Optical apparatus and imaging apparatus

Abstract

To provide an optical device and an imaging apparatus capable of acquiring satisfactory polarization information over the entire photographing screen.SOLUTION: The image pickup apparatus includes, in order from an object side to an image side, a first phase difference plate, a second phase difference plate including a liquid crystal layer and capable of changing a phase difference to a plurality of phase differences, a polarization plate, an optical system having a maximum angle of view of 14 degrees or more, and an image pickup element. 0.4 ≤ Iang / I0 ≤ 1.2 is satisfied, where I0 and Iang are amounts of change in intensity of transmitted light on the image sensor when a polarizing direction of incident light on an optical axis of the optical system and at an angle of view of 14 degrees or more is changed in each of a plurality of states of retardation set by the second retardation plate.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to an optical device and an imaging device, and more particularly to an optical device capable of acquiring polarization information and an imaging device having the same. [Background technology]

[0002] It has been known that observing the polarization state of light from a subject can enhance and detect certain features of the subject. For example, by attaching a polarizing filter to the front of the lens of a single-lens reflex camera and photographing the subject by changing the direction of the transmitted polarization, it is possible to obtain effects such as highlighting the texture of the subject, such as color and contrast, and emphasizing or reducing the appearance of reflected light from the surface of water, etc.

[0003] One method for acquiring polarization information from a subject is to rotate a polarizing plate to acquire multiple images. Furthermore, Patent Document 1 discloses a method for acquiring images with different polarization states by using a polarizing element composed of a wavelength plate, a variable phase plate made of liquid crystal, and a polarizing plate, and changing the phase difference imparted by the variable phase plate without rotating the polarizing plate. It also describes an example in which polarization information is acquired from multiple images with different polarization states captured using the polarizing element, and a composite image different from the captured images is created based on the acquired polarization information.

[0004] Furthermore, Patent Document 2 describes an example in which, when an image is captured using a polarizing element and an imaging element, the amount of phase change imparted by a variable phase plate is reduced to reduce the influence of angular characteristics. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Patent No. 6482308 specification [Patent Document 2] Patent No. 6679366 specification Summary of the Invention [Problem to be solved by the invention]

[0006] However, in Patent Documents 1 and 2, when a phase difference is applied, there exists a phase difference state in which polarization information of a subject at oblique incidence cannot be correctly acquired due to the angular characteristics of the liquid crystal. Therefore, if the polarization state is acquired with such a phase difference, there is a possibility that coloring will occur in the output image or an image with incorrect polarization information will be output.

[0007] An object of the present invention is to realize an imaging device that acquires polarization information without losing color information or polarization information over the entire captured image. [Means for solving the problem]

[0008] In order to achieve the above object, an imaging device of the present invention includes, in order from the object side to the image side, a first retardation plate, a second retardation plate including a liquid crystal layer and capable of changing a phase difference in a plurality of ways, a polarizing plate, an optical system with a maximum angle of view of 14 degrees or more, and an imaging element, and is capable of acquiring the polarization state of an object by changing the phase difference of the second retardation plate and performing multiple images, wherein, in each state of the plurality of phase differences set by the second retardation plate, when the polarization direction of incident light on the optical axis of the optical system and at an angle of view of 14 degrees or more is changed, the change amounts of transmitted light intensity at the imaging element are defined as I0 and Iang, respectively: 0.4 ≦ Iang / I0 ≦ 1.2 It is characterized by satisfying the following. [Effects of the Invention]

