JP2020187153A - Optical low-pass filter and imaging device - Google Patents

Abstract

To provide an optical low-pass filter formed of a two-layer optical anisotropic element that can reduce occurrence of a false color and moire evenly in two oblique directions while maintaining the resolution in a high spacial frequency in a vertical or horizontal direction.SOLUTION: An optical low-pass filter 6 comprises first and second optical anisotropic elements 1 and 2 arranged sequentially from a light incidence side to a light projection side and each separating an incident light beam to an ordinary ray and an extraordinary ray. In a direction view perpendicular to an optical plane of the first and second optical anisotropic elements, a first extraordinary light beam is separated from the incident light beam at the first optical anisotropic element. The bending angle θe of a second extraordinary light beam separated in such a way that it is bent from the first extraordinary light beam at the second optical anisotropic element satisfies the condition: 100°≤θe≤130°.SELECTED DRAWING: Figure 2

Description

The present invention relates to an optical low-pass filter suitable for an imaging device.

An optical low-pass filter is used in an image pickup device that uses a solid-state photographing element such as a CCD sensor or a CMOS sensor in order to suppress the occurrence of false color and moire in the captured image. The optical low-pass filter limits image information at high frequencies above the Nyquist frequency by controlling the distribution of point images formed by light.

Patent Document 1 is configured in two layers by using a birefringent optical element that separates light rays in the horizontal (horizontal) direction and a birefringent optical element that separates light rays in the vertical (vertical) direction, and one incident light beam. An optical low-pass filter that separates the above into four is disclosed. In a general imaging device, when the separation width of the light rays of each birefringent optical element is a [μm], the contrast disappears at the cutoff spatial frequency 1 / (2a). The separation width a is set so that the cutoff spatial frequency is close to the Nyquist spatial frequency of the image sensor.

On the other hand, Patent Document 2 discloses an optical low-pass filter in which the angle formed by the light ray separation directions of the two layers of birefringent optical elements that separate the light rays is 60 °. In this optical low-pass filter, a vertically and horizontally symmetrical MTF is realized by making the four point images formed by the four separated light rays symmetrical in the direction of an oblique 45 °.

Japanese Unexamined Patent Publication No. 10-054960 Japanese Unexamined Patent Publication No. 2004-126384

In the optical low-pass filter disclosed in Patent Document 1, the cutoff space frequency in the diagonal direction is √2 times the cutoff space frequency in the vertical and horizontal directions. However, it is desirable to cut at a lower frequency because moiré in the diagonal direction appears as a false color. On the other hand, since moire in the vertical and horizontal directions can be regarded as a resolution, it is desirable to increase the cutoff spatial frequency as much as possible. Further, in the optical low-pass filter disclosed in Patent Document 1, a retardation plate as a birefringent plate optical element for depolarizing light is required between the two layers of birefringent plate optical elements.

On the other hand, in the optical low-pass filter disclosed in Patent Document 2, the cutoff spatial frequencies in the vertical direction and the horizontal direction are the same. However, the low-pass effect is different in the two diagonal directions. In particular, there is no cutoff spatial frequency in one of the two diagonal directions, and there is a possibility that strong color moiré will occur.

The present invention is composed of two layers of optically anisotropic elements so that false colors and moire can be reduced evenly in two diagonal directions while maintaining resolution up to a high spatial frequency in the vertical or horizontal direction. An optical low-pass filter is provided.

The optical low-pass filter as one aspect of the present invention has first and second optically anisotropic elements that are arranged in order from the light incident side to the light emitting side and separate the incident light rays into normal light rays and abnormal light rays, respectively. .. In a directional view orthogonal to the element planes of the first and second optically anisotropic elements, the first optically anisotropic element separates the first abnormal ray from the incident ray, and further, the second optical anisotropy. It is characterized in that the bending angle θe when the second abnormal light ray is separated so as to bend from the first abnormal light ray in the sex element satisfies the condition that 100 ° ≦ θe ≦ 130 °.

An image pickup device including the above-mentioned optical low-pass filter and an image pickup device that photoelectrically converts an optical image formed by light passing through the optical low-pass filter also constitutes another aspect of the present invention.

According to this embodiment, in an optical low-pass filter composed of two layers of optically anisotropic elements, false colors and false colors are evenly distributed in two diagonal directions while maintaining resolution up to a high spatial frequency in the vertical or horizontal direction. The occurrence of moire can be reduced.

