JP7540915B2 - Imaging device - Google Patents

Description

The present invention relates to an imaging device using a Fresnel zone aperture.

Research is being conducted on imaging devices that use Fresnel Zone Apertures (FZAs). These imaging devices can capture images of objects without using an imaging optical system, and can therefore be made thin. In these imaging devices, the FZA is placed in front of the imaging surface of the image sensor. The FZA is concentric, has a pitch that is inversely proportional to the distance from the center, and has a periodic grayscale pattern that corresponds to the distance from the center. The light from the object is imaged by the image sensor as an image of a grayscale pattern whose intensity is modulated by the FZA. The grayscale pattern obtained by the image sensor is then superimposed on the FZA pattern to generate moiré fringes, and a reconstructed image of the object is obtained by Fourier transforming the moiré fringes.

In such imaging devices, a technology known as the space division method has been proposed in which multiple FZAs with different phases of the shading patterns are arranged on a plane perpendicular to the optical axis. In this space division method, the moiré fringes obtained for each FZA are added and Fourier transformed to cancel the moiré fringe noise (see, for example, Patent Documents 1 and 2).

JP 2018-61109 A International Publication No. 2018/055831

In the above technology, multiple FZAs are arranged in a plane perpendicular to the optical axis, so as the object approaches the image sensor, the difference in direction from the object to each FZA becomes greater. Therefore, if the distance between the object and the image sensor becomes too close, an imaging device using the above technology will not be able to obtain a reconstructed image of the object.

Therefore, the present invention aims to provide an imaging device that can take close-up images of an object without using an imaging optical system.

According to one aspect of the present invention, an imaging device is provided. The imaging device includes a polarizer having a transmission axis along a predetermined direction and converting light from an object into linearly polarized light along the predetermined direction when the light passes through the polarizer; a birefringent element having a slow axis at 45° to the predetermined direction and concentrically arranged multiple rings centered on the optical axis, in which the retardation imparted to the linearly polarized light output from the polarizer varies by a predetermined amount between two adjacent rings in the radial direction from the center of the multiple rings; a quarter-wave plate having a slow axis parallel or perpendicular to the predetermined direction and converting the light output from the birefringent element into linearly polarized light in a direction corresponding to the retardation of the ring of the multiple rings of the Fresnel zone opening through which the light passed; and a pixel group for each pixel group including a predetermined number of adjacent pixels. The polarized image sensor is provided with pixel polarizers arranged so that the directions of the transmission axes of the pixels are equiangularly spaced, receives light emitted from the quarter-wave plate, and generates an image in which the value of each pixel corresponds to the relationship between the direction of the transmission axis of the pixel polarizer and the retardation of the ring of the Fresnel zone opening through which the light incident on that pixel is transmitted, and a calculation unit extracts the values of the individual pixels included in each group of pixels having pixel polarizers with the same transmission axis direction from the generated image to generate a plurality of pattern images having a shading pattern according to the shape of the Fresnel zone opening due to light from the object and in which the shading pattern phases are different from each other, and generates a reproduced image of the object based on the generated plurality of pattern images.

In this imaging device, the birefringent element preferably has a liquid crystal layer containing liquid crystal molecules oriented along the slow axis, and a first transparent electrode and a second transparent electrode facing each other along the optical axis with the liquid crystal layer in between, and the first transparent electrode has a plurality of ring-shaped partial electrodes arranged concentrically corresponding to each of the plurality of ring zones, and the voltage applied between the plurality of partial electrodes and the second transparent electrode is adjusted to impart to the linearly polarized light a retardation that varies by a predetermined amount between two adjacent ring zones along the radial direction from the center of the plurality of ring zones.

The imaging device according to the present invention has the advantage of being able to take close-up images of an object without using an imaging optical system.