[0009] According to the present invention, it is possible to realize an imaging device that acquires polarization information without losing color information or polarization information over the entire captured image. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 1 is a diagram illustrating a configuration of an imaging device. [Figure 2] FIG. 2 is a diagram illustrating the axial direction of a polarization acquisition means. [Figure 3]FIG. 10 is a diagram showing an example of the azimuth dependence of the incident polarization state and intensity. [Figure 4] FIG. 10 is a diagram showing the relationship between the axis orientation of a polarizing plate and the transmittance. [Figure 5] 10 is a diagram showing the transmittance dependency of the polarization acquisition means of the present invention on incident polarized light. FIG. [Figure 6] FIG. 10 is a diagram showing the transmittance dependency on the variable phase difference of the polarization acquisition means of the present invention. [Figure 7] 10A and 10B are diagrams showing the variable phase difference of the polarization acquisition means of the present invention and the transmittance dependence on incident polarized light. [Figure 8] FIG. 1 is a diagram illustrating a configuration of an imaging device. [Figure 9] FIG. 2 is a diagram illustrating the configuration of a variable retardation plate. [Figure 10] 1 is a schematic diagram illustrating the configuration of an imaging device according to a first embodiment of the present invention. [Figure 11] 10 is a diagram showing a change in transmitted light intensity at an incident angle of 0 degrees with respect to a variable phase difference in Example 1. FIG. [Figure 12] FIG. 10 is a diagram showing a change in transmitted light intensity at the maximum angle of view with respect to the variable phase difference in Example 1. [Figure 13] FIG. 10 is a diagram showing the change in transmitted light intensity at an incident angle of 0 degrees and at the maximum angle of view with respect to a variable phase difference in the comparative example. DETAILED DESCRIPTION OF THE INVENTION

[0011] Preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings.

[0012] FIG. 1 is a simplified schematic diagram illustrating the configuration of an imaging device 100 of the present invention. The imaging device 100 includes a polarization acquisition means 4 consisting of a λ / 4 plate 1, a variable retardation plate 2, and a polarizing plate 3, an optical system 5 that forms an image of light from a subject, and an imaging element 6. The z direction in the figure indicates the optical axis direction, and the x and y directions are coordinate axes that are orthogonal to a plane perpendicular to the z direction. Below, the y direction is defined as the upward direction, and the x direction is defined as the direction perpendicular to the paper surface. Note that in FIG. 1, the polarization acquisition means 4 is located on the light incident side of the optical system 5, but the present invention is not limited to this. The polarization acquisition means 4 may be located on the light incident side of the imaging element 6. For example, it may be located between the optical system 5 and the imaging element 6, or, if the optical system 5 is composed of multiple optical elements, it may be located somewhere between the optical system 5 and the imaging element 6.

[0013] Figure 2 shows the slow axis directions of the λ / 4 plate 1 and variable retarder 2, as well as the polarization transmission axis direction of the polarizer 5. Figure 2 shows the arrangement of each element in the xy plane as viewed from the optical system 1 side, with the axis of each element parallel to the bold arrow in Figure 2. Furthermore, all of the axes are also arranged parallel to the xy plane. The angle φ between the x-axis and the slow axis direction of the λ / 4 plate 1 and the transmission axis direction of the polarizer 3 is φ=90°, and the slow axis direction of the variable retarder 4 is φ=45°. The imaging device 100 captures multiple images while changing the phase difference of the variable retarder 2, thereby obtaining images with different polarization states. This is described in detail below.

[0014] The azimuthal dependence of the polarization intensity of light reflected from an object can be obtained by observing the light intensity while changing the transmission axis of the polarizer. For example, consider the case where the polarization of light reflected from an object can be expressed as shown in Figures 3(a) and 3(b). In Figure 3(a), the x- and y-axes represent two directions perpendicular to the z-direction (= the light's direction of travel), as in Figure 1. The ellipse indicated by the thick dotted line represents the azimuthal dependence of the amplitude of the polarization state. Figure 3(b) shows a graph plotting the angle φ with the x-direction on the horizontal axis and the polarized light intensity I(φ) on the vertical axis. The different arrows in Figure 3(a) represent the amplitude at each angle φ, and the square of this is plotted as the light intensity. In this example, the intensity of the polarized light oscillating 45° relative to the x-axis is strongest. For such an object, acquiring information on φ at 45° or at 135°, which is perpendicular to φ, allows for the capture of an image that best emphasizes the object's features. While polarized light generally contains both intensity and phase information, the imaging device of the present invention focuses on acquiring the polarization directions with maximum and minimum amplitudes and their intensities.

