JPS623202A - Optical filter - Google Patents

Abstract

PURPOSE:To decrease return distortions by forming double refracting transparent plates having the sepn. distance equal to the pitch in such a manner that the sepn. distance component in the scanning direction is half the pitch and that the sepn. distance component in the opposite direction is also half the pitch. CONSTITUTION:Rays 251, 252 are double refracted in the 2nd crystal plate 22 and are so separated that the sepn. distance component in the u-axis direction attains P22u equal to a/2 and the sepn. distance component in the v-axis direction attains P22v equal to b/2 (=a/2), namely, said rays are separated by the distance P22 (=a/sq. rt. 2) in the direction +45 deg. with respect to the u-axis by which the rays are separated to 251 and 253 as well as 252 and 254. The sepn. directions are in the direction 45 deg. with resepct to the respective polarization directions of the rays 251, 252 and therefore half the respective components of the rays 251, 2524 remain and half the same are separated. The intensities of the rays 251-254 are 1/4 the intensity of the original unit luminous flux and equal to each other. The return components from the carrier are thereby decreased and the components of the return distortion part are decreased.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to an optical filter, and more particularly to an optical filter suitable for a solid-state image sensor having a configuration in which a large number of pixels are aligned and orthogonally arranged in the horizontal and vertical directions.

2. Description of the Related Art At present, some single-tube color television cameras incorporate a CCDCD solid-state image element 1 shown in FIG. 8 instead of a decoy image tube. In the figure, reference numeral 2 denotes a large number of photodiodes which are pixels and photoelectric conversion elements, and are arranged orthogonally arranged in the horizontal direction indicated by U at pitch a and vertically indicated by black at pitch b. 3 is a manual transfer CCD section, and 4 is a horizontal COD shift register. An optical image of the subject is formed on the surface of this solid-state image element 1, and photoelectrically converted by each photodiode 2. The photocharges generated in each photodiode 2 are read out one after another, and a video signal is taken out from the solid state element.

Problem to be Solved by the Invention In this case, since the photodiodes 2 are arranged at intervals, the optical image is generated at a frequency corresponding to pitch a in the U direction, and at a pitch b in the ■ direction. It will be sampled spatially at the corresponding frequency,
vR is an object whose horizontal and vertical spatial frequencies are zero.
When imaging, the video signal is extracted in the form of a modulated wave 5 represented by an arc in FIG. At this point, another modulated wave 6
, 7°8 are generated as shown by the circles centered on the carriers 5a, 7a, and 8a, respectively, and since the aperture ratio of the photodiode is relatively low, the level of the modulated wave 6.7 is high and it is different from the modulated wave 5. When they overlap, aliasing distortion 9.10 occurs in the spatial frequency region indicated by hatching in the figure.This aliasing distortion occurs when, for example, when an object with a thin striped pattern is imaged, the lines making up the striped pattern are shaped like a beat. As described above, in the solid-state image sensor 1, a unique aliasing distortion occurs due to the spatial sampling of the optical image.
Suppressing the aliasing component from 7a to reduce aliasing distortion 9.10
need to be reduced.

By the way, the lens optical system has a low-pass filter characteristic as shown by the curve (■) in FIG. 10, and the high-frequency components at the back of the optical image have a low F. The number of pixels of the current solid-state imaging probe 1 is as follows:
400 pixels in 3-inch size ″′: Because it is elementary, the spatial sampling frequency is not high enough as shown by fl. Therefore, the spatial frequency is higher than the Nyquist limit, which is 1/2 the frequency of f1 above. Also, the MTF value is relatively large, but aliasing distortion cannot be suppressed, and an optical filter is required.The optical filter applied to the dancing image tube generally has an MTF value as shown by the curve ■ in Fig. 6. In the figure, the trap point fu.

The MTF value in the spatial frequency range between fu2, which is 1/2 of the frequency from fu to fu, is large, and the suppression of the aliasing component from the carrier 6a in FIG. 9 is insufficient. Therefore, the solid-state image sensor 1 requires a specially designed optical filter.

An object of the present invention is to provide an optical filter that solves the above problems.

Means for Solving the Problems The present invention provides a first birefringent transparent plate in which the scanning direction of the solid-state image sensor is the separation direction and has a separation distance equal to the pitch a of the pixels of the solid-state image sensor in the scanning direction; A second method for separating pixels such that the separation distance component in the scanning direction is a/2 and the separation distance component in one of the directions orthogonal to the scanning direction is 1/2 of the pitch b of the pixels of the solid-state image sensor in the direction orthogonal to the scanning direction. A birefringent transparent plate, a third birefringent transparent plate whose separation direction is a direction opposite to the one direction in the orthogonal scanning direction, and a separation distance equal to the pitch b, and separation in the scanning direction. A fourth birefringent transparent plate separated so that the distance component is a/2 and the separation distance component in the direction orthogonal to the scanning direction opposite to the one direction is b/2 is superposed. It is the composition.

Effect: The above configuration separates a unit incident light beam into 12 light beams, and both the MTF characteristics in the scanning direction and the direction perpendicular to the scanning function to sufficiently attenuate the aliasing component from the carrier.