1 is a schematic diagram illustrating the configuration of an imaging device according to an embodiment of the present invention. 1 is a diagram showing the relationship between the transmission axis of a polarizer, the slow axis of a birefringent element, and the slow axis of a quarter-wave plate. This figure shows the relationship between the retardation of each ring zone of a birefringent element and the polarization state of each ring zone transmitted through the birefringent element before and after the light emitted from the birefringent element enters the quarter-wave plate. 1 is a diagram showing an example of the intensity of light from an object that has been transmitted through an annular zone of the FZA of a birefringent element and a pixel polarizer provided in each pixel of a polarization image sensor. FIG. 1 is a diagram showing an example of the relationship between an FZA annular zone through which light from an object has passed and values obtained for each pixel in a pixel group of a polarization image sensor. FIG. FIG. 11 is a schematic side cross-sectional view of a birefringent element according to a modified example.

Below, an imaging device according to one embodiment will be described with reference to the drawings. This imaging device obtains a reconstructed image of an object to be imaged without using an imaging optical system by arranging a birefringent element constituting the FZA in front of the imaging surface of the image sensor. In addition, this imaging device uses a polarized image sensor as the image sensor. In the polarized image sensor, a polarizer is provided for each pixel in front of the imaging element that generates a signal for that pixel. Then, for each pixel group including a predetermined number of adjacent pixels, the polarizers are arranged so that the directions of the transmission axes of the polarizers of the individual pixels included in that pixel group are equiangularly spaced from each other. In addition, in the birefringent element constituting the FZA, retardation changes by a predetermined amount between two adjacent ring zones along the radial direction from the center of the FZA. Furthermore, a polarizer is arranged closer to the object side than the birefringent element, and a quarter-wave plate is arranged between the birefringent element and the polarized image sensor. Then, for each set of pixels in which the transmission axis of the polarization image sensor is provided in the same direction, the values of the individual pixels included in that set are extracted to generate a plurality of pattern images having a grayscale pattern corresponding to the shape of the FZA due to the light from the object, and in which the phases of the grayscale patterns are different from each other. Therefore, this imaging device can reproduce an image of the object in which the noise of the moiré fringes is canceled based on the moiré fringes between the grayscale patterns shown in each of the plurality of pattern images and a reference pattern corresponding to the shape of the FZA. In addition, since this imaging device can generate a plurality of pattern images in which the phases of the grayscale patterns are different from each other using one FZA, it can take close-up images of the object.

Figure 1 is a schematic diagram of an imaging device according to one embodiment of the present invention. As shown in Figure 1, the imaging device 1 has, for example, a polarizer 11, a birefringent element 12, a quarter-wave plate 13, and a polarization image sensor 14 along the optical axis OA, in that order from the object 10 side. The polarizer 11, the birefringent element 12, and the quarter-wave plate 13 may be arranged in close contact with each other, or may be arranged at a predetermined interval from each other. The quarter-wave plate 13 and the polarization image sensor 14 are also arranged at a predetermined interval. Furthermore, the imaging device 1 has a calculation device 15 for reconstructing an image of the object 10 based on the image obtained by the polarization image sensor 14.

The polarizer 11 has a transmission axis along a predetermined direction, and transmits only the polarized light component having a polarization plane along the direction of the transmission axis out of the light emitted from the object 10 or the light from a light source (not shown) reflected or scattered by the object 10 (hereinafter simply referred to as light emitted from the object 10). In other words, the polarizer 11 converts the light emitted from the object 10 and transmitted through the polarizer 11 into linearly polarized light having a polarization plane along the direction of the transmission axis. The linearly polarized light emitted from the polarizer 11 is then incident on the birefringent element 12.

The birefringent element 12 constitutes the FZA. The birefringent element 12 is configured by providing a birefringent film having a retardation and shape corresponding to each zone of the FZA on a substrate that transmits light from the object 10, such as glass or a light-transmitting resin.

In this embodiment, the birefringent element 12 has a slow axis inclined at 45° with respect to the transmission axis of the polarizer 11 in a plane perpendicular to the optical axis OA. The birefringent element 12 is arranged so that the optical axis OA passes through the center of the FZA. The birefringent element 12 is configured so that the retardation given to the transmitted light, that is, the phase difference of the extraordinary ray with respect to the ordinary ray transmitted through the birefringent element 12, changes by a predetermined amount between two adjacent rings in the radial direction from the center of the FZA, and the pitch of each ring in the radial direction from the center of the FZA is inversely proportional to the distance from the center of the FZA. The predetermined amount of retardation that changes between the rings can be, for example, π/2, but is not limited to this. When the predetermined amount of retardation that changes between the rings is π/2, the retardation of each of the four consecutive rings along the radial direction is 0, π/2, π, and 3π/2. Therefore, the linearly polarized light from polarizer 11 has a different polarization state depending on which of the four consecutive ring zones along the radial direction is transmitted.