The figure which shows the structure of the digital camera provided with the optical low-pass filter which is the Example of this invention. The figure which shows the structure of the optical low-pass filter of an Example. The figure which shows the direction of the optical axis of a uniaxial birefringent optical element and ray separation. The figure explaining the light ray separation of the optical low-pass filter of an Example and a comparative example, and the light ray intensity after the separation. The figure which shows the separation path of an abnormal light ray in Examples 1 to 3 of this invention. The figure which shows the point image distribution formed by the optical low-pass filter of Example 1. FIG. The figure which shows the spatial frequency characteristic of the 2D MTF of the optical low-pass filter of Example 1. FIG. The figure which shows the spatial frequency characteristic of MTF in each direction of the optical low-pass filter of Example 1. FIG. The figure which shows the point image distribution formed by the optical low-pass filter of Example 2. The figure which shows the spatial frequency characteristic of the 2D MTF of the optical low-pass filter of Example 2. The figure which shows the spatial frequency characteristic of MTF in each direction of the optical low-pass filter of Example 2. FIG. The figure which shows the point image distribution formed by the optical low-pass filter of Example 3. The figure which shows the spatial frequency characteristic of the 2D MTF of the optical low-pass filter of Example 3. FIG. The figure which shows the spatial frequency characteristic of MTF in each direction of the optical low-pass filter of Example 3. FIG. The figure which shows the point image distribution formed by the optical low-pass filter of the comparative example. The figure which shows the spatial frequency characteristic of the 2D MTF of the optical low-pass filter of the comparative example. The figure which shows the spatial frequency characteristic of MTF in each direction of the optical low-pass filter of a comparative example. The figure explaining the light ray separation of a general optical low-pass filter and the light ray intensity after the separation.

Hereinafter, examples of the present invention will be described with reference to the drawings. FIG. 1 shows the configuration of a digital camera 100 as an image pickup apparatus provided with an optical low-pass filter 6 according to an embodiment of the present invention. The image pickup light incident on the image pickup optical system 5 from a subject (not shown) passes through the optical low-pass filter 6 and reaches the image pickup element 7. The image sensor 7 photoelectrically converts (impresses) an optical image formed by the image pickup light that has passed through the optical low-pass filter 6. The optical low-pass filter 6 imparts a low-pass effect to the imaged light by controlling the distribution of the point image formed by the imaged light to limit the optical image information at a high frequency equal to or higher than the Nyquist frequency.

FIG. 2 shows the configuration of the optical low-pass filter 6. The optical low-pass filter 6 is arranged with the first birefringent optical element (first optically anisotropic element) 1 in this order from the object side (subject side or light incident side) to the image side (imaging element side or light emitting side). It has a configuration in which a second birefringent optical element (second optically anisotropic element) 2 is laminated. The first and second birefringent optical elements 1 and 2, respectively, separate incident light rays by their birefringence. In this embodiment, the optical low-pass characteristic is realized by the combination of the first and second birefringence optical elements 1 and 2.

The first and second birefringence optics 1 and 2 are rectangular with long and short sides, respectively (however, a part may be cut out so that the front and back sides and orientation of the birefringence optics can be seen). It is formed as a parallel flat plate. As shown in FIG. 2, the first direction (horizontal direction) in which the long side of each birefringent optical element (that is, the long side of the image sensor 7 shown in FIG. 1) extends is defined as the x-axis direction, and the short side extends. The second direction (vertical direction) is the y-axis direction. In the image sensor 7, the x-axis direction and the y-axis direction are pixel array directions, which coincide with the direction in which the long side and the short side of each birefringence optical element extend. In each birefringence optical element, the xy plane parallel to the x-axis and y-axis directions is referred to as an element surface (optical surface). The direction orthogonal to the element surface and the image pickup surface of the image pickup device 7 is called the optical axis direction. The thickness of the first and second birefringent optical elements 1 and 2 shown in FIG. 2 in the optical axis direction is actually about several hundred μm.

3 (a) and 3 (b) show the orientation of the optical axis of each birefringent optical element (1, 2). Each birefringent optical element is composed of uniaxial crystals such as quartz and lithium niobate. When each birefringent optical element is viewed from the optical axis direction as shown in FIG. 3 (a), the optical axis of the uniaxial crystal (indicated by both thick arrows) is φ = 0 ° (y-axis direction) with respect to the x-axis direction. It faces the direction of -90 °). Further, as shown in FIG. 3B, when each birefringent optical element is viewed from the y-axis direction, the optical axis of the uniaxial crystal is only an angle Ψ with respect to the optical axis direction (90 ° −Ψ with respect to the element surface). Only) leaning. In the following description, the angle Ψ is referred to as the tilt angle. The tilt angle Ψ is generally set in the range of 45 ° ± 20 °.

As shown in FIGS. 3C and 3D, the light rays incident on the birefringent optical element as a parallel flat plate whose optical axis is tilted with respect to the element surface are two rays, an ordinary ray and an abnormal ray. Is separated into. Specifically, as shown in FIG. 3C, the light rays 8 incident on the birefringent optical element from the optical axis direction are the normal light rays 9 that pass straight through the birefringent optical element and the abnormal light rays 10 that pass diagonally. Separate into and. The direction in which the normal ray 9 and the abnormal ray 10 are separated (the x-axis direction in FIG. 3C) is referred to as the ray separation direction.

The normal ray 9 is polarized light whose electric field oscillates in a direction orthogonal to the ray separation direction, and the abnormal ray 10 is polarized light whose electric field oscillates in a direction parallel to the ray separation direction. That is, the normal ray 9 and the abnormal ray 10 are linearly polarized light whose polarization directions are orthogonal to each other. The separation width (hereinafter referred to as the light ray separation width) a of the normal ray 9 and the abnormal ray 10 in the ray separation direction is the magnitude Δn of the refractive index anisotropy peculiar to the material of the birefringent optical element and the inclination angle Ψ of the optical axis. It is uniquely determined from the thickness d of the birefringent optical element (parallel flat plate). Therefore, if the material of the birefringence optical element and the inclination angle Ψ are determined, the light ray separation width a is proportional to the thickness d. By stacking such birefringence optical elements, point images separated into 2 Nth powers with respect to the number of layers N are formed on the image sensor 7.

