JP2020106563A - Image capturing device - Google Patents

Abstract

To provide a polarization acquisition element capable of accurately acquiring polarization information without reducing resolution even when wide wavelength plates are used.SOLUTION: A polarization acquisition element for acquiring different polarization states is provided. The polarization acquisition element comprises a laminate-type 1/4λ plate obtained by laminating a material with birefringence while shifting a fast or slow axis in an in-plane direction, a variable phase plate that allows for switching the retardation between two or more values, and a polarizing plate, and is arranged such that an angle between a fast or slow axis of the variable phase plate and a transmission axis of the polarizing plate is roughly 45 degrees, and that an angle between the fast or slow axis of the variable phase plate and a fast or slow axis of the 1/4λ plate at a specific wavelength is roughly 45 degrees. The maximum wavelength λmax and the minimum wavelength λmin of a used wavelength range satisfies a condition expressed as: λmax/λmin>2.SELECTED DRAWING: Figure 1

Description

The present invention relates to an element and an image pickup apparatus, and more particularly to an element and an image pickup apparatus capable of acquiring polarization information.

It is known that predetermined characteristics of a subject can be emphasized and detected by observing the polarization state of light from the subject. For example, by attaching a polarization filter to the front of the lens of a single-lens reflex camera and observing the transmitted polarization direction, you can emphasize the texture such as color and contrast of the subject, and emphasize the reflection of reflected light such as water surface. You can obtain effects such as reducing. In addition, it is used in an inspection device or the like for capturing an image of a plurality of polarization components having different polarization directions and detecting an edge or a defective portion of a subject.

There are various methods as an imaging device for imaging polarization information from a subject. Patent Document 1 discloses a configuration of an image sensor that extracts polarization information from a plurality of pixels by forming a wire grid polarization plate that transmits different polarized light to each pixel on the solid-state image sensor. Further, Patent Document 2 discloses a method of acquiring polarization information from a plurality of images captured while changing the phase difference of the phase difference plate, which is composed of a ¼λ plate, a phase difference plate capable of changing the phase difference, and a polarizing plate. It is disclosed.

JP 2012-80065 A JP, 2016-145924, A

In Patent Document 1, although polarization information can be obtained from a single image, a plurality of pixels are assigned to acquire polarization information, so there is a problem that resolution is lost. Further, in Patent Document 2, there is a problem that it is difficult to simultaneously acquire polarization information over a wide wavelength range because the polarization acquisition performance deteriorates due to the wavelength dispersion due to the phase difference of the ¼λ plate.

In order to solve the above problems, the polarization acquisition element according to the present invention,
A polarization acquisition element for acquiring different polarization states,
A laminated type 1/4 λ plate in which a material having birefringence is laminated by shifting the fast axis or the slow axis in the in-plane direction,
A variable phase plate that can change the phase difference into two or more values,
And a polarizing plate,
The angle formed by the fast axis or slow axis of the variable phase plate and the transmission axis of the polarizing plate is arranged at about 45 degrees,
An angle formed by the fast axis or slow axis of the variable phase plate and the fast axis or slow axis at a specific wavelength of the laminated ¼λ plate is arranged at about 45 degrees,
The maximum wavelength λmax and the minimum wavelength λmin in the operating wavelength range are
λmax/λmin>2
It is characterized by satisfying.

According to the present invention, it is possible to provide a polarization acquisition element that can acquire polarization information without lowering resolution and that can acquire polarization information with high accuracy even when the wavelength plate is wide.