EXAMPLE Next, an example of the optical filter according to the present invention will be described.

FIG. 1 shows an optical filter 20 that is applied to the solid-state imaging element 1 in which the horizontal pitch a and the vertical pitch b are equal. The optical filter 20 is indicated by arrow 2 in FIG.
A first crystal plate 21 whose separation direction is in the 1A direction (the same direction as the scanning direction in the solid-state image sensor 1), a second crystal plate 22 whose separation direction is in the arrow 22A direction, and a second crystal plate 22 whose separation direction is in the arrow 23A direction (the same direction as the scanning direction in the solid-state image sensor 1); A third crystal plate 23 whose separation direction is the scanning orthogonal direction (-V axis direction) perpendicular to the scanning direction in 1 and a fourth crystal plate 24 whose separation direction is the direction of arrow 24A are stacked. This is the structure ゛.

The first crystal plate 21 has a thickness tl such that the separation distance is a, the second crystal plate 22 has a thickness tl such that the separation distance is a/72, and the third crystal plate 23 has a thickness tl such that the separation distance is a/72. The fourth crystal plate 24 has a thickness t3 so that the resulting branch distance becomes , , b (=a), and the fourth crystal plate 24 has a thickness t4 so that the resulting separation distance becomes a/J2.

Unit light flux 25 of circularly polarized light incident on this optical filter 20
is first birefringent in the first crystal plate 21, and as shown by the vector O in FIGS. 3 and 4 (A), the light ray 2 is separated by a distance P2121 equal to the pitch a in the U-axis direction.
It is divided into 5+ and 252. Two separated rays 2
5+, 252, as shown by the arrow in FIG. 4(A),
Both are linearly polarized lights, and the polarization directions are orthogonal to each other.

The light beams 25+, 252 are transmitted to the second crystal plate 22. P2 is birefringent, and the separation distance component in the U-axis direction is equal to a/2, as shown by the vector O in Figures 3 and 4 (B).
2u, ■Axial separation distance component is b/2 (=a/2)
PZ equal to! V, i.e. +4 to the U axis
The distance P22 (=a/V'2> is separated in the 5° direction, and the beams are divided into rays 251 and 253, and 252 and 254.)
Since the separation direction is 45 degrees with respect to each polarization direction of the light rays 25+ and 252, the light rays 25+ and 25
1/2 of each component of 2 remains and 1/2 is separated, and the intensity of each light ray 251 to 254 becomes 1/4 of the intensity of the original unit light beam 25 and is equal to each other.

The separated light rays 25+ to 254 are birefringent in the third crystal plate 23, and as shown by the vector 0 in FIGS. =a
) is separated by a distance llI P 23, and the ray 251~
254 and 255-258. In this case, since the separation direction is at 45 degrees with respect to the polarization direction of each light ray 251 to 254, each light ray 251 to 254 is a component of each component of light 1G 25 + to 254, as in the above case. 1
/2 remains, 1/2 is separated, each ray 251-258
The intensity is 1/8 of the intensity of the unit luminous flux 25, and is equal to each other.

The separated light rays 251 to 258 are birefringent in the fourth crystal plate 24, and as shown in FIGS. 3 and 4 (D), the separation distance component in the U-axis direction becomes a/2. equal to P24U, the separation distance component in the -V axis direction is P24V equal to b/2, that is, -45 with respect to the U axis.
The distance 1111 P 24 (=a/J2> is separated in the degree direction, and the beams are separated into rays 251 to 25+6. Each ray 251 to
258 is 1/2 of its components, as in the above case.
remains, and 1/2 is separated. Also, rays 25+, 25
2.

Ray 25s, 25to separated from 253.25g.

2511.2514 are the rays 25s, 25a,
252257, and as a result, the unit luminous flux 25 is 1
It is separated into two rays. Eight of these rays are 251°2
53, 234, 25s, 2
512, 25 Den 3, 25+5251
The intensity of rays 6 is 1/16 of the intensity of the original unit beam 25, and the intensity of the remaining four rays is 1/16 of the intensity of the unit beam 25.
/8.

MTF of optical filter 20 that separates light beams as described above
The characteristics are expressed by the following formula, where U is the spatial frequency in the horizontal direction and ■ is the spatial frequency in the vertical direction. When calculating by substituting specific values for ('' and This is a curve.

The MTF characteristic in the U@ direction is obtained by plotting the absolute value of the MTF value at each spatial frequency on the U axis in FIG. 5, and is as shown by the curve ■ in FIG. 6. ful corresponds to the frequency of the carrier 6a, and fu2 corresponds to fU+/
It is 2. From Figure 6, the MTF value gradually decreases as the horizontal spatial frequency increases, and reaches zero at fu2.
It can be seen that beyond this, the MTF value increases somewhat, then decreases again, and reaches zero at fu, and the MTF value between fu2 and ful is kept small.