The light that passes through the birefringent element 12 and has a different polarization state for each ring zone enters the quarter-wave plate 13.

The quarter-wave plate 13 has a slow axis in the same direction as the transmission axis of the polarizer 11, and converts the light from the birefringent element 12 into linearly polarized light in a direction that corresponds to the retardation of the transmitted ring zone.

Figure 2 shows the relationship between the transmission axis of the polarizer 11, the slow axis of the birefringent element 12, and the slow axis of the quarter-wave plate 13. Figure 3 shows the relationship between the retardation of each annular zone of the birefringent element 12 and the polarization state of each annular zone of the birefringent element 12 before and after the light emitted from the birefringent element 12 enters the quarter-wave plate 13.

2, the polarizer 11 and the quarter-wave plate 13 are arranged so that the transmission axis 11a of the polarizer 11 and the slow axis 13a of the quarter-wave plate 13 are parallel to each other. In contrast, the slow axis 12a of the birefringent element 12 is tilted 45° clockwise when viewed from the object 10 side with respect to the transmission axis 11a of the polarizer 11 and the slow axis 13a of the quarter-wave plate 13.

As shown in table 300 in FIG. 3, the retardations of the four ring zones 1 to 4 in the FZA formed by the birefringent element 12 are 0, π/2, π, and 3π/2, arranged in order from the center along the radial direction. In this case, the polarization state of the light transmitted through ring zone 1 has a retardation of 0, so regardless of whether it is transmitted through the quarter-wave plate 13 or not, the light does not change from the polarization state before entering the birefringent element 12 and becomes linearly polarized light along the transmission axis 11a of the polarizer 11.

In contrast, the polarization state of the light that passes through the annular zone 2 is right-handed circularly polarized light because the retardation is π/2 and the slow axis 12a of the birefringent element 12 is tilted 45° rightward with respect to the transmission axis 11a of the polarizer 11. Furthermore, the polarization state of this light becomes linearly polarized light parallel to the slow axis 12a of the birefringent element 12 by passing through the quarter-wave plate 13.

In addition, the polarization state of the light that has passed through the annular zone 3 has a retardation of π and the slow axis 12a of the birefringent element 12 is inclined at 45° with respect to the transmission axis 11a of the polarizer 11, so that the light becomes linearly polarized, perpendicular to the direction of the polarization before entering the birefringent element 12, i.e., the direction of the transmission axis 11a of the polarizer 11. Therefore, the polarization state of this light does not change even when it passes through the quarter-wave plate 13.

The polarization state of the light that passes through the annular zone 4 is counterclockwise circularly polarized light because the retardation is 3π/2 and the slow axis 12a of the birefringent element 12 is tilted 45° clockwise with respect to the transmission axis 11a of the polarizer 11. Furthermore, the polarization state of this light becomes linearly polarized light perpendicular to the slow axis 12a of the birefringent element 12 by passing through the quarter-wave plate 13.

In this way, the light transmitted through the quarter-wave plate 13 becomes linearly polarized light in a direction that corresponds to the retardation of the ring zone that was transmitted through the birefringent element 12. The light transmitted through the quarter-wave plate 13 is then received by the polarization image sensor 14.

The polarization image sensor 14 receives light emitted from the object 10 and transmitted through the polarizer 11, the birefringent element 12, and the quarter-wave plate 13. The polarization image sensor 14 then generates an image in which the value of each pixel corresponds to the relationship between the direction of the transmission axis of the polarizer (hereinafter, referred to as the pixel polarizer to distinguish it from the polarizer 11) provided in that pixel, and the retardation of the ring of the FZA through which the light incident on that pixel has passed.