Schematic diagram of polarization acquisition element Schematic of laminated 1/4λ plate The phase difference Δ _LC of the variable phase plate and the transmitted light intensity I of the polarization acquisition element when the axial direction of the laminated 1/4 λ plate is 85° The phase difference Δ _LC of the variable phase plate and the transmitted light intensity I of the polarization acquisition element when the axial direction of the laminated 1/4 λ plate is 80° The phase difference Δ _LC of the variable phase plate and the transmitted light intensity I of the polarization acquisition element when the axial direction of the laminated 1/4 λ plate is 75° Axial displacement and minimum polarization degree of laminated 1/4 λ plate Lag amount of phase difference and minimum polarization degree of laminated 1/4λ plate Schematic diagram of the laminated 1/4 lambda plate of Example 1. The phase difference Δ(λ) with respect to the wavelength λ of the laminated ¼λ plate of the first embodiment. Axial direction θ(λ) with respect to wavelength λ of the laminated 1/4 λ plate of Example 1 2 is a schematic diagram of an image pickup apparatus according to a second embodiment. Pixel array of the solid-state image sensor of Example 2 Outline of polarization acquisition element Axial direction of polarization acquisition element Phase difference Δ _LC of variable phase plate of polarization acquisition element and transmitted light intensity I of polarization acquisition element Incident elliptical polarization and transmitted light intensity I(θ) The phase difference Δ _LC of the variable phase plate and the transmitted light intensity I of the polarization acquisition element when the phase difference of the ¼ λ plate is 0.38 The phase difference Δ _LC of the variable phase plate and the transmitted light intensity I of the polarization acquisition element when the phase difference of the ¼ λ plate is 0.50 Incident polarized light and detected polarized light (phase difference of 1/4 λ plate at 0.38) Incident polarization and detection polarization (when the phase difference of 1/4 λ plate is 0.50) Outline of 1/4λ plate of Comparative Example 1 Phase difference Δ(λ) with respect to the wavelength λ of the ¼λ plate of Comparative Example 1 Axial direction θ(λ) with respect to the wavelength λ of the ¼λ plate of the first embodiment Outline of 1/4λ plate of Comparative Example 2 Phase difference Δ(λ) with respect to the wavelength λ of the ¼λ plate of Comparative Example 2 Axial direction θ(λ) with respect to the wavelength λ of the ¼λ plate of the second embodiment

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings.

FIG. 1 is a schematic diagram showing a simple configuration of a polarization acquisition element 100 of the present invention.

As shown in FIG. 1, the polarization acquisition element 100 includes a laminated ¼λ plate 10, a variable phase plate 20, and a polarizing plate 30.

FIG. 2 is a schematic diagram showing a simple configuration of the laminated 1/4 λ plate 10.

The laminated ¼λ plate has a structure in which a plurality of layers (n layers in FIG. 2) of materials having birefringence are laminated. Further, when the laminated structure is layer 1, layer 2,... Layer n, the layers are arranged so that the axial directions of the adjacent layers are different from each other. That is, for the integer k that satisfies 1<k≦n, the axial directions θ _k−1 and θ _k of the layer k−1 and the layer k are different from each other. Here, the “axis” indicates the fast axis or the slow axis of the phase plate, and the “axial direction θ” means that the angle between the “axis” and the x axis in the xy plane is 0 degrees or more and less than 180 degrees. Show. In the following description, the axes of the ¼λ plate and the variable phase plate are described as the direction of the fast axis.

Further, the phase difference and the axial direction of the laminated 1/4λ plate 10 have wavelength dependence. Here, the phase difference at the wavelength λ of the laminated ¼λ plate 10 is expressed as Δ(λ) and the axial direction θ(λ), and the phase difference of each layer of the laminated ¼λ plate 10 is Δ ₁ (λ). delta ₂ (lambda), when expressed as ···· Δ _n (λ), the following relation (1) holds with Jones matrix.

R(-θ(λ))C(Δ(λ))R(θ(λ))
=R(-[theta] _n )C([Delta] _n ([lambda]))R([theta] _n )... R(-[theta] ₂ )C([Delta] ₂ ([lambda]))R([theta] ₂ ).
R(-[theta] ₁ )C([Delta] ₁ ([lambda]))R([theta] ₁ ) (1)
Note that R(θ) and C(Δ) in the equation (1) are represented by the following equations (2) and (3), respectively.

In the present invention, by appropriately setting the phase difference Δ(λ) and the axial direction θ(λ) of the laminated 1/4 λ plate 10 as described above, and using them in combination with the variable phase plate 20 and the polarizing plate 30, It has been found that the polarization acquisition performance can be exhibited for a wide range of wavelengths.

Hereinafter, before describing the embodiments of the present invention, the polarization information acquisition using the variable retardation plate described in JP-A-2016-145924 will be described first, and then the configuration example of the present invention will be described in more detail. ..

Regarding Acquisition of Polarization Information Using Variable Phase Plate FIG. 13 shows an outline of a configuration example of the polarization acquisition element 200 disclosed in JP-A-2016-145924. In addition, FIG. 14 shows an example of the axial direction of each element constituting the polarization acquisition element 200.

