which consists of a wavelength-dispersive lens, in the optical path between the photosynthesis means and the interference fringe imaging means, provided such that the size of the Gabor zone plate formed for each light point of the object is uniform, regardless of the wavelength of the incident light, by generating a focal length corresponding to the wavelength of the incident light for each light point of the object.

2. The incoherent digital holography imaging apparatus according to claim 1, characterized in that the glass material of the wavelength-dispersive lens is made of flint glass.

3. The incoherent digital holography imaging apparatus according to claim 1 or 2, further comprising a calculation unit that performs calculations to form a reconstructed image of the entire object based on the obtained interference fringes.

4. The incoherent digital holography imaging apparatus according to any one of claims 1 to 3, characterized in that the light splitting means and the photosynthesis means are configured by the same beam splitter.

5. The incoherent digital holography imaging apparatus according to claim 4, characterized in that the phase distribution imparting means comprises a concave mirror that reflects either the first divided light or the second divided light to the beam splitter, and a plane mirror that reflects the other to the beam splitter.

6. In an incoherent digital holography imaging method, incoherent light from an object is split into two beams consisting of a first-segmented beam and a second-segmented beam; the radius of curvature of at least one of the wavefronts of the first-segmented beam and the second-segmented beam is modulated to impart a relative spherical phase distribution to the wavefronts of the two split beams; the two beams consisting of the first-segmented beam and the second-segmented beam are then combined, and the interference fringes formed by the interference of the combined first-segmented beam and the second-segmented beam are imaged, An incoherent digital holography imaging method characterized by using an optical element consisting of a wavelength-dispersive lens provided in the optical path between a photosynthesis means for combining the two aforementioned lights and an interference fringe imaging means for imaging the interference fringes, thereby generating a focal length corresponding to the wavelength of the incident light for each light point of the object, and adjusting so that the size of the Gabor zone plate formed for each light point of the object is uniform regardless of the wavelength of the incident light.

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