In Specific Examples 1 to 3 described later, each birefringent optical element is represented by an inclination angle Ψ and a light ray separation width a.

With reference to FIGS. 18A to 18C, light ray separation by a general two-layer birefringent optical element will be described. ◯ in FIGS. 18A to 18C indicate light rays seen from the direction of the optical axis. The directions and lengths of the solid double-headed arrows indicate the polarization direction and intensity of the separated light rays, respectively.

As shown in FIG. 18 (a), the light ray 401 incident on the first birefringence optical element is a normal light ray 401'at the same position as the incident light ray 401 as shown in FIG. It is separated from the abnormal light ray 402 that deviates in the 0 ° direction (right direction in the figure), which is the light ray separation direction of the birefringence optical element of 1. When the incident ray 401 is an unpolarized ray, the intensity of the separated normal ray 401'and the anomalous ray 402 are equal to each other, and each has half the intensity of the incident ray 401.

Next, as shown in FIG. 18C, when the light ray 401'and the light ray 402 are incident on the second birefringence optical element, the light ray 401'is the ordinary ray 401 ″ and the second birefringence optical element. It is separated into an abnormal ray 403 that deviates in the 45 ° direction (upper right direction in the figure), which is the ray separation direction. Further, the ray 402 is a normal ray 402'and an abnormal ray 404 that also deviates in the 45 ° direction. In this way, the incident ray 401 is separated into four rays 401 ″, 402 ′, 403, 404. The intensities of 401 ″, 402 ′, 403, and 404 after separation are equal to each other, and each has an intensity of 1/4 that of the incident light ray 401.

On the other hand, in this embodiment, since the light ray separation direction of the second birefringence optical element is different from the 45 ° direction, the light intensity after separation is different from each other. Here, the case where the light ray separation direction of the second birefringence optical element is the 30 ° direction will be described with reference to FIGS. 4A to 4C. Also in FIGS. 4A to 4C, ◯ indicates a light ray viewed from the optical axis direction, and the direction and length of the solid double-headed arrow indicate the polarization direction and intensity of the light ray after separation, respectively.

As shown in FIG. 4 (a), the light ray 501 incident on the first birefringence optical element is a normal light ray 501'at the same position as the incident light ray 501 as shown in FIG. It is separated into an abnormal ray (first abnormal ray) 502 that deviates in the 0 ° direction (right direction in the figure), which is the ray separation direction of the birefringent optical element 1. When the incident ray 501 is an unpolarized ray, the intensity of the separated normal ray 501'and the anomalous ray 502 are equal to each other, and each has half the intensity of the incident ray 501.

Next, as shown in FIG. 4C, when the light ray 501'and the light ray 502 are incident on the second birefringent optical element, the light ray 501'is the ordinary ray 501 ″ and the second birefringent optical element. It is separated into an abnormal ray 503 that deviates in the ray separation direction of 30 ° (upper right direction in the figure). At this time, the ray separation direction of the second birefringence optical element is set with respect to the polarization direction of the ray 501'. Since it is tilted by 60 °, the intensities of the normal light ray 501 ″ and the abnormal light ray 503 after separation are different from each other. When the normal ray 501'before separation is represented by a vector (double arrow of a broken line), the vector is separated into two orthogonal line segments in the ray separation direction. That is, the normal ray 501 "and the abnormal ray 503 are separated by an amplitude ratio of sin60 ° (= √3 / 2): cos60 ° (= 1/2), which is the square of the amplitude ratio 3/4: 1. Separated at a strength ratio of / 4.

On the other hand, the light ray 502 shown in FIG. 4B is an abnormal light ray (third) deviated from the normal light ray 502'in the direction of 30 ° by the second birefringence optical element as shown in FIG. 4C. 2 abnormal light rays) 504 and separated. At this time, since the ray separation direction of the second birefringence optical element is tilted by 30 ° with respect to the polarization direction of the ray 502, the intensities of the separated ordinary ray 502'and the abnormal ray 504 are different from each other. When the ray 502 before separation is represented by a vector (double arrow of a broken line), the vector is separated into two orthogonal line segments in the ray separation direction. That is, the normal ray 502'and the abnormal ray 504 are separated by an amplitude ratio of sin30 ° (= 1/2): cos30 ° (= √3 / 2), which is the square of the amplitude ratio of 1/4: 3. Separated at a strength ratio of / 4.

Of the four rays 501 ″, 503, 502 ′, 504 after being separated so as to be located at the apex of the rhombus in this way, the ray 501 ″ located at the diagonal apex of the rhombus with the longer diagonal line. And the light ray 504 have an intensity of 3/8 with respect to the intensity of the incident light ray 501. Further, the light rays 503 and the light rays 502'located at the diagonal vertices having the shorter diagonal line have an intensity of 1/8 with respect to the intensity of the incident light rays 501.