In the polarization acquisition element 200, the quarter-wave plate 11 and the variable phase plate 21 are arranged so that the axes thereof are substantially 45 degrees, and the quarter-wave plate 11 and the polarizing plate 31 are substantially parallel to each other in transmission axis. ..

By changing the phase difference of the variable phase plate 21 to 0λ, 1/4λ, 1/2λ..., The polarization direction of the light transmitted through the polarization acquisition element 200 can be changed.

FIG. 15 shows the relationship between the phase difference Δ _LC of the variable phase plate 21 and the light intensity I transmitted through the polarization acquisition element 10 when linearly polarized light enters the polarization acquisition element 200. Note that FIG. 15 shows the light intensity in an ideal state in which the influence of reflection and absorption on the surface of each element is ignored.

It can be seen from FIG. 15 that the relationship between the phase difference Δ _LC and the light intensity I changes depending on the direction of the incident linearly polarized light. Assuming that the direction of linearly polarized light is α, the relationship between the phase difference Δ _LC and the light intensity I satisfies the following expression (4).

I=cos ² (π/4−πΔ _LC −α) (4)
Similarly, when the elliptically polarized light shown in FIG. 16 is incident, the relationship between the phase difference Δ _LC and the light intensity is
The following expression (5) is satisfied.

I=Acos ² (π/4−πΔ _LC −α)+B... (5)
From the formula (5), the polarization information A, B, and α can be calculated by changing the Δ _LC to three or more values and measuring the light intensity I.

In the above description, the case where the phase difference Δ _1/4λ =0.25 of the 1/4λ plate 11 is described. Hereinafter, it is assumed that the phase difference of the ¼λ plate 11 has wavelength dependency, and the case where the phase difference Δ _1/4λ deviates from 0.25 will be considered. In Patent Document 2, there is no specific description that the phase difference of the ¼λ plate 11 gives to the polarization acquisition performance.

17 to 18 show the relationship between the phase difference Δ _LC of the variable phase plate 21 and the light intensity I transmitted through the polarization acquisition element 11 when linearly polarized light enters the polarization acquisition element 200. In FIG. 17, the phase difference Δ _1/4λ of the 1/4λ plate 11 is indicated by a broken line when the design value is (0.25), and the solid line is indicated when the phase difference Δ _1/4λ is 0.38. To do. Further, in FIG. 18, the case where the phase difference Δ _{1/4 λ} is 0.50 is indicated by a solid line. From FIGS. 17 to 18, it can be seen that when the phase difference Δ _{1/4 λ} deviates from the design value, the fluctuation range of the light intensity I with respect to the phase difference Δ _LC of the variable phase plate 21 decreases according to the angle of the incident polarized light. Further, it can be seen that the peak position of the light intensity I (that is, the value of Δ _LC having the maximum value and the minimum value) also changes according to the incident polarization angle.

19 to 20 show incident polarized light and detected polarized light when the phase difference Δ _1/4λ of the 1/4λ plate is 0.38 and 0.50, respectively. Here, the detected polarization is the incident polarization estimated from the light intensity I shown in FIGS. 19 to 20 based on the equation (5).

It can be seen from FIGS. 19 to 20 that the phase difference Δ _{1/4 λ} deviates from the design value and the calculated polarization degree or angle of the light deviates, and correct polarization information cannot be acquired. Further, it can be seen that when Δ _{1/4 λ} becomes 0.50, it becomes impossible to detect the polarization direction α.

From the above, the polarization acquisition element 200 shown in FIG. 13 cannot correctly acquire polarization information when the 1/4 λ plate 11 has wavelength dispersion. This problem becomes noticeable when acquiring polarization information in a wide wavelength range, and it is difficult to correctly acquire polarization information in a wide range such as visible to near infrared. In particular, when the polarization information is acquired in the wavelength range in which the maximum wavelength λmax and the minimum wavelength λmin of the used wavelength range satisfy λmax/λmin>2, it is difficult to correctly acquire the polarization information in the normal 1/4λ.

