JPH03139983A - Image sensor - Google Patents

Abstract

PURPOSE:To reduce the generation of aliases without complexing an optical system by providing each picture element with a peripheral part whose sensitivity is reduced from the center part in a prescribed direction. CONSTITUTION:It is defined that the total length of the center part 1a and peripheral part 1b of each picture element 1 in the scanning direction is b1, the length of the center part 1a in the scanning direction is c1, the sensitivity of the center part la is To, and the sensitivity of the peripheral part 1b is T1 lower than the To. Since the sensitivity distribution of respective picture elements constituting an image sensor is controlled, the value of a transmission function in a space frequency band exceeding the Nyquist frequency of the image sensor can be suppressed, the aliases of image information picked up by the image sensor can be reduced and a clear image can be obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an image sensor that improves aliasing in image information to be captured.

[Prior Art] As is generally known, when image information of a subject is discretely collected by a pixel array image sensor (hereinafter simply referred to as an image sensor) such as a solid-state image sensor using a CCD, the image sensor Among the spatial frequency components of the subject, frequency components exceeding the sampling frequency of the image sensor (Nyquist frequency) cannot be collected. Frequency components exceeding this sampling frequency become noise called aliasing (aliasing distortion), and this noise adversely affects the image information collected by the image sensor by generating moire fringes and false colors. In order to prevent such negative effects, by lowering the cutoff frequency of the optical system located in front of the image sensor than the sampling frequency of the image sensor, it is possible to prevent frequency components exceeding the sampling frequency of the image sensor from being transmitted to the image sensor. For this reason, conventional methods have been used such as installing an optical low-pass filter directly through the image sensor or at the pupil position of the optical system.

[Problems to be Solved by the Invention] In the conventional photographing device described above, an optical low-pass filter is installed, which makes the optical system complicated, and
There was a drawback that it was necessary to adjust the position of this optical filter.

The image sensor of the present invention has been made in view of such problems, and an object thereof is to provide an image sensor that can reduce aliasing without complicating the optical system.

[Means for Solving the Problems] In the image sensor of the present invention, each pixel has a peripheral portion where the sensitivity is lowered in a predetermined direction with respect to the center portion.

Alternatively, a thin film filter having a lower transmittance at the periphery in a predetermined direction relative to the center of each pixel may be provided, so that the periphery has substantially lower sensitivity relative to the center of each pixel. good.

Further, when the two types of image sensors described above are one-dimensional image sensors, the direction in which the transmittance of the pixels changes may be perpendicular to the arrangement direction of each pixel.

Furthermore, the pixels constituting the one-dimensional image sensor have a substantial sensitivity T. and the actual sensitivity T sandwiching this center
1, the scanning pitch interval in the direction perpendicular to the arrangement direction of each pixel of each pixel is 81, the length of the pixel in the vertical direction is b1, and the length of the center of the pixel in the vertical direction. When the length is C4, 1, . It is preferable to satisfy 6al<Ig<2.6a1 0.3al<cl<1.3a1 0, 3<Tl/To<0, and 8.

Further, when the image sensor is an image sensor or a staggered one-dimensional image sensor, the direction in which the substantial sensitivity of each pixel decreases may coincide with the pixel arrangement direction.

Further, the pixels constituting the staggered one-dimensional image sensor have a substantial sensitivity T. It consists of a central part and a peripheral part surrounding this central part with substantial sensitivity T1, where the pixel arrangement pitch is C2, the length of each pixel in the arrangement direction is b2, the practical sensitivity of the central part is T0, and the peripheral part is When the actual sensitivity of the area is] 1 and the length of the center is C2, 1.6a2 <bl <2.6a2 0.3a2 <cl<1.3a2 0.3<T* /"ro <o, It is preferable to satisfy the following conditions.

Further, a first pixel column consisting of a plurality of pixels arranged at an arrangement pitch of 2a2, and a first pixel column consisting of a plurality of pixels arranged at an arrangement pitch of 2a2 and arranged in parallel with the first pixel column shifted by C2 in the pixel arrangement direction. Staggered array 1 consisting of two pixel columns
In a dimensional image sensor, each pixel is one of the pixels.
- has a peripheral part where the sensitivity is substantially lower in the pixel arrangement direction and in the direction perpendicular to this arrangement direction with respect to the part, and the interval between the first pixel column and the second pixel column is set to al and 1, ,
The length of each pixel in the direction perpendicular to the arrangement direction is b1, the length of the center of each pixel in the direction perpendicular to the arrangement direction is CL, the length of each pixel in the arrangement direction is b2, the center of each pixel When the length in the arrangement direction of the pixel is C2, the actual sensitivity at the center of each pixel is 'I''0, and the actual sensitivity at the periphery is T1, 1.6at < bl < 2.6at 0.3al It is desirable to satisfy <c)<1..3a1 1.6a2 <t+2 <2.6a2 0, 3a2 <C2<1, 3 C2.

[Operation] To control the sensitivity distribution on the pixels constituting the image sensor (Please note that the transfer function 1 in this image sensor
The characteristics of 4TF(v) can be changed. Therefore, by controlling the sensitivity distribution of each pixel L constituting the image sensor as in the present invention, the transfer function MT of the image sensor in a spatial frequency band exceeding Nyquis 1 to frequency is
It becomes possible to suppress the value of F(ν), reduce aliasing of image information captured by the image sensor, and obtain a clear image.

Next, the present invention will be explained with reference to the drawings.