As described above, in the polarization image sensor 14, for each pixel group including a predetermined number of adjacent pixels, the pixel polarizers provided on the front surface of the image sensor that generates the signals of the individual pixels included in that pixel group are arranged so that the directions of the transmission axes of the pixel polarizers are equiangularly spaced from one another. Therefore, even for light that has passed through the same ring zone of the FZA, the amount of light that transmits through the pixel polarizer for each pixel will differ depending on the direction of the transmission axis of the pixel polarizer. As a result, in the image generated by the polarization image sensor 14, the value of each pixel will be a value that depends on the relationship between the direction of the transmission axis of the pixel polarizer and the retardation of the ring zone of the FZA through which the light incident on that pixel has passed.

Figure 4 is a diagram showing an example of the intensity of light from the object 10 that has passed through the FZA annular zone of the birefringent element 12 and the pixel polarizer provided in each pixel of the polarization image sensor 14. Figure 5 is a diagram showing an example of the relationship between the FZA annular zone through which light from the object 10 has passed and the pixel values obtained for each pixel in each pixel group of the polarization image sensor 14. In Figures 4 and 5, the annular zones 1 to 4 of the FZA (hereinafter sometimes simply referred to as FZA) of the birefringent element 12 are the same as the annular zones 1 to 4 shown in Figure 3. That is, the retardations of the annular zones 1 to 4 are 0, π/2, π, and 3π/2, respectively. In this embodiment, each pixel group 400 in the polarization image sensor 14 has four pixels 401 to 404 arranged in two pixels each in the vertical and horizontal directions. The upper left pixel 401 is provided with a pixel polarizer having a transmission axis oriented in a direction perpendicular to the transmission axis of the polarizer 11 in a plane perpendicular to the optical axis OA, as shown by an arrow 401a. The upper right pixel 402 is provided with a pixel polarizer having a transmission axis oriented in a direction rotated 45° counterclockwise with respect to the transmission axis of the polarizer 11 in a plane perpendicular to the optical axis OA, as shown by an arrow 402a. The lower right pixel 403 is provided with a pixel polarizer having a transmission axis oriented in a direction perpendicular to the transmission axis of the polarizer 11 in a plane perpendicular to the optical axis OA, as shown by an arrow 403a. The lower left pixel 404 is provided with a pixel polarizer having a transmission axis oriented in a direction rotated 45° clockwise with respect to the transmission axis of the polarizer 11 in a plane perpendicular to the optical axis OA, as shown by an arrow 404a. That is, in this example, the pixel polarizers provided on the front surface of the image sensor that generates signals for each pixel included in the pixel group are arranged so that the directions of the transmission axes of the pixel polarizers are spaced at 45° intervals. The arrangement of the pixels included in each pixel group of the polarization image sensor 14 is not limited to the above example. For example, the pixels included in each pixel group may be arranged in a line along a predetermined direction. The number of pixels included in each pixel group is also not limited to four, and may be, for example, six or eight. In this case, the pixel polarizers provided in each pixel may be arranged so that the directions of the transmission axes of the pixel polarizers provided in each pixel are spaced at 30° intervals or 22.5° intervals.

As shown in table 500 in FIG. 5, the polarization direction of the light transmitted through zone 1 of FZA is parallel to the transmission axis of polarizer 11, so the polarization direction of the light transmitted through zone 1 is perpendicular to the transmission axis of the pixel polarizer provided in pixel 401. Therefore, the intensity of the light transmitted through zone 1 and incident on pixel 401 is lower than the intensity of the light transmitted through zone 1 and incident on other pixels in pixel group 400. In contrast, the polarization direction of the light transmitted through zone 1 is parallel to the transmission axis of the pixel polarizer provided in pixel 403. Therefore, the intensity of the light transmitted through zone 1 and incident on pixel 403 is higher than the intensity of the light transmitted through zone 1 and incident on other pixels in pixel group 400. In addition, the polarization direction of the light transmitted through zone 1 is at 45° with the transmission axis of the pixel polarizer provided in pixel 402 and the transmission axis of the pixel polarizer provided in pixel 404. Therefore, the intensity of the light that passes through zone 1 and enters pixel 402 or pixel 404 is higher than the intensity of the light that passes through zone 1 and enters pixel 401, and lower than the intensity of the light that enters pixel 403.

Similarly, for light that has passed through other rings of the FZA, the intensity of the light differs for each pixel in the pixel group 400 depending on the relationship between the transmission axis of the pixel polarizer provided in that pixel and the polarization direction of the light after passing through the quarter-wave plate 13 due to the retardation of that ring of the FZA. Furthermore, for each of rings 1 to 4 of the FZA, the polarization direction after passing through that ring and the quarter-wave plate 13 is rotated by 45°. Therefore, even if pixels are provided with pixel polarizers with the same transmission axis, the intensity of the light incident on that pixel will differ depending on the retardation of the ring through which the light has passed. The polarization image sensor 14 then generates an image for each pixel having a value that corresponds to the intensity of the light received.

The polarization image sensor 14 outputs the generated image to the computing device 15.

The calculation device 15 is an example of a calculation unit, and generates a reconstructed image of the object 10 based on the image received from the polarization image sensor 14. To this end, the calculation device 15 has a communication interface, a memory, and a processor.

The calculation device 15 is connected to the polarization image sensor 14 via a communication interface and receives images from the polarization image sensor 14. Furthermore, the calculation device 15 may be connected to another device (not shown) or a display device (not shown) via the communication interface. The calculation device 15 may then output the generated reconstructed image of the object 10 to another device via the communication interface, or display it on a display device.

The memory is, for example, a non-volatile or volatile semiconductor memory, and stores various data used to generate a reconstructed image of the object 10 from the image received from the polarization image sensor 14, and various data generated during the calculations to generate the reconstructed image of the object 10. For example, the memory stores a reference pattern of shading according to the shape of the FZA, which is used to generate moiré fringes between the shading pattern caused by the light from the object 10. The memory may also store the image received from the polarization image sensor 14, or the reconstructed image of the object 10.

The processor is, for example, a CPU and its peripheral circuits. The processor may also include a numerical calculation circuit. The processor then generates a reconstructed image of the object 10 from the image received from the polarization image sensor 14.

As explained above with respect to the polarization image sensor 14, in the image generated by the polarization image sensor 14, the value of each pixel corresponds to the relationship between the direction of the transmission axis of the pixel polarizer and the retardation of the FZA annular zone through which the light incident on that pixel is transmitted.

Therefore, the processor of the calculation device 15 generates a pattern image by extracting the values of each pixel included in each set of pixels in which a pixel polarizer having the same transmission axis is provided from the image received from the polarization image sensor 14. This results in multiple pattern images. As described above, when the directions of the transmission axes of the pixel polarizers of the individual pixels included in each pixel group differ from each other by 45°, four pattern images are obtained. Each pattern image has a shading pattern according to the shape of the FZA due to the light from the object 10, and the phases of the shading patterns along the radial direction from the center are different from each other. That is, multiple shading patterns similar to the shading patterns obtained by each of the multiple FZAs arranged on a plane perpendicular to the optical axis by the above-mentioned space division method are obtained. However, in this embodiment, the positions of the pixels corresponding to the light that originated from the object 10 and passed through the same position of the FZA are shifted by at most one pixel between each pattern image, so even if the distance between the object 10 and the imaging device 1 is close, the intensity of the light that originates from the object 10 and travels in a specific direction is represented by pixels at approximately the same position in each thinned image.

Therefore, the processor can generate a reconstructed image of the object 10 by performing a calculation on these multiple pattern images to obtain a reconstructed image of the object from multiple FZA shading patterns using a spatial division method described in JP 2018-61109 A or WO 2018/055831. That is, the processor can generate a reconstructed image of the object 10 in which the noise of the moiré fringes is canceled by adding and Fourier transforming the moiré fringes between each of the multiple FZA shading patterns caused by light from the object 10, which are out of phase with each other and are represented in each pattern image, and the FZA reference pattern.

The computing device 15 outputs the generated reconstructed image of the object 10 to another device via a communication interface, or displays it on a display device.

As described above, this imaging device uses a polarized image sensor as an image sensor. Therefore, for one FZA in which retardation changes by a predetermined amount between two adjacent rings along the radial direction from the center, this imaging device extracts the values of each pixel included in each set of pixels in which a pixel polarizer having the same transmission axis of the polarized image sensor is provided, and generates multiple pattern images having a shading pattern according to the shape of the FZA due to light from the object and in which the phases of the shading patterns are different from each other. Then, this imaging device generates a reproduced image of the object based on the multiple pattern images. In this imaging device, even if the distance between the object and the imaging device is close, the light emitted from the object and traveling in a predetermined direction is represented by pixels at approximately the same position between each pattern image, so that even if the distance between the object and the imaging device is close, a reproduced image of the object can be generated. Therefore, this imaging device can take a close-up image of the object. In addition, this imaging device can obtain a reproduced image of the object from an image obtained by one imaging. Therefore, unlike the so-called time-division method, which requires multiple shots to obtain multiple shading patterns with different phases, this imaging device can generate a reconstructed image of an object even if the object changes over time.

According to a modified example, the quarter-wave plate 13 may be arranged so that the slow axis of the quarter-wave plate 13 is perpendicular to the transmission axis of the polarizer 11 in a plane perpendicular to the optical axis OA. In this case, as in the above embodiment, the polarization direction of the light emitted from the quarter-wave plate 13 corresponds to the retardation of the FZA annular zone through which the light has passed, so that the value of each pixel of the image generated by the polarization image sensor 14 corresponds to the direction of the transmission axis of the pixel polarizer of that pixel and the retardation of the FZA annular zone through which the light incident on that pixel has passed. Therefore, as in the above embodiment, multiple pattern images are obtained from the image generated by the polarization image sensor 14, so that the imaging device can generate a reproduced image of the object 10.

According to another variant, the birefringent element 12 may be configured as a liquid crystal optical element.

Figure 6 is a schematic side cross-sectional view of the birefringent element 12 according to this modified example. The birefringent element 12 has a liquid crystal layer 20 and transparent substrates 21 and 22 that face each other along the optical axis OA and are arranged approximately parallel to each other with the liquid crystal layer 20 in between. The birefringent element 12 also has a transparent electrode 23 arranged between the transparent substrate 21 and the liquid crystal layer 20, and a transparent electrode 24 arranged between the liquid crystal layer 20 and the transparent substrate 22. The liquid crystal molecules 25 of the liquid crystal layer 20 are sandwiched between the transparent substrates 21 and 22, sealed in the area surrounded by the seal member 26, and homogeneously oriented so that the long axis direction is inclined at 45° with respect to the transmission axis of the polarizer 11.

The transparent substrates 21 and 22 are made of a material that is transparent to the light from the object 10, such as glass or resin. The transparent electrodes 23 and 24 are made of a material called ITO, which is made by adding tin oxide to indium oxide.

One of the transparent electrodes 23 and 24 (second transparent electrode) is provided so as to cover the entire area through which linearly polarized light emanating from the object 10, passing through the polarizer 11, and entering the birefringent element 12 passes. The other of the transparent electrodes 23 and 24 (first transparent electrode) is arranged concentrically around the optical axis OA and has a plurality of annular partial electrodes. Each of the plurality of annular partial electrodes corresponds to an individual annular zone of the FZA constituted by the birefringent element 12. That is, each partial electrode is formed so that the width of each partial electrode along the radial direction from the optical axis OA is equal to the width of the corresponding annular zone of the FZA. In addition, each partial electrode is insulated from each other. Then, the calculation device 15 applies a voltage between the partial electrode and the transparent electrode facing it for each partial electrode such that the retardation by the liquid crystal layer 20 becomes the retardation of the corresponding annular zone of the FZA. As a result, the birefringent element 12 can constitute an FZA similar to that of the above embodiment. In addition, a driving device (not shown) that applies a voltage to the liquid crystal layer 20 of the birefringent element 12 may be provided separately from the computing device 15.