Configuration Example of the Present Invention In order to solve the above problems, the polarization acquisition element 100 of the present invention uses the laminated type 1/4λ plate 10. As described above, the laminated ¼λ plate 10 has a structure in which materials having birefringence are laminated in different axial directions. Normally, the phase difference of the wave plate changes with respect to the wavelength, but in the laminated 1/4λ plate, the axial direction changes with respect to the wavelength in addition to the phase difference. Therefore, by optimizing the amount of change in the phase difference with respect to the wavelength and the amount of change in the axial direction, respectively, the polarization information can be correctly acquired in a wide band in which the maximum wavelength λmax and the minimum wavelength λmin in the use wavelength range satisfy λmax/λmin>2. be able to. Here, in the present invention, it is assumed that the visible and near-infrared or the visible and near-ultraviolet wavelength ranges are acquired, and specifically, a wide wavelength range in which λmin is 430 nm or less and λmax is 900 nm or more. Polarization information can be acquired correctly with.

Hereinafter, consider a case where the axial direction θ _{1/4λ of} the laminated 1/4λ plate 10 deviates from the design value (hereinafter, 90 degrees).

3 to 4 show the relationship between the phase difference Δ _LC of the variable phase plate 20 and the light intensity I transmitted through the polarization acquisition element 10 when linearly polarized light enters the polarization acquisition element 100. In FIG. 3, when the axial direction θ _1/4λ of the laminated type 1/4λ plate 10 is as designed (90 degrees), a broken line is shown, and when the axial direction θ _1/4λ is 85 degrees, a solid line is shown. To do. Further, FIG. 4 shows a case where the axial direction θ _{1/4 λ} is 80 degrees, and FIG. 5 shows a case where the axial direction θ _{1/4 λ} is 75 degrees.

3 to 5 that the peak position of the light intensity I with respect to the phase difference Δ _LC of the variable phase plate 21 changes when the axial direction θ _{1/4 λ} deviates from the design value (90 degrees). However, as compared with the case where the phase difference Δ _{1/4 λ} changes (FIGS. 17 to 18), it can be seen that the change in the peak position according to the incident polarization angle is small. In a region where the change in the peak position depending on the incident polarization angle is small, the amount of change in the peak position can be regarded as a constant (constant) that does not depend on the incident polarization angle.

When the amount of change in the peak position is a constant, the polarization states A and B to be acquired are not affected when the polarization information A, B, and α are calculated from the three different Δ _LC using Expression (5). On the other hand, the polarization state α is affected, but since the amount of change in the peak position is a constant, it can be corrected by measuring the amount of change in the peak position in advance. The correction method is, for example, holding the deviation amount calculated by comparing the angle obtained from equation (5) with the known linearly polarized light and the actual angle, and holding the deviation amount as the correction term, and based on the deviation amount at the time of measurement, It suffices to correct α.

From the above, it can be seen that the change in the peak position caused by the deviation of the axial direction θ _{1/4 λ} from the design value is permissible when acquiring the polarization information.

Next, consideration will be given to the reduction of the fluctuation range caused by the deviation of the axial direction θ _{1/4 λ} from the design value. It is understood from FIGS. 3 to 5 that the variation width becomes more remarkable as the axial direction θ _{1/4 λ} deviates from the design value. However, as compared with the case where the phase difference Δ _{1/4 λ} changes (FIGS. 17 to 18 ), it can be seen that the variation width is less reduced.

Since the decrease in the fluctuation range depends on the incident polarization direction, it cannot be uniformly corrected like the above-mentioned peak position. However, if the variation width is largely reduced, the accuracy of the polarization information A, B, and α will be reduced if the polarization information is acquired without correction. Therefore, in order to prevent the deterioration of the polarization acquisition accuracy, it is necessary to set the axial direction θ _{1/4 λ} within a range in which the fluctuation range is less decreased.

Here, the reduction of the fluctuation range can be evaluated using, for example, the degree of polarization (DOP). The degree of polarization (DOP) can be expressed by the following equation (6).

DOP=(Imax-Imin)/(Imax+Imin) (6)
Further, Imax and Imin in the equation (6) can be obtained by Imax=A+B and Imin=B using the polarization information A and B calculated from the light intensity I.