In this embodiment, the arrangement of the first and second birefringence optical elements is defined using the angle θe as shown in FIG. 4 (d). With respect to the angle θe, in the optical axis direction view (direction view orthogonal to the element surface), the first anisotropy ray is separated from the incident light ray by the first optically anisotropic element, and the second optically anisotropic element is further separated. This is the bending angle when the second abnormal light ray is separated so as to bend from the first abnormal light ray. In other words, it is the bending angle of the separation path in which the abnormal light rays are separated by the first and second birefringent optical elements in the optical axis direction.

In FIG. 4D, a part of the incident light ray 501 on the first birefringent optical element is separated into the abnormal light ray 502 in the 0 ° direction, which is the ray separation direction, by the first birefringent optical element. .. Further, the light ray 502 is separated into an abnormal light ray 504 in the light ray separation direction of 30 ° by the second birefringence optical element. The bending angle of this separation path is θe = 120 °. The bending angle θe is expressed in the range of 0 ° ≦ θe ≦ 180 °.

4 (e) to 4 (h) show that the positions of the light rays after separation by the first and second birefringence optical elements are the same as those in FIG. 4 (c), but the separation path and the bending angle θe are shown in FIG. 4 (e). The case different from d) is shown. Θe shown in FIG. 4H is 30 °.

As shown in FIG. 4 (e), the light ray 601 incident on the first birefringent optical element is a normal light ray 601'at the same position as the incident light ray 601 as shown in FIG. It is separated into an abnormal ray 602 that deviates in the 180 ° direction (left direction in the figure), which is the ray separation direction of the birefringent optical element. When the incident ray 601 is an unpolarized ray, the intensity of the separated normal ray 601'and the anomalous ray 602 are equal to each other, and each has half the intensity of the incident ray 601.

Next, as shown in FIG. 4 (g), when the light rays 601'and the light rays 602 are incident on the second birefringence optical element, the light rays 601' are ordinary rays 601 ″ and the second birefringence optical element. It is separated into an abnormal ray 603 that deviates in the 30 ° direction (upper right direction in the figure), which is the ray separation direction. At this time, as in FIG. 4 (c), the normal ray 601 ″ and the abnormal ray 603 are 3/4: Separated at a strength ratio of 1/4.

On the other hand, the light ray 602 shown in FIG. 4 (f) is a normal light ray 602'by the second birefringence optical element and an abnormal light ray 604 displaced in the 30 ° direction with respect to the normal light ray 602'as shown in FIG. 4 (g). Is separated into. At this time, similarly to FIG. 4C, the normal ray 602'and the abnormal ray 604 are separated at an intensity ratio of 1/4: 3/4.

Of the four rays 601 ″, 603, 602 ′, 604 after being separated so as to form the apex of the rhombus, the ray 601 ″ located at the diagonal apex of the rhombus with the shorter diagonal line. And the ray 604 have an intensity of 3/8 with respect to the intensity of the incident ray 601. Further, the ray 603 and the ray 602'located at the diagonal apex having the longer diagonal line have an intensity of 1/8.

As described above, due to the difference in the bending angle θe described above, the relationship of intensity is different even if the positions of the four light rays after separation are the same. Therefore, in order to obtain a desired intensity relationship (intensity distribution) for the four separated light rays, it is necessary to appropriately set the bending angle θe. Specifically, the bending angle θe is
100 ° ≤ θe ≤ 130 ° (1)
It is desirable to satisfy the above conditions. Further, it is desirable that the light ray separation widths of the first and second optically anisotropic elements are equal to each other.

Further, it is more desirable that the bending angle θe satisfies the following equation (1)'.
110 ° ≤ θe ≤ 128 ° (1)'
Further, it is more desirable that the bending angle θe satisfies the following equation (1) ″.
115 ° ≤ θe ≤ 125 ° (1) ″
Further, it is desirable that the direction θ0 of the bisector of the bending angle θe is parallel to either the horizontal direction or the vertical direction, which are the two pixel arrangement directions orthogonal to each other in the image sensor 7. As a result, the imaging surface phase-difference AF described later can be performed satisfactorily.

5 (a) to 5 (d) show the separation paths of abnormal light rays by the first and second birefringent optical elements 1 and 2 in the optical low-pass filter according to the first embodiment when viewed from the optical axis direction. Table 1 shows a numerical example 1 corresponding to the first embodiment.

As shown in FIG. 5A, the light ray separation direction of the first birefringence optical element 1 is 60 ° with respect to the x-axis direction. Further, as shown in FIG. 5B, the light ray separation direction of the second birefringence optical element 2 is a direction of 120 ° with respect to the x-axis direction.

As shown in FIG. 5 (c), the incident light ray is separated into an ordinary ray 501'and an abnormal ray 502 (the thin double arrow indicates the polarization direction of each ray) in the first birefringent optical element 1, and then anomalies. As shown in FIG. 5D, the light ray 502 is separated into an ordinary light ray 502'and an abnormal light ray 504 by the second birefringence optical element 2. As a result, the bending angle θe of the separation path of the abnormal rays 502 and 504 becomes 120 °.