[0015] As mentioned above, information on the azimuth dependence of polarization intensity in Figure 3(b) can be obtained by observing the light intensity I(φ) while rotating the polarizer. Figure 4 shows the difference in the behavior of transmitted light when the polarizer axis direction is changed. (a)(b)(c)(d) show the polarizer transmission axis at 90, 0, 135, and 45 degrees relative to the x-axis. The intensity and polarization direction of the emitted light after passing through the polarizer are indicated by the length and orientation of the arrows on the polarizer output side. The azimuth dependence of the intensity of emitted light passing through polarizers with different transmission axis directions reflects the azimuth dependence of the polarization intensity of the incident light. Therefore, the azimuth dependence of polarization intensity obtained using a method like Figure 4 can be used to estimate the direction with the maximum polarization intensity.

[0016] In contrast, the imaging device of the present invention acquires similar information by varying the phase difference of a single retarder between multiple values ​​without rotating the polarizer. Figure 5 shows how the intensity of transmitted light changes depending on the direction of incident polarization φ in the polarization acquisition means 4 of the present invention. (a) (b) (c) (d) show the polarization behavior when the phase difference of the variable retarder 2 is 0.25λ and the vibration direction of the incident polarized light is φ = 90°, 45°, 0°, or 135°. The direction and length of the arrows before and after transmission through each element indicate the polarization direction and intensity. For example, in the configuration of Figure 5(a), the light is transmitted without change in polarization state because it is parallel to the slow axis of the λ / 4 plate 1. It is converted to right-handed circular polarization by the variable retarder 2 and then transmitted by the polarizer 6 as linear polarization with an intensity approximately 50% of the incident light. In contrast, in the state shown in Figure 5(b), the light is converted to left-handed circular polarization by the λ / 4 plate 1 and then converted to linear polarization with an azimuth of 90° by the variable retarder 2, and is transmitted through the polarizer 5 with almost no loss. Similarly, Figures 5(c) and 5(d) transmit approximately 50% and 0%, respectively. Comparing the behavior of Figures 5(a) to (d), it can be seen that when the phase difference of the variable retarder 2 is 0.25λ, polarized light with φ=45° has the highest transmittance. If the polarization direction resulting in this maximum transmittance is designated as φo, the imaging device of the present invention acquires polarization information by varying φo by changing the phase difference of the variable retarder 2.

[0017] Figures 6(a), 6(b), 6(c), and 6(d) show the behavior of polarization in the direction φo where the transmittance is maximized when the phase difference of the variable retarder 2 is 0, 0.25λ, 0.5λ, and 0.75λ, respectively. As shown in Figure 6, transmittance is maximized when the incident polarization state is φo = 90° in (a), φo = 45° in (b), φo = 0° in (c), and φo = 135° in (d). By acquiring images while varying the phase difference of the variable retarder 2, we can obtain polarization information that is approximately equivalent to that obtained by rotating the transmission axis of the polarizer 3 in each direction. Figure 7 shows the relationship between the incident polarization direction φ and the transmittance T(φ) of the polarization acquisition means when the phase difference of the variable retarder 2 is changed. The different line types correspond to the different phase differences of the variable retarder, i.e., Figure 6(a), (b), (c), and (d). It can be seen that the polarization direction φ at which the transmittance is maximized changes depending on the phase difference of the variable phase difference plate.

[0018] Furthermore, in the present invention, the slow axes of the λ / 4 plate 1 and variable retardation plate 2, and the slow axis of the variable retardation plate 2 and the transmission axis of the polarizer 3, are arranged so as to form an angle of 45°, thereby minimizing the influence of the phase information of the incident polarized light. With this arrangement, for example, when perfectly circularly polarized light is incident, the λ / 4 plate 1 converts it into linearly polarized light with an azimuth of 45°, and the transmitted intensity is uniform regardless of the subsequent phase difference of the variable retardation plate 2. Even in the case of elliptically polarized light, a value corresponding to the azimuth dependency of the intensity of the incident polarized light can be obtained, so information about the intensity can be obtained.