As a result of diligent studies, in order to sufficiently secure the accuracy of polarization information, it is necessary to suppress the decrease in polarization degree to less than 10%, more preferably less than 5%, and more preferably less than 2%. I found out. FIG. 6 shows the relationship between the deviation amount δθ=θ _1/4λ −90 in the axial direction θ _{1/4λ and} the minimum degree of polarization obtained when linearly polarized light is incident. Since the degree of polarization depends on the angle of incident polarization and δθ, the minimum degree of polarization is shown.

From FIG. 6, if the deviation amount δθ in the axial direction θ _{1/4 λ} is −12 degrees or more and 12 degrees or less, the decrease in the polarization degree is less than 10%, and if −8 degrees or more and 8 degrees or less, the polarization degree It can be seen that the amount of decrease is less than 5%, and the amount of decrease in polarization degree can be suppressed to less than 2% if it is -5 degrees or more and 5 degrees or less. In order to satisfy the above condition for the wavelength in the use wavelength range, the axial direction θ(λ) at an arbitrary wavelength λ in the use wavelength range is |θ(λ)−90|<12(deg)
Preferably satisfies
|θ(λ)-90|<8 (deg)
More preferably,
|θ(λ)-90)|<5 (deg)
It is more preferable to satisfy.

On the other hand, when the phase difference Δ _1/4λ of the 1/4λ plate deviates from the design value, the fluctuation range and the peak position of the light intensity I change according to the incident polarization angle, as shown in FIGS. In the case of the axial direction θ _{1/4 λ} , the shift amount of the peak position could be regarded as a constant, but in the case of the phase difference Δ 1/4 λ, both the peak position and the fluctuation range change depending on the incident polarization angle. .. Therefore, in order to acquire the polarization information with sufficient accuracy, it is necessary to further suppress the decrease in fluctuation range. In the present invention, it has been found that in order to obtain sufficient accuracy, it is necessary to suppress the fluctuation range to less than 5%, more preferably less than 2%.

FIG. 7 shows the relationship between the deviation amount δΔ _1/4λ = Δ _1/4λ −0.25 of the phase difference Δ _1/4λ when linearly polarized light is incident and the minimum degree of polarization obtained. From FIG. 7, the phase difference Δ(λ) of the 1/4λ plate with respect to an arbitrary wavelength λ in the used wavelength range is
│Δ(λ)-0.25│<0.05
It is preferable to satisfy |Δ(λ)-0.25|<0.02
It is more preferable to satisfy.

In the present invention, in order for the phase difference Δ(λ) of ¼λ to satisfy the above condition in a wide wavelength range of λmax/λmin>2, it is necessary to allow the deviation from the axial direction θ(λ) and the design value. I found out. The amount of displacement in the axial direction is preferably larger than 1 deg, more preferably 2 deg or more. From the above, the axial direction θ (λ) is
1<|θ(λ)−90|<12 (deg)
Preferably satisfies
1<|θ(λ)−90|<8 (deg)
It is more preferable to satisfy 1<|θ(λ)−90)|<5 (deg)
It is more preferable to satisfy.

In the above description, the design value in the axial direction θ(λ) is assumed to be 90 degrees, but the design value is not limited to 90 degrees. The ¼ λ plate of the present invention may be installed in a direction forming 45 degrees with respect to the variable phase plate, and the design value changes depending on the direction of the variable phase plate. Therefore, it is preferable that the axial direction is set within the above range for an arbitrary design value θ ₀ (deg),
1(deg)<|θ(λ)−θ ₀ |<12(deg)
It is preferable to satisfy 1 (deg)<|θ(λ)−θ ₀ |<8 (deg)
It is more preferable to satisfy 1(deg)<|θ(λ)−θ ₀ |<5(deg)
It is more preferable to satisfy.

Further, in order to create a ¼ λ plate satisfying the above conditions of θ(λ) and Δ(λ) in a wavelength range satisfying λmax/λmin>2, the ¼λ plate should have 5 or more layers. Is preferred.
This is because if the number of layers is less than 5, it is difficult to satisfy the condition of Δ(λ) in the wavelength range of the wide band.

Further, as a result of earnest studies, in order to satisfy the above-mentioned conditions of θ(λ) and Δ(λ), the layer n which is arranged closest to the polarizing plate and the most polarizing plate among the layers constituting the ¼λ plate. The phase difference Δ _n and the phase difference Δ ₁ of the layer 1 arranged on the side farther from the

It has been found that it is preferable to satisfy .. .DELTA.n-1 of the layers 2 to n-1 excluding the layer n and the layer 1, respectively.