FIG. 6 shows the arrangement and intensity of four point images formed by the four light rays separated by the optical low-pass filter of Numerical Example 1. The size (area) of the point image in FIG. 6 indicates the intensity of the point image. That is, the larger the size of the point image, the higher the intensity.

The four point images are arranged at the vertices of a rectangle (diamond) whose diagonal line in the y-axis direction is longer than that in the diagonal line in the x-axis direction. The length of the diagonal line in the y-axis direction is √3 times the length of the diagonal line in the x-axis direction. The strength of the two point images located at the diagonal vertices in the x-axis direction is three times as strong as that of the two point images located at the diagonal vertices in the y-axis direction. Since the light ray separation width is narrow in the x-axis direction, the intensity of the point image after separation is high, which cancels out the intensity difference of the low-pass filter effect with the y-axis direction.

FIG. 7 shows a two-dimensional MTF of the four point images shown in FIG. The horizontal axis and the vertical axis indicate the MTF standardized by the Nyquist frequency of the image sensor 7. The contour lines indicating MTF 0.8 are substantially circular in the central portion of FIG. 7, and the intensities of the low-pass filter effects are equal to each other in the y-axis direction and the x-axis direction in the low spatial frequency region.

Further, the cutoff space frequency exists up to about 60 ° in the x-axis direction, and this cutoff space frequency is about 1.5 times the Nyquist frequency. There is no cutoff spatial frequency in the y-axis direction. In order to reduce the intensity difference of the low-pass filter effect in the x-axis direction and the y-axis direction, parameters such as the light ray separation width and the thickness of each birefringent optical element may be adjusted while checking the MTF. However, the inventor empirically knows that the standard deviations σx and σy of a plurality of point images in the x-axis direction and the y-axis direction can be compared without calculating the MTF one by one. By adjusting the parameters so that the standard deviations σx and σy of the point image obtained by weighting with intensity are σx = σy in the x-axis direction and the y-axis direction, the low-pass in the x-axis direction and the y-axis direction is performed. The intensity difference of the filter effect can be reduced.

Even when σx and σy are different from each other, it is preferable that σx / σy satisfies the condition of the following equation (2).

1 / 1.4 (= 0.7) ≤ σx / σy ≤ 1.4 (2)
By satisfying the condition of equation (2), it is possible to use σx and σy for adjusting the MTF in the high frequency range while suppressing the difference between the MTF in the x-axis direction and the y-axis direction in the low frequency range within an allowable range. it can. Table 1 shows the standard deviations σx, σy and their ratios σx / σy in Numerical Example 1.

FIG. 8A shows the MTFs in the x-axis direction and the y-axis direction among the two-dimensional MTFs shown in FIG. 7, and FIG. 8B shows the MTFs in the + 45 ° and −45 ° directions as two diagonal directions. Is shown. In the low frequency range, the MTF in the x-axis direction and the MTF in the y-axis direction coincide with each other, and the MTF in the ± 45 ° direction is equal to each other.

In recent image processing, false colors in the oblique direction tend to be reduced by an optical low-pass filter to allow luminance moiré in the x-axis and y-axis directions. Therefore, it is important that the low-pass filter effect in the diagonal direction is high. Further, some recent digital cameras perform AF (autofocus) by the imaging surface phase difference detection method. In this imaging surface phase-difference AF, a plurality of pixels for phase difference detection that divide the exit pupil of the imaging optical system and output a pair of signals are provided in the image sensor that images the subject image, and the phase difference of the paired signals. Focusing of the imaging optical system is performed according to the defocus amount calculated from. In the imaging surface phase-difference AF, it is important that the low-pass filter effect in the division direction (phase difference detection direction) of the exit pupil is high. Therefore, it is desirable that the direction of the bisector of the bending angle θe is parallel to the horizontal direction or the vertical direction, which is often the phase difference detection direction.

Moire occurs when the subject to be focused has a repeating pattern with a spatial frequency higher than the Nyquist frequency of the image sensor. At this time, the strength of the moire formed by the light rays passing through the divided pupil regions does not always occur at the same position. Then, there is a possibility that the laterality of the moiré pattern that happens to occur is erroneously detected as defocus. Therefore, it is necessary to suppress moire by giving a high low-pass filter effect in the phase difference detection direction of the imaging surface phase difference AF.

The optical low-pass filter of this embodiment can obtain a strong low-pass filter effect in the x-axis direction corresponding to the lateral direction when the phase difference is detected in the lateral direction in the image plane phase difference AF. On the other hand, in the y-axis direction corresponding to the vertical direction, a constant MTF is left, and although it is a false resolution, a feeling of resolution can be left, which is preferable.

The above-mentioned advantages of the imaging surface phase-difference AF can be obtained particularly in a digital camera using an image sensor having a number of pixels exceeding 10 million pixels. That is, the pixel pitch p [μm] in the pixel array direction orthogonal to each other in the image sensor in which the color filters are provided in the Bayer array is determined.
1.0 ≤ p ≤ 7.0 (3)
It is obtained in a digital camera that satisfies the above conditions. When the pixel pitch is larger than the upper limit of the equation (3), the number of pixels of the image sensor is small and the performance of the imaging optical system is relatively high. Therefore, the meaning of the optical low-pass filter satisfies the equation (3). Will be different from. On the other hand, when the pixel pitch is less than the lower limit of the equation (3), the optical low-pass filter as in this embodiment is not required because the resolving power of the imaging optical system is insufficient. In the numerical example 1, the pixel pitch p is set to 5 μm.