[0019] To find the direction in which the polarization intensity is maximum from the obtained input value, the input value can be treated as polarization intensity and analyzed using an appropriate function (for example, a sine function) for the azimuth dependence of the intensity of the incident polarized light. Here, the following determinant holds, where I(φi) is the intensity of the polarized light incident at azimuth φi, Δj is the phase difference of the variable retardation plate, Tij is the transmittance of the polarization acquisition means 6 at the variable phase difference Δj with respect to the incident polarization intensity I(φi), and Tj is the transmitted light intensity of all incident light at the variable phase difference Δj.

[0020]

number

[0021] The subscript j in Tj corresponds to the variable phase difference j, and each j can be thought of as corresponding to the transmission axis direction of a certain polarizer. Furthermore, Tij is a quantity that can be uniquely determined once the vibration direction of the incident linearly polarized light and the configuration of the polarization acquisition means are determined. Therefore, by first acquiring Tij and then changing the variable phase difference Δj to acquire the transmitted light intensity Tj, the azimuth dependence of the incident polarized light can be determined by analyzing the transmitted light intensity plot against the vibration direction of the incident polarized light.

[0022] Using the above method, it is possible to obtain information on the azimuth dependence of polarization intensity using a single variable retardation plate. However, this cannot be achieved by simply adding polarization acquisition means 4 to an imaging device such as a conventional single-lens reflex camera and interchangeable lenses. For example, using it directly in an imaging device may cause the following problems.

[0023] In imaging devices such as digital single-lens reflex cameras, an optical low-pass filter is placed near the image sensor to prevent moiré and false colors. Figure 8 shows a schematic diagram of the configuration of an imaging device of the present invention in such a case. The optical low-pass filter 7 is typically placed immediately in front of the image sensor 6. While the operating principles of optical low-pass filters will not be described in detail here, they typically utilize polarization properties, such as a multi-layered birefringent medium or a polarizing diffraction element. Simply placing the polarization acquisition means 4 on the light incident side of the optical system 5 may result in the desired optical low-pass filter effect being inhibited due to the influence of the polarization acquisition means 4. Furthermore, using a system in which polarizing plates are placed directly in front of the sensor makes it impossible to acquire polarization information because the polarization information of the subject is lost when it passes through the optical low-pass filter.

[0024] On the other hand, as the simplest countermeasure in a system where the polarization acquisition means 4 is arranged on the light incident side of the optical system 5 as in the present invention, a λ / 4 plate may be inserted between the polarizing plate 3 and the optical system 5 to obtain circular polarization. However, since the λ / 4 plate has wavelength dispersion and does not become uniform circular polarization over the entire visible light range, a phase shift due to wavelength may appear as a color change in the image. Therefore, if a λ / 4 plate is used, it is desirable to use an achromatic λ / 4 plate designed to minimize the phase difference in the visible wavelength band which is the wavelength used. As another method, by arranging the optical ray separation direction on the optical system side (when it has a laminated structure) of the optical low-pass filter and the transmission axis direction of the polarizing plate 6 to form 45 degrees, the characteristics of the optical low-pass filter and the characteristics of the polarization acquisition means can be made compatible. As imaging devices of the present invention, there are a method of arranging an achromatic λ / 4 plate and a method of making the axial direction of the optical low-pass filter and the axial direction of the polarizing plate form 45 degrees. Either method may be used, but the latter is simpler.