It has been found that it is preferable to satisfy In addition, if the phase difference of each layer satisfies the above conditions, the layer 1 and the layer n, or the layers 2 to n-1 may have different phase differences. However, in order to make the structure simpler, it is more preferable that the layer 1 and the layer n have the same retardation and the layers 2 to n-1 have the same retardation. Further, the material forming the layers 1 to n is not particularly limited as long as it has birefringence, and any material can be selected and used. However, in order to make the structure simpler, it is more preferable that the layers 1 to n are made of the same material.

In order to acquire an image using the above-mentioned polarization acquisition element, an imaging device having an optical system, a polarization acquisition element, and a solid-state imaging element may be used. By setting the phase difference of the variable phase plate of the polarization acquisition element to three or more different states and acquiring images in each state, a plurality of images with different polarization states are captured. At this time, the wavelength range used by the polarization acquisition element may be matched with the wavelength range acquired by the imaging device.

In order to acquire the polarization information, the polarization information A, B, and α are calculated by using the polarization information acquisition device having the image pickup device and the image processing means based on the plurality of captured images and the formula (5). Good. Further, the polarization information A, B, and α are calculated for each pixel of the solid-state image sensor. The image processing means may be provided inside the apparatus as a processing unit inside the image pickup apparatus. In that case, a series of processes from image acquisition to calculation of polarization information can be performed in the imaging device. The image processing means may be separately provided outside the imaging device, and an information processing device such as a PC may be used as the image processing means. In that case, a plurality of images taken by the imaging device may be stored in a storage medium (for example, a flash memory, an SD card, a CF card, etc.) once and read into an external image processing means. Alternatively, the image data may be directly transmitted from the imaging device to the image processing means by using a wireless technology such as WiFi.

Further, it is desirable that the polarization information acquisition device has a correction processing unit. The correction processing means corrects the polarization information α using the correction coefficient. The correction coefficient may be acquired by, for example, injecting a known polarized light and comparing the measured angle α with the angle of the incident polarized light, as described above. Note that the correction coefficient does not have to be acquired each time shooting is performed. If a value acquired in advance is stored in the correction processing unit and the stored correction coefficient is read and used at the time of shooting. Good.

As the solid-state image sensor, for example, CMOS or CCD can be used. In addition, a filter (color filter) that limits the wavelength of light that passes through each pixel of the solid-state image sensor may be provided to obtain information on light in different bands for each pixel. In the case of acquiring information of light in a plurality of bands as described above, that is, in the case of providing a plurality of color channels, examples of color channels include, for example, red, green, blue, near infrared (RGBIR) and red, green, blue. , Near ultraviolet (RGBUV), or a combination thereof. The wavelength band used by the polarization acquisition element may correspond to the wavelength band of the color channel acquired by the solid-state image sensor. For example, in the case of RGBIR described above, the wavelength band used is about 400 nm to 1100 nm. When a plurality of color channels are provided, it is preferable that the correction means described above corrects α by using an optimum correction coefficient for each color channel.

In the above description, the polarization acquisition element is assumed to be in an ideal state, but in an actual element, the transmittance is lower than the ideal state due to the influence of surface reflection, absorption, scattering and the like. Therefore, if necessary, a process of measuring the transmittance of the polarization acquisition element in advance and correcting the deviation from the ideal state may be added to the correction means.

The polarization acquisition element 100 according to the first embodiment of the present invention includes a laminated ¼λ plate 10, a variable phase plate 20 and a polarizing plate 30, as shown in FIG. The laminated ¼λ plate 10 has a structure in which five layers of materials having uniaxial birefringence are laminated by changing the axial direction.

The variable phase plate of the present invention is made of VA type liquid crystal. The liquid crystal molecules can impart a predetermined phase difference to the transmitted light by controlling the orientation of the liquid crystal molecules according to the applied voltage applied to the upper and lower electrode layers.

Further, the polarizing plate of this example is an absorption type polarizing plate. When a polarizing plate such as a wire grid polarizer that reflects unnecessary light is used as the polarizing plate, the reflected unnecessary light becomes stray light or ghost light and adversely affects the image. Is not preferable. In order to prevent the light from becoming ghost light as much as possible, it is preferable that the polarizing plate has a characteristic of absorbing 50% or more of the polarized light vibrating in the direction orthogonal to the transmission axis with respect to the used wavelength range.