Further, when the light ray separation width of the first and second optically anisotropic elements 1 and 2 is a [μm],
0.9 ≤ p / a ≤ 1.5 (4)
It is preferable to satisfy the above conditions. In this case, the light ray separation width a is preferably adjusted in the range of 3.33 [μm] to 5.55 [μm]. Therefore, it is preferable to use an image sensor having a pixel pitch p of about 3 [μm] to 5 [μm]. The larger the light ray separation width a, the stronger the low-pass filter effect, and the MTF can be reduced from the lower frequency range. On the other hand, the smaller the light ray separation width a, the weaker the low-pass filter effect, and the MTF remains up to a higher frequency range.

When an image sensor having a small pixel pitch that is lower than the lower limit of the formula (4) is used, the MTF of the image pickup optical system itself is insufficient, so that the optical low-pass filter itself becomes unnecessary. On the other hand, when an image sensor having a large pixel pitch that exceeds the upper limit of the equation (4) is used, moire cannot be visually recognized as a sense of resolution, and there is a possibility that it may be perceived as a mere adverse effect. In this case, a low-pass filter effect stronger than the low-pass filter effect obtained by this example and other examples described later is required.

The first and second birefringent optical elements 1 and 2 shown in FIGS. 5A and 5B are the same birefringent optical elements whose front and back sides are inverted around the x-axis, and in order to make the front and back sides easy to understand. It has a shape in which diagonal notches are provided at one of the four corners of the rectangle. This eliminates the need to use two different types of birefringent optical elements.

Next, the optical low-pass filter of the second embodiment will be described. Table 1 shows a numerical example 2 corresponding to this embodiment. The optical low-pass filter of Example 2 (Numerical Example 2) has the same ray separation width a and bending angle θe of the separation path as the low-pass filter of Example 1, and the direction in which the bisector of the bending angle θe extends is the x-axis. The direction is 90 ° with respect to the direction. Further, in this embodiment as well, the pixel pitch p of the image sensor is set to 5 μm as in the first embodiment.

In this embodiment as well, as in the first embodiment, the incident light rays on the optical low-pass filter are separated into four light rays by the first and second birefringent optical elements. The four point images formed by these four light rays are shown in FIG. In FIG. 9, an abnormal ray (first abnormal ray) is separated from an incident ray incident on the first birefringence optical element at a position on the right side in the drawing at an upper position. Further, the abnormal light ray (second abnormal light ray) is separated from the abnormal light ray by the second birefringent optical element at the position on the left side. The bending angle θe of the separation path of the abnormal light beam is 120 ° as in Example 1.

On the other hand, from the ordinary light rays that have passed through the first birefringent optical element, the abnormal light rays are separated into the lower position by the second birefringent optical element. As a result, the four point images are located at the vertices of the oblique shape (diamond). In this rhombus, the length of the diagonal line in the x-axis direction is longer than the length of the diagonal line in the y-axis direction. Then, as in the first embodiment, the two point images located at the diagonal vertices with the shorter diagonal length (y-axis direction) each have an intensity of 3/8 of the incident light ray, and the diagonal length The two point images located at the diagonal vertices on the longer side (x-axis direction) each have an intensity of 1/8 of the incident light ray.

As shown in Table 1, the standard deviations σx and σy of the point images in the x-axis direction and the y-axis direction are σx = σy. Since the light ray separation width is narrow in the y-axis direction, the intensity of the point image after separation is high, which cancels out the difference in intensity of the low-pass filter effect with the x-axis direction.

FIG. 10, similar to FIG. 7, shows a two-dimensional MTF of the four point images shown in FIG. Further, FIGS. 11 (a) and 11 (b) similar to FIGS. 8 (a) and 8 (b) show the MTF in the x and y axis directions and the MTF in the ± 45 ° direction among the two-dimensional MTFs shown in FIG. 11, respectively. Is shown. The contour lines indicating MTF 0.8 are substantially circular in the central portion of FIG. 10, and the intensities of the low-pass filter effects are equal to each other in the y-axis direction and the x-axis direction in the low spatial frequency region.

Further, in FIG. 11A, the MTF in the x-axis direction and the MTF in the y-axis direction coincide with each other in the low frequency region. In FIG. 11B, the MTFs in the ± 45 ° direction are equal to each other and have a cutoff spatial frequency of about 1.4 times the Nyquist frequency. In FIG. 11A, the cutoff spatial frequency is about 1.4 times the Nyquist frequency in the y-axis direction, but the MTF is not completely reduced in the x-axis direction, and the contrast of about 50% remains. ing.

The optical low-pass filter of this embodiment is preferably used in a digital camera in which the phase difference detection direction in the imaging surface phase difference AF is the y-axis direction or a digital camera in which the phase difference detection direction is both the x and y-axis directions.