[0025] The variable retardation plate used in the present invention is an element using liquid crystal. Fig. 9 shows a schematic view of the variable retardation plate 2 using liquid crystal. The variable retardation plate 2 has a structure in which a liquid crystal layer 11 is sandwiched between a substrate 8, an electrode layer 9, and an alignment film 10. The liquid crystal layer 11 is a VA mode liquid crystal, and the liquid crystal molecules 12 are aligned following the alignment film 10. By controlling the tilt angle θ of the liquid crystal according to the applied voltage applied to the electrode layer thereon, a predetermined phase difference is imparted to the transmitted light. The circular portion in Fig. 9 shows an enlarged view of the liquid crystal layer. In this example, when the applied voltage is 0 [V], the liquid crystal molecules 12 are aligned almost perpendicular to the plate surface. By changing the applied voltage to 0 [V], A [V], and B [V] (A < B), the tilt angle θ of the liquid crystal molecules min from θ MAX to θ MAX can be changed. The phase difference of the variable retardation plate is imparted by the refractive index anisotropy of the liquid crystal molecules, and its magnitude changes depending on the incident angle of the light ray and the angle of the optical axis (tilt angle θ) of the refractive index anisotropy of the liquid crystal molecules. At this time, the phase difference when the tilt angle is θ MAX is the maximum phase difference Δ min and the phase difference when θ minThen, the amount of phase change is the difference between the maximum and minimum phase differences, Δ MAX -Δ min It is expressed as:

[0026] Furthermore, by controlling the applied voltage, a phase difference value greater than or equal to the minimum phase difference and less than or equal to the maximum phase difference can be achieved. In principle, any phase difference within this range can be achieved. However, when considering the driving speed and the angular characteristics of the variable phase plate, the maximum and minimum phase differences are more advantageous than other cases. Therefore, when changing the phase difference during measurement to two or more values, it is preferable to set the phase difference so that it includes the maximum and minimum phase differences.

[0027] Because liquid crystals have angular characteristics, when used as a variable retarder, the transmitted light intensity, which changes depending on the incident polarization direction φ, differs depending on the angle of incidence on the variable retarder. The orientation in which the liquid crystal molecules tilt when a voltage is applied to the liquid crystal—i.e., the polarization direction φo that results in maximum transmittance at oblique incidence along the variable retarder's slow axis—shifts compared to an incidence angle of 0°. Although this results in a deviation from the correct polarization direction, the amplitude of the transmitted light intensity can generally be obtained satisfactorily, making it possible to correct the polarization information. On the other hand, for oblique incidence along an orientation other than the variable retarder's slow axis, even if good transmitted light intensity amplitude is obtained at an incidence angle of 0°, there are cases in which the amplitude is poor when the incident polarization direction φ is changed due to the influence of angular characteristics. In such cases, it is difficult to determine the polarization direction φo that results in maximum transmittance, and therefore, incorrect polarization information is added to the image when a polarized image is synthesized. Furthermore, because the transmitted light intensity does not reach 0% for any incident polarization direction, coloring occurs in that region when a polarized image is generated.

[0028] Therefore, it is preferable not to use the above-mentioned phase difference to obtain polarization information, but to set an appropriate phase difference that allows the amplitude of the transmitted light intensity to be obtained. Specifically, if the amount of change, which is the difference between the maximum transmittance and the minimum transmittance of the transmitted light intensity at an incident angle of 0 degrees, i.e., on the optical axis of the optical system, is I0, and the amount of change in the transmitted light intensity at an angle of view of 14 degrees or more is Iang, then: 0.4 ≦ Iang / I0 ≦ 1.2 (2) In addition, when the retardation has wavelength dispersion, it is preferable that the above condition be satisfied over the entire wavelength band used.

[0029] It is preferable that the present invention uses the VA type liquid crystal described in FIG. 9, but the present invention is not limited to this and can be applied to various types of liquid crystal such as the TN type and the OCB type.

[0030] Since the polarization acquisition means of the present invention is used in an imaging device, it is desirable that the polarizer be an absorptive polarizer. Using a polarizer that reflects unwanted light, such as a wire-grid polarizer, would reflect the polarized light on the side that is being cut, causing stray light and ghosting, which would adversely affect the image, making it undesirable for the configuration of the imaging device. Therefore, it is desirable to use an absorptive polarizer that absorbs unwanted light. More preferably, to minimize the effects of the aforementioned ghosting, the polarizer should have the property of absorbing 50% or more of the polarized light vibrating in a direction perpendicular to the transmission axis across the entire visible wavelength range, which is the wavelength used. Examples of such polarizers include films made from stretched resin members containing iodine compounds. However, the material is not limited to these, and any absorptive polarizer may be used.