Here, FIG. 8 shows the axial direction and phase difference of each layer (layer 1, layer 2,... Layer 5 from the light incident side) constituting the laminated 1/4λ plate. The phase difference is shown at 600 nm. 9 shows the phase difference Δ(λ) with respect to the wavelength λ of the laminated ¼λ plate 10, and FIG. 10 shows the axial direction θ(λ) with respect to the wavelength λ of the laminated ¼λ plate 10. The phase difference Δ(λ) and the axial direction θ(λ) of the laminated 1/4λ plate 10 were calculated using the equation (1).

It can be seen from FIG. 9 that the laminated ¼λ plate 10 of this example satisfies |Δ(λ)−0.25|<0.02 at 400 nm to 1200 nm. Further, from FIG. 10, for an arbitrary wavelength λ of 400 nm to 1200 nm,
2(deg)<|θ(λ)−90|<5(deg)
It turns out that From this, it is understood that the polarization acquisition element of the present invention can accurately acquire polarization information with respect to the used wavelength of 400 to 1200 nm.
[Comparative Example 1]
The polarization acquisition element of Comparative Example 1 of the present invention has the same configuration as the polarization acquisition element of Example 1 except for the ¼λ plate. The ¼λ plate of this comparative example (schematically shown in FIG. 21) is a wave plate having a single-layer structure made of the same birefringent material as the material forming each layer of the laminated 1/4λ plate of Example 1. Similar to the first embodiment, the phase difference Δ(λ) and the axial direction θ(λ) of the ¼λ plate are shown in FIGS. 22 and 23. From FIG. 23, in the ¼λ plate of this comparative example, θ(λ)=0 at 400 to 1200 nm, and no axial deviation occurs. On the other hand, from FIG. 22, it can be seen that |Δ(λ)−0.25|>0.1 and the phase difference shift increases. Normally, in a wave plate composed of a single layer of a material having wavelength dispersion, the change in the phase difference with respect to the wavelength becomes large as shown in FIG. Therefore, when acquiring the polarization information in a wide wavelength range as in the present invention, the accuracy of acquiring the polarization information decreases, which is not suitable.
[Comparative example 2]
The polarization acquisition element of Comparative Example 2 of the present invention also has the same configuration as the polarization acquisition element of Example 1 except for the ¼λ plate. The ¼λ plate of this comparative example has a structure in which two kinds of materials having different wavelength dispersions are laminated so that the angles formed by their axes are perpendicular to each other. The structure of the wave plate of this comparative example is shown in FIG. Further, similar to Example 1 and Comparative Example 1, the phase difference Δ(λ) and the axial direction θ(λ) of the ¼λ plate are shown in FIGS. 25 and 26. From FIG. 26, the 1/4 λ plate of this comparative example has θ(λ)=0 at 400 to 1200 nm as in Comparative example 1, and no axial deviation occurs. On the other hand, from FIG. 25, |Δ(λ)−0.25|<0.05 is obtained in the range of 400 to 750 nm, and it is understood that the polarization information can be acquired with high precision in this wavelength range. However, when the wavelength range exceeds 800 nm, |Δ(λ)−0.25|>0.05, and in the range of 400 to 1200 nm, |Δ(λ)−0.25|>0.10, which is a phase difference shift. It can be seen that According to this comparative example, a wavelength plate in which two layers of different birefringent materials are laminated so that their axial directions are perpendicular to each other can obtain polarization information with sufficient accuracy in a wavelength range where λmax/λmin<2. It can be seen that the acquisition accuracy deteriorates in a wide band exceeding λmin>2.

Example 2 of the present invention An image pickup apparatus and a polarization information acquisition apparatus will be described.

First, an outline of the image pickup apparatus 1000 is shown in FIG.

The image pickup apparatus 1000 of the present invention includes an interchangeable lens 101, a polarization acquisition filter 100, and a digital camera 102. The polarization acquisition filter 100 includes the above-mentioned polarization acquisition element, and the digital camera 102 includes a CMOS sensor in which pixels are two-dimensionally arranged.