Next, the optical low-pass filter of Example 3 will be described. Table 1 shows a numerical example 3 corresponding to this embodiment. The optical low-pass filter of Example 3 (Numerical Example 3) differs from the low-pass filter of Example 1 in the ray separation width and the bending angle θe of the division path. The direction in which the bisector of the bending angle θe extends is the direction of 0 ° with respect to the x-axis direction, as in the first embodiment. Further, in this embodiment as well, the pixel pitch p of the image sensor is set to 5 μm as in the first embodiment.

In this embodiment as well, as in the first embodiment, the incident light rays on the optical low-pass filter are separated into four light rays by the first and second birefringent optical elements. The four point images formed by these four light rays are shown in FIG.

In FIG. 12, an abnormal ray (first abnormal ray) is separated from an incident light ray incident on the first birefringent optical element at a lower position in the drawing at a right position. Further, the abnormal light ray (second abnormal light ray) is separated from the abnormal light ray by the second birefringent optical element at the upper position. The bending angle θe of the separation path of the abnormal light beam is 110 °.

On the other hand, from the ordinary light rays that have passed through the first birefringence optical element, the abnormal light rays are separated to the left position by the second birefringence optical element. As a result, the four point images are located at the vertices of the oblique shape (diamond). In this rhombus, the length of the diagonal line in the y-axis direction is longer than the length of the diagonal line in the x-axis direction. Then, as in the first embodiment, the two point images located at the diagonal vertices with the shorter diagonal length (x-axis direction) each have an intensity of 3/8 of the incident light ray, and the diagonal length The two point images located at the diagonal vertices on the longer side (y-axis direction) each have an intensity of 1/8 of the incident light ray.

In this embodiment, the standard deviations σx and σy of the point images in the x-axis direction and the y-axis direction are different from each other, and σx / σy = 0.74. That is, the low-pass filter effect in the x-axis direction is weaker than the low-pass filter effect in the y-axis direction.

FIG. 13, similar to FIG. 7, shows a two-dimensional MTF of the four point images shown in FIG. Further, FIGS. 14 (a) and 14 (b) similar to FIGS. 8 (a) and 8 (b) show the MTF in the x and y axis directions and the MTF in the ± 45 ° direction among the two-dimensional MTFs shown in FIG. 12, respectively. Is shown. As shown in FIG. 14A, the MTF values differ from each other in the x-axis direction and the y-axis direction in the low frequency range. As described above, in this embodiment, the low-pass filter effect in the x-axis direction is weak, so that the MTF in the x-axis direction is slightly higher than the MTF in the y-axis direction. At about 1.3 times the Nyquist frequency, the MTFs in the x-axis direction and the y-axis direction coincide with each other, and the MTF in the y-axis direction is 40% or less, so that the low-pass filter effect in the x-axis direction and the y-axis direction is effective. The balance as a whole is good.

In the above Examples 1 to 3, the birefringent optical element is represented by the separation width, but in general, the birefringent optical element is represented by the thickness of the birefringent optical element with the tilt angle Ψ of the optical axis fixed. I often do it. As the tilt angle Ψ of the optical axis, 45 °, which maximizes the light ray separation width with respect to the thickness, is often used. However, in this embodiment, an inclination angle Ψ other than 45 ° may be used within the range of the following equation (5).
25 ° ≤ Ψ ≤ 40 ° (5)
Table 2 shows the thickness d of the birefringent optical element in each embodiment when the inclination angles Ψ are 45 °, 40 °, and 35 °. When the inclination angle Ψ is 40 ° or 35 °, the thickness d is larger than when it is 45 °, but the inclination of the optical axis is shallow, so the birefringence optical element is cut out from the uniaxial crystal material (bulk). The height can be lowered, and a low-priced bulk can be used. However, although the required bulk height decreases as the inclination angle Ψ becomes shallower, it becomes necessary to increase the thickness of the birefringent optical element in order to reduce the light ray separation width, which in turn increases the cost. Therefore, it is preferable to set the inclination angle Ψ in the range of the equation (5), and more preferably in the range of 30 ° or more and 40 ° or less.
<Comparison example>
Next, as a comparative example, the optical low-pass filter disclosed in Patent Document 2 will be described. Here, too, the pixel pitch p of the image sensor is set to 5 [μm]. Further, for comparison with the examples, the entire four point images disclosed in Patent Document 2 are rotated so as to be symmetrically arranged in the x-axis direction and the y-axis direction, and the MTF in the 45 ° direction is the example. Is equal to. Table 3 shows numerical examples of the optical low-pass filter disclosed in Patent Document 2.

FIG. 15 shows the arrangement and intensity of a point image formed by light rays separated by an optical low-pass filter disclosed in Patent Document 2. In FIG. 15, when an incident light ray is incident on the first birefringent optical element at a position on the left side, the abnormal light ray (first abnormal light ray) is separated into an upper position by the first birefringent optical element. Further, the abnormal light ray (second abnormal light ray) is separated from the abnormal light ray by the second birefringent optical element at the position on the right side. At this time, the bending angle θe of the separation path of the abnormal light beam is 60 °.