[0031] Although the images obtained by the imaging device of the present invention each contain different polarization information, they can be used as they are without undergoing any arithmetic processing, such as image processing. However, by performing arithmetic processing between images with different polarization information, it is possible to obtain an image in which the features of the subject are more emphasized pixel by pixel. For example, by generating an image using only the lowest or highest light intensity values ​​among the acquired data, an image in which the scattered light component of the subject or the specular reflection component from the subject is emphasized can be obtained. Note that the polarized light intensity value here may be the direct value of the image acquired by the polarization acquisition means, or it may be an interpolated or extrapolated value obtained from polarization analysis. Here, interpolation and extrapolation refer to the use of estimated values ​​from the analysis results to emphasize or suppress the differences in the obtained polarization intensity. By optically acquiring object information of the subject in this way, an image in which the features are emphasized or suppressed can be obtained. Alternatively, by combining these, it is possible to generate an image that matches the photographer's intention. Furthermore, an image may have different polarization information or an emphasis effect for each region of the image. For example, by combining images with different polarization states for the main subject and background (such as the sky), it is possible to obtain effects such as making the background color uniform, or obtaining an image that emphasizes both the background and the main subject.In addition, by performing various processes that utilize the intensity dependency of the polarization of the subject, it is possible to obtain an image that meets the purpose. [Example]

[0032] An imaging device according to a first embodiment of the present invention will be described.

[0033] FIG. 10 shows a schematic diagram of the configuration of an imaging device 300 of the present invention. The device configuration of the imaging device 300 is substantially the same as that shown in FIG. 8. It comprises a λ / 4 plate 1 made of a stretched film, a variable retardation plate 2 made of a liquid crystal element, polarization acquisition means 4 made of an absorptive polarizer 3 made of a stretched film, an optical system 5 that forms an image of light from a subject, and an imaging element 6. The optical system 5 has a focal length of 34.2 mm, and the imaging element 6 is a full-size sensor. The refractive index anisotropy Δn of the liquid crystal at wavelengths of 450 nm, 550 nm, and 650 nm is 450=0.1034, △n 550 =0.0996, △n 650 =0.0947, and the liquid crystal layer thickness d=5.0 μm. λ is the difference between the extraordinary refractive index ne and the ordinary refractive index no of the liquid crystal at wavelength λ, |ne - no|.

[0034] Furthermore, an optical low-pass filter 7 is disposed between the image sensor 6 and the optical system 5. The multiple image signals obtained by the image sensor 6 are temporarily stored in a signal recording unit 13. After that, the signals are processed and output as multiple images 14, or they are output as one or multiple images 16 after passing through a signal processing unit 15.

[0035] In the imaging device 300 of the present invention, when capturing an image of a subject, multiple images are captured at once while the phase difference of the variable retarder 2 is changed over time. The multiple images with different polarization information are temporarily stored in the signal recording unit 13. There are two methods for outputting the images: a first method in which the images with different polarization information are output as multiple images 14 as they are, and a second method in which the multiple images are processed by the signal processing unit as described above to extract the features of the subject and output image 16. When output using the first method, the multiple images can be processed separately using an external processing device such as a PC, making it possible to obtain images that require more complex calculations. Furthermore, with the second method, the desired image can be obtained quickly by having the signal processing unit 15 perform processing to extract predetermined features in advance.

[0036] The λ shown for the retardation of each retarder is designed for light with a wavelength of 550 nm, which has a high luminosity. The slow or fast axis of the λ / 4 plate 1 and the transmission axis of the polarizer 5 are aligned parallel to the y direction, while the slow or fast axis of the variable retarder 2 is aligned at 45° with respect to the x direction. The variable retarder 2 has five retardation values: Δ=0λ, 0.15λ, 0.45λ, 0.60λ, and 0.75λ, with a phase shift of 0.75λ. The retardation shown here is the retardation when light is incident on the variable retarder at an incident angle of 0°. Figures 11(a)-(e) show the transmitted light intensity as a function of the direction φ of incident polarization for wavelengths of 450 nm, 550 nm, and 650 nm, respectively, for retardations of 0λ, 0.15λ, 0.45λ, 0.60λ, and 0.75λ at an incident angle of 0°. Here, the various plot points are obtained by simulation, and the types of lines connecting the plot points are obtained by the above-mentioned fitting.

[0037] 12(a) to 12(e) show how the transmitted light intensity changes under the same conditions as above when the slow axis of the variable retardation plate is set to 0 degrees, with an orientation of 259 degrees and an incident angle of 32.5 degrees. Table 1 shows the change I0, which is the difference between the maximum and minimum transmittance of the transmitted light intensity obtained by fitting at each wavelength, and Iang / I0, which is calculated from Iang. Due to oblique incidence characteristics, the change in phase difference also increases as the incident angle increases. Therefore, if the maximum angle of view satisfies equation (2), intermediate angles of view also tend to satisfy it. Therefore, focusing on the maximum angle of view, Iang is the change in transmitted light intensity at 32.5 degrees, which is the maximum angle of view of the image pickup device in this embodiment.

[0038] [Table 1]

[0039] 11, 12, and Table 1, the amplitude of the transmitted light intensity is obtained for each phase difference and each wavelength, and it can be seen that equation (2) is satisfied for each wavelength and each phase difference. Therefore, φo can be obtained for each phase difference, and by analyzing the transmitted light intensity plot against the vibration direction of the incident polarized light, the azimuth dependence of the incident polarized light can be obtained.

[0040] Below, a comparative example will be shown as an inappropriate phase difference that does not satisfy the formula (2).

[0041] As a comparative example, in an imaging device having the same configuration as in Example 1, when the phase difference of the variable retardation plate is Δ=0.25λ, the transmitted light intensity at an incident angle of 0 degrees, an azimuth of 259 degrees, and an incident angle of 32.5 degrees is shown in FIGS. 13(a) and 13(b), and Table 2 shows Iang / I0 for each wavelength.

[0042] [Table 2]

[0043] Figure 13(b) shows that the amplitude of the change in transmitted light intensity at λ450nm is small. This is because the variable retarder is made of liquid crystal, and the angular characteristics and wavelength dispersion of the liquid crystal result in a phase difference for obliquely incident light that is different from the phase difference at an incident angle of 0° at λ550nm. At an incident angle of 0° at λ550nm, Δ=0.25λ results in a phase difference such that, at an azimuth of 259° at λ450nm and an incident angle of 33°, the four states of incident polarization (φ=90°, 45°, 0°, and 135°) are linearly polarized and left- and right-handed circularly polarized after passing through the variable retarder. Therefore, the transmitted light intensity for these four states of incident polarization after passing through polarizer 5 is approximately 50%, making it impossible to obtain amplitude. Since a phase difference of Δ=0.25λ does not allow us to obtain φo at λ450nm, it is impossible to calculate the azimuth dependence of the incident polarization.

[0044] Furthermore, the small amplitude of the change in transmitted light intensity results in erroneous polarization information, such as the presence of a dominant diffuse component, even though a polarized component is actually present in the captured image. Therefore, when a polarized image is generated, the oblique incidence region and wavelengths with a small amplitude phase difference are added as diffuse components. While λ450 nm has been used as an example so far, the above-described change in transmitted light intensity also occurs in this region and in the vicinity of this wavelength. Table 2 shows that equation (2) is not satisfied even at λ550 nm. Therefore, in the polarized image captured and generated under the conditions of the comparative example, a blue-green color appears in the region with an azimuth of 259 degrees and an incident angle of 33 degrees and in the surrounding area. [Example]

[0045] An imaging device according to a second embodiment of the present invention will be described. The element configuration of the imaging device according to the present invention is substantially the same as that shown in the first embodiment, but the variable retardation plate 2 is different. The refractive index anisotropy Δn 450 =0.1082, △n 550 =0.1014, △n 650 =0.0984, and the thickness of the liquid crystal layer d=6.0 μm.

[0046] The variable retarder 2 has four phase differences, Δ=0λ, 0.50λ, 0.63λ, and 0.89λ, with a phase change of 0.89λ. Table 3 shows Iang / I0 for each wavelength. Here, Iang is the difference between the maximum transmittance and minimum transmittance of hyperpolarized light intensity at an azimuth of 259 degrees and an incident angle of 32.5 degrees. The incident angle of 32.5 degrees is the maximum angle of view of the imaging device in this example.

[0047] [Table 3] [Example]

[0048] An image pickup device according to a third embodiment of the present invention will be described. The element configuration of the image pickup device according to the present invention is substantially the same as that shown in the first embodiment. The focal length of the optical system 5 is 50 mm, and the refractive index anisotropy Δn 450 =0.0669, △n 550 =0.0507, △n 650 =0.0042, and the thickness of the liquid crystal layer d=10.0 μm.

[0049] The variable retarder 2 has three phase differences, Δ=0λ, 0.38λ, and 0.75λ, with a phase change of 0.75λ. Table 4 shows Iang / I0 at each wavelength. Here, Iang is the difference between the maximum transmittance and minimum transmittance of the hyperpolarized light intensity at an azimuth of 259 degrees and an incident angle of 23.4 degrees. The incident angle of 23.4 degrees is the maximum angle of view of the imaging device in this example.

[0050] [Table 4]

[0051] Although the preferred embodiments of the present invention have been described above, the present invention is not limited to these embodiments, and various modifications and changes are possible within the scope of the gist of the present invention. [Explanation of symbols]

[0052] 1 λ / 4 plate 2 Variable phase plate 3 Polarizing Plate 4. Polarization acquisition method 5 Optical system 6. Image sensor 7 Optical low-pass filter 8 PCB 9 Electrode layer 10 Orientation film 11 Liquid crystal layer 12 Liquid crystal molecules 13 Signal Recording Unit 15 Signal processing section 100, 200, 300 imaging device

Claims

1. An imaging device having, in order from the object side to the image side, a first retardation plate, a second retardation plate including a liquid crystal layer and capable of changing a phase difference in a plurality of ways, a polarizing plate, an optical system having a maximum angle of view of 14 degrees or more, and an imaging element, wherein the imaging device can acquire the polarization state of an object by changing the phase difference of the second retardation plate and taking an image a plurality of times, wherein, in each state of the plurality of phase differences set by the second retardation plate, when the polarization direction of incident light on the optical axis of the optical system and at an angle of view of 14 degrees or more is changed, the change amounts of transmitted light intensity at the imaging element are defined as I0 and Iang, respectively: 0.4 ≦ Iang / I0 ≦ 1.2 An imaging device characterized by satisfying the following.

2. 2. The imaging device according to claim 1, wherein the first retardation plate provides a phase difference of λ / 4 between a polarized component in the slow axis direction and a polarized component in the fast axis direction.

3. 3. The imaging device according to claim 1, wherein a slow axis direction or a fast axis direction of the first retardation plate is parallel to a polarization direction of the polarized light component extracted by the polarizing plate, and a slow axis direction or a fast axis direction of the second retardation plate is inclined by 45 degrees with respect to the polarization direction.

4. The ratio of Δn of the second wave plate is 0.85 < Δn450 / Δn550 < 1.35 0.80 < Δn650 / Δn550 < 1.15 4. The imaging device according to claim 1, wherein the following is satisfied: however, Δn450 is the difference between the ordinary refractive index no and the extraordinary refractive index ne at λ450 nm Δn550 is the difference between the ordinary refractive index no and the extraordinary refractive index ne at λ550 nm Δn650 is the difference between the ordinary refractive index no and the extraordinary refractive index ne at λ650 nm

5. When the maximum phase difference of the second wave plate is 0.75λ, the plurality of phase differences Re set by the second phase plate are 0.222λ ≦ Re ≦ 0.375λ 5. The imaging device according to claim 1, wherein the imaging device does not include:

6. 6. The imaging device according to claim 1, wherein a plurality of images are acquired while changing the phase difference of the variable retardation plate, and polarization information of the subject is acquired from the plurality of images.

Images