In order to acquire the polarization information, the polarization acquisition filter 100 is installed on the object side (light incident side) of the digital camera 102. The polarization acquisition filter 100 of the present embodiment is provided with a mechanism that can be attached to and detached from the interchangeable lens 101 and the digital camera 102, and is arranged between the optical lens 101 and the digital camera 102 or on the object side of the optical lens 101. As described above, by having the detachable mechanism, it can be used in combination with various interchangeable lenses 101 and digital cameras 102 having a common detachable mechanism.

The solid-state image sensor of the present invention has four color channels (blue, green, red, near infrared). FIG. 12 shows an example of a pixel array in the solid-state image sensor of the digital camera 102 of the present invention. Note that R, G, B, and IR in FIG. 12 correspond to red, green, blue, and near-infrared wavelength regions, respectively. A color polarization image is acquired by acquiring an image with the imaging device of the present embodiment.

The polarization information acquisition device of the present invention calculates polarization information A, B, and α by an image processing unit (not shown) using the acquired color polarization image. The polarization information is calculated for each color channel and each pixel. Further, the polarization information acquisition device of the present invention has a correction processing means. The correction processing means corrects α of the polarization information calculated based on the correction coefficients δα _R , δα _G , δα _B , and δα _IR corresponding to each color channel.

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 can be made within the scope of the gist thereof.

10 laminated 1/4λ plate, 20 variable phase plate, 30 polarizing plate,
100 polarization acquisition element, 101 interchangeable lens, 102 digital camera,
1000 imaging device

Claims (11)

A polarization acquisition element for acquiring different polarization states,
The maximum wavelength λmax and the minimum wavelength λmin in the operating wavelength range are
λmax/λmin>2
When satisfying,
A laminated type 1/4 λ plate in which a material having birefringence is laminated by shifting the fast axis or the slow axis in the in-plane direction,
A variable phase plate that can change the phase difference into two or more values,
And a polarizing plate,
The angle formed by the fast axis or slow axis of the variable phase plate and the transmission axis of the polarizing plate is arranged at about 45 degrees,
Polarized light characterized in that an angle formed by a fast axis or a slow axis of the variable phase plate and a fast axis or a slow axis at a specific wavelength of the laminated type ¼λ plate is arranged at about 45 degrees. Acquisition element. The in-plane retardation Δ(λ) of the laminated type ¼λ plate is |Δ(λ)−0.25|<0.05 with respect to an arbitrary wavelength λ within the used wavelength range.
The polarized light acquisition element according to claim 1, wherein The angle θ(λ) of the fast axis of the laminated ¼λ plate is 1 (deg)<|θ(λ)−θ ₀ |<12 (deg) with respect to an arbitrary wavelength λ within the wavelength range used.
The polarized light acquisition element according to claim 1 or 2, characterized in that:
However, θ ₀ is a design value of the fast axis. The polarization acquisition element according to any one of claims 1 to 3, wherein the laminated ¼λ plate has five or more layers. A phase difference Δa (nm) between a layer arranged on the most variable phase plate side and a layer arranged on the farthest side from the variable phase plate side among the layers constituting the laminated type ¼λ plate is
The polarization acquisition element according to any one of claims 1 to 4, which satisfies the following condition. The phase difference Δb of the layers excluding the layer arranged closest to the variable phase plate and the layer arranged closest to the variable phase plate among the layers constituting the laminated ¼λ plate has
The polarization acquisition element according to any one of claims 1 to 5, which satisfies the following condition. The maximum wavelength λmax and the minimum wavelength λmin are
λmin<430 (nm)
λmax>850 (nm)
The polarization acquisition element according to any one of claims 1 to 6, which satisfies the following condition. Optical system,
A polarization acquisition element according to any one of claims 1 to 7,
A solid-state image sensor,
An imaging device comprising: An image pickup apparatus according to claim 8;
Image processing means,
Have
The polarization information acquisition device, wherein the image processing means calculates polarization information A, B, and α from images acquired in three or more different states of the polarization acquisition element.
However, A is the maximum light intensity, B is the minimum light intensity, and α is the angle having the maximum light intensity. Furthermore, it has a correction processing means,
The polarization information acquisition apparatus according to claim 9, wherein the correction processing unit corrects the α using a correction coefficient. 11. The polarization information acquisition device according to claim 10, wherein α is corrected for each color channel.