In this case, especially the MT between horizontal spatial frequencies fu2 to fu1
Since the F value is kept small, the high-frequency aliasing component from the Kirriya 6a is effectively suppressed, and the level of the generated aliasing distortion 9 becomes small.

(2) The axial MTF characteristic is obtained by plotting the absolute value of one MTF value at each spatial frequency on the V axis in FIG. 5, as shown by the curve (2) in FIG. 7.

It becomes dark. fvl corresponds to the frequency of the carry r7a, and fv2 is fV+/2. From FIG. 7, similar to the horizontal MTF characteristics shown in FIG. 6, the MTF value gradually decreases as the vertical spatial frequency increases, and fv2
, it becomes zero, and when it exceeds this value, it increases somewhat and then decreases again, and f
It becomes zero at v+, and it can be seen that the MTF value between fv+ and fv2 is kept small.

As in the above case, firstly, the vertical spatial frequency fv2
The MTF value between ~fv is kept small, and the second
Since the MTF value of vertical spatial frequency fV2 is small, ■
The aliasing strain in the axial direction is also sufficiently reduced.

Therefore, by using the optical filter 10 described above, both the aliasing distortion appearing in the U-axis direction and the aliasing distortion appearing in the V-axis direction are suppressed and reduced.

Note that there is a place with a high MTF value in the diagonal direction in FIG. 5, but this place is far enough away from the origin and the spatial frequency is also quite high. For such high spatial frequency components, as shown in FIG. 10, the characteristics of the lens optical system effectively act to lower the MTF value, so there are few problems.

Note that the optical filter of the present invention has a horizontal pitch a and a vertical pitch b of the photodiodes 2 as described above.
The second . Of course, by appropriately changing the separation direction of the fourth crystal plate, it is possible to obtain a configuration suitable for an a/b solid-state imaging device.

Furthermore, the order in which the first to fourth crystal plates 21 to 24 are arranged is not limited to that in the above embodiment, and unless odd-numbered crystal plates or even-numbered crystal plates are adjacent to each other, the order of arrangement is not limited to the above embodiment. The order may be arbitrary.

Effects of the Invention As described above, according to the optical filter according to the present invention, in both the scanning direction and the direction perpendicular to the scanning direction of the solid-state imaging device, the spatial frequency of carriers generated during imaging by the solid-state &f image toxin particles and the spatial frequency of this spatial frequency can be adjusted. Because the MTF characteristic is small in the band between 1/2 spatial frequency and small in the vicinity of the latter spatial frequency,
The aliasing component from the carrier is reduced, and the aliasing distortion component is also reduced, and the aliasing distortion can be sufficiently reduced in both the scanning direction and the direction orthogonal to the scanning direction, making it suitable for solid-state imaging devices. have

[Brief explanation of drawings]

FIG. 1 is a perspective view showing the structure of an embodiment of the optical filter according to the present invention, FIG. A diagram showing the state of light beam separation by the optical filter in Figure 1, and Figure 4 (A
) to (D) are diagrams showing the state of light beam separation by each crystal plate constituting the optical filter in FIG. 1, respectively. FIG. 5 is a diagram showing two-dimensional MTF characteristics of the optical filter in FIG. 1, and FIG. and FIG. 7 are diagrams showing MTF characteristics in the horizontal direction and vertical direction, respectively, and FIG. 8 is a diagram showing the CC to which the optical filter of the present invention can be applied.
FIG. 9 is a diagram schematically showing an example of a solid-state image sensor D.
A diagram schematically showing the appearance of carriers, modulated waves, and aliasing distortion when an object is imaged by the solid-state imaging device shown in the figure. Figure 10 is a typical M with a lens optical system.
FIG. 3 is a diagram showing TF characteristics. 1... CCD solid-state image sensor, 2... Photodiode, 5, 6.7, 8. ...Modulated waves, 6a, 7a. 8a...Carrier, 9,10...aliasing distortion, 20
- Optical filter, 21... First crystal plate, 22.
...Second crystal plate, 23...Third crystal plate, 24...
・Fourth crystal plate, 25...Unit luminous flux, 251-25 electric 6...Separated rays. Patent applicant: Victor Japan Co., Ltd. Figure 3 P2 MU (No.)

Claims (1)

[Claims] An optical filter applied to a solid-state imaging device in which a large number of pixels are arranged at a pitch a in the scanning direction and at a pitch b in a direction orthogonal to the scanning direction, the scanning direction being a separation direction and the pitch a A first birefringent transparent plate having an equal separation distance and a separation distance component in the scanning direction of a/2.
The second birefringent transparent plate is separated such that the separation distance component in one of the directions perpendicular to the scanning direction is b/2, and the second birefringent transparent plate is separated such that the separation distance component in one of the directions perpendicular to the scanning direction is b/2, and the direction opposite to the one direction among the directions perpendicular to the scanning direction is a third birefringent transparent plate having a separation distance equal to the pitch b in the separation direction and a separation distance component in the scanning direction;
/2, and a fourth separation distance component in a direction opposite to the one direction among the scanning orthogonal directions is b/2.
An optical filter characterized by being made by overlapping birefringent transparent plates of.