On the other hand, the abnormal light beam is separated from the normal light ray passing through the first birefringence optical element to the lower position by the second birefringence optical element. As a result, the four point images are located at the vertices of the oblique shape (diamond). In this rhombus, the length of the diagonal line in the x-axis direction is longer than the length of the diagonal line in the y-axis direction.

Then, in this comparative example, unlike Examples 1 to 3, the two point images located at the diagonal vertices of the longer diagonal length (y-axis direction) each have an intensity of 3/8 of the incident light ray. The two point images located at the diagonal vertices with the shorter diagonal length (x-axis direction) each have an intensity of 1/8 of the incident light ray. As described above, in this comparative example, the arrangement of the four point images is the same as in Examples 1 to 3, but the intensity relationship of these point images is different from Examples 1 to 3.

Further, in this comparative example, the ratio of the standard deviations of the point images in the x-axis direction and the y-axis direction is σx / σy = 0.333, and σy is considerably larger than σx. Therefore, the effect of canceling the strength difference of the low-pass filter effect in the x-axis direction and the y-axis direction cannot be obtained, and the difference in MTF in the y-axis direction with respect to the x-axis direction becomes remarkable.

FIG. 16, similar to FIG. 7, shows a two-dimensional MTF of the four point images shown in FIG. Further, FIGS. 17 (a) and 17 (b) similar to FIGS. 8 (a) and 8 (b) show the MTF in the x and y axis directions and the MTF in the ± 45 ° direction among the two-dimensional MTFs shown in FIG. 16, respectively. Is shown. As shown in FIG. 17B, the MTFs in the ± 45 ° direction are equal to each other, but as shown in FIG. 17A, the MTF values are significantly different between the x-axis direction and the y-axis direction. In particular, in the low frequency region, a difference of about 3 times in MTF occurs, and resolution deterioration in the y-axis direction is not acceptable. Therefore, the low-pass filter effect equivalent to that of the optical low-pass filters of Examples 1 to 3 cannot be obtained only by rotating the optical low-pass filter of Patent Document 2.

Each of the above-described examples is only a representative example, and various modifications and changes can be made to each of the examples in carrying out the present invention.

1 First birefringence optical element (first optically anisotropic element)
2 Second birefringence optical element (second optically anisotropic element)
6 Optical low-pass filter

Claims (10)

An optical low-pass filter that is arranged in order from the light incident side to the light emitting side and has first and second optically anisotropic elements that separate the incident light rays into normal light rays and abnormal light rays.
In the direction view orthogonal to the optical plane of the first and second optically anisotropic elements, the first abnormal ray is separated from the incident ray by the first optically anisotropic element, and further, the second The bending angle θe when the second abnormal light ray is separated so as to bend from the first abnormal light ray by the optically anisotropic element.
100 ° ≤ θe ≤ 130 °
An optical low-pass filter characterized by satisfying the above conditions. The optical low-pass filter according to claim 1, wherein the light separation widths of the first and second optically anisotropic elements are equal to each other. The optical low-pass filter is used together with an image sensor that photoelectrically converts an optical image formed by light passing through the optical low-pass filter.
When the two pixel arrangement directions orthogonal to each other in the image sensor are the first direction and the second direction,
The optical low-pass filter according to claim 1 or 2, wherein the direction θ0 of the bisector of the bending angle θe is parallel to either one of the first and second directions. When the standard deviations of the plurality of point images formed on the image sensor by the plurality of light rays emitted from the optical low-pass filter in the first and second directions are σx and σy, respectively.
1 / 1.4 ≤ σx / σy ≤ 1.4
The optical low-pass filter according to claim 3, wherein the optical low-pass filter is characterized by satisfying the above conditions. When the light ray separation width of the first and second optically anisotropic elements is a [μm] and the pixel pitch in the pixel arrangement directions orthogonal to each other in the image pickup element is p [μm].
0.9 ≤ p / a ≤ 1.5
The optical low-pass filter according to claim 3 or 4, wherein the condition is satisfied. Color filters are provided in the image sensor in a Bayer array.
The pixel pitch p [μm] of the image sensor is
1.0 ≤ p ≤ 7.0
The optical low-pass filter according to any one of claims 3 to 5, wherein the optical low-pass filter satisfies the above-mentioned condition. The optics according to any one of claims 1 to 6, wherein the same optically anisotropic element whose front and back sides are inverted is used as the first and second optically anisotropic elements. Low-pass filter. The optical low-pass filter according to any one of claims 1 to 7.
An image pickup device including an image pickup device that photoelectrically converts an optical image formed by light passing through the optical low-pass filter. The image sensor has pixels that divide the exit pupil of the image pickup optical system and output a pair of signals.
The imaging device uses the pair of signals to perform autofocus by an imaging surface phase difference detection method.
The imaging apparatus according to claim 8, wherein the direction in which the exit pupil is divided is parallel to the direction of the bisector of the bending angle θe. The inclination Ψ of the optical axis of the first and second optically anisotropic elements with respect to the direction orthogonal to the image pickup surface of the image pickup element is
25 ° ≤ Ψ ≤ 40 °
The imaging device according to claim 7 or 8, wherein the image pickup apparatus according to claim 7 or 8, wherein the image pickup apparatus is satisfied.