According to this modification, the calculation device 15 or the drive device can appropriately set the retardation of each ring of the FZA by adjusting the voltage applied to the liquid crystal layer 20 according to the wavelength of the object 10 itself or the light source illuminating the object 10. As a result, the imaging device 1 can generate a reproduced image in which the feature of interest of the object 10 can be easily identified according to the wavelength of the object 10 itself or the light source illuminating the object 10. For example, the imaging device 1 may be used to capture biometric information in a biometric authentication device based on biometric information such as the vein pattern of a finger, hand or wrist, or a fingerprint or palm print. In this case, when the fingerprint or palm print of a finger, hand or wrist is captured as the object 10, the object 10 is illuminated with white light. Therefore, the calculation device 15 or the drive device may adjust the voltage applied to the liquid crystal layer 20 so that the retardation between two adjacent rings of the FZA is shifted by a predetermined amount for any wavelength contained in the white light. On the other hand, when the vein pattern of a finger, hand, or wrist is photographed as the object 10, the object 10 is illuminated with near-infrared light. Therefore, the computing device 15 or the driving device adjusts the voltage applied to the liquid crystal layer 20 so that the retardation between two adjacent zones of the FZA shifts by a predetermined amount for any wavelength contained in the near-infrared light. In this way, by adjusting the voltage applied to the liquid crystal layer 20, the imaging device 1 can generate reconstructed images of a fingerprint or palm print and a vein pattern for the same part of a single user, and the biometric authentication device can perform biometric authentication using both the fingerprint or palm print and the vein pattern.

The imaging device according to the above embodiment or modified example is not limited to application to biometric authentication devices, but is also suitable for use in various devices that require a thin imaging device, such as smartphones or sensor devices for IoT. In particular, this imaging device is suitable for use in devices that require close-up photography of an object.

As described above, those skilled in the art can make various modifications to suit the implementation form within the scope of the present invention.

REFERENCE SIGNS LIST 1 imaging device 10 object 11 polarizer 12 birefringent element 13 quarter-wave plate 14 polarization image sensor 15 arithmetic unit 20 liquid crystal layer 21, 22 transparent substrate 23, 24 transparent electrode 25 liquid crystal molecule 26 sealing member

Claims (2)

A polarizer having a transmission axis along a predetermined direction, which converts light from an object into linearly polarized light along the predetermined direction when the light passes through the polarizer;
a birefringent element that has a slow axis that is at an angle of 45° to the predetermined direction, and that constitutes a Fresnel zone aperture having a plurality of concentric rings centered on an optical axis, and in which retardation imparted to the linearly polarized light output from the polarizer varies by a predetermined amount between two adjacent rings in a radial direction from the center of the plurality of rings;
a quarter-wave plate having a slow axis parallel or perpendicular to the predetermined direction, and converting the light emitted from the birefringent element into linearly polarized light in a direction corresponding to the retardation of a zone through which the light passes among the plurality of zones of the Fresnel zone aperture;
a polarization image sensor that receives light that has exited the quarter-wave plate, and generates an image in which the value of each pixel corresponds to the relationship between the direction of the transmission axis of the pixel polarizer and the retardation of a ring of the Fresnel zone aperture through which light incident on the pixel has passed, the ring of the Fresnel zone aperture being one of the plurality of rings; and
a calculation unit that extracts values of individual pixels included in each set of pixels, each set having the pixel polarizer having the same transmission axis direction, from the image to generate a plurality of pattern images having a shading pattern corresponding to the shape of the Fresnel zone opening due to the light from the object and in which the shading patterns have mutually different phases, and generates a reproduced image of the object based on the generated plurality of pattern images;
An imaging device having the above configuration. The imaging device according to claim 1, wherein the birefringent element has a liquid crystal layer including liquid crystal molecules oriented along the slow axis direction, and a first transparent electrode and a second transparent electrode facing each other with the liquid crystal layer sandwiched between them along the optical axis, the first transparent electrode has a plurality of ring-shaped partial electrodes arranged concentrically corresponding to each of the plurality of ring zones, and the voltage applied between the plurality of partial electrodes and the second transparent electrode is adjusted to impart retardation to the linearly polarized light that varies by a predetermined amount between two adjacent ring zones in a radial direction from the center of the plurality of ring zones.

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