FIG. 1(a) is a plan view of a one-dimensional array image sensor (linear array sensor) showing an embodiment of the present invention. The image sensor 10 is composed of pixels 1 arranged at a predetermined arrangement pitch a2. Pixel arrangement direction
By scanning the image sensor 10 relative to the image in the scanning direction Y perpendicular to the image sensor 10 at the scanning pitch al as shown by the broken line, the arrangement pitch a2 in the X direction and Y
It is possible to capture image information corresponding to when an image is captured by a two-dimensionally arrayed image sensor configured by arranging pixels l- at an array pitch al in the direction.

FIG. 1(b) is a plane IA showing the principle configuration of one pixel 1 of the image sensor according to the present invention, and the length in the scanning direction Y including the center portion 1a and peripheral portion 1b of the pixel 1 is b.
1, and the length of the central portion 1a in the scanning direction Y is 01.

FIG. 1 <c> is such a sensitivity distribution diagram of pixel 1,
The sensitivity of the central part 1a is To, and the sensitivity of the peripheral part 1b is central part 1.
Sensitivity T of a.

It has a lower T1.

The transfer function MTF(ν) in the scanning direction Y due to the aperture of the pixel 1 of the image sensor according to the present invention is expressed by equation (1), where ν is the spatial frequency on the pixel 1.

Txbt+(To-Tt)ct fycl Also, the Nyquist frequency ν in the scanning direction Y is expressed as (Equation 21).

1/N = 1/ (2at)
t2) By the way, in the conventional image sensor, the following relationship exists in FIG. 1(a).

b1=cl=al (3-4
>To = 1
(3-2)
"T"t = 0 (3
-3) From this relationship, based on equation (1), the transfer function +, +
When rF(ν) is calculated, a transfer function characteristic diagram as shown in FIG. 2(a) is obtained for a conventional general image sensor. Further, FIG. 2(b) corresponds to FIG. 2(a), and FIG. 2(c) is a plan view of one pixel 1, and FIG. 2(c) is a sensitivity distribution diagram for one pixel.

In FIG. 2(a), the horizontal axis is the spatial frequency ν in the Y direction on the pixel 1. As shown in FIG. 2, even at a spatial frequency higher than the Nyquist frequency νN of pixel 1, the transfer function 1. Since the value of IT F (ν) is large, the spatial frequency component exceeding the Nyquist frequency ν becomes aliased. Therefore, the transfer function in the spatial frequency component exceeding this Nyquist frequency νN is 1.1
By lowering the value of 1'F (v), it is possible to improve aliasing.

If you simply want to change the MTF characteristics, you can also change the ratio of the scanning pitch a of pixel 1 and the length bl in the scanning direction Y, and change the transfer function MTF of spatial frequency components exceeding the Nyquist wave number ν. It is also possible to improve the aliasing by reducing the value of (ν>).However, as shown in FIG. 3, when the pixel 1 has uniform sensitivity, the aliasing is not improved sufficiently.

Figure 3 shows the length bl of pixel 1 in the scanning direction Y and the scanning pitch a.
FIG. 3 is a characteristic diagram of the transfer function MTF(ν) when the value is doubled by 1; At the Nyquist frequency νN, MTF(ν)=
0, and an improvement in the characteristics is recognized compared to the case shown in Figure 2.However, near the spatial frequency 3ν/2, the MTF (ν
) is increasing in the negative direction, and this component results in aliasing, which is still insufficient.

Therefore, in this invention, by changing the distribution characteristics of the actual sensitivity of each pixel apart from the length of the pixel 1,
This makes it possible to improve aliasing by reducing the value of the transfer function 14TF(ν) in the spatial frequency band exceeding the Nyquist frequency νN.

[Example] Hereinafter, the present invention will be explained based on examples.

First of all, the deputy director asked me about the image sensor according to the present invention.
Each constant of one pixel for the seventh embodiment is shown, and the conventional example and comparative example described above in FIGS. 2 and 3 are also shown.

As shown in the table, the constants of pixel 1 are the length b1 of the peripheral part 1b in the scanning direction Y, the length C1 of the central part 1a, the sensitivity T0 of the central part 1a, and the sensitivity 'rl of the peripheral part 1b9. The characteristics of the transfer function MTF(shi) obtained in each example are shown in FIGS. 4 to 10.

In addition, in Figures 3 to 10, (a) is a characteristic diagram of the transfer function MTF (ν), (b) is a corresponding plan view of pixel 1, and (C) is a sensitivity diagram of pixel 1. It is a distribution map.

Table Generally speaking, the closer the value of the transfer function 147F(ν) in the spatial frequency band above the Nyquist frequency νN is to 0, the less the aliasing is, and the value of the transfer function MTF(ν) in the spatial frequency band below the Nyquist frequency νN is The closer it is to 1, the better the image can be captured. Therefore, it is expected to realize characteristics of the transfer function MTF(ν) that are close to such characteristics.

In FIG. 4, the length of the central portion 1a of the pixel 1 in the scanning direction Y is the same as the scanning pitch, and the length of the peripheral portion 1b is 2a1.
, when the sensitivity of the central portion 1a is 1 and the sensitivity of the peripheral portion 1b is 0.5. In this case, the Nyquist frequency ν. The value of the transfer function MTF (ν) in the spatial frequency band exceeding ν is reduced, so aliasing is suppressed, and in the spatial frequency band below the Nyquist frequency ν, the transfer function 14TF (ν) is relatively small. A value close to 1 has been achieved.

Hereinafter, FIGS. 5 to 10 show the transfer function 1 when each constant of pixel 1 is changed and set. The characteristics of ITF(ν) are shown, and as in FIG. 4, it can be seen that the 14TF value in the band above the Nyquist frequency νN is well reduced.

As can be seen from the characteristic diagrams of the transfer function MTF (ν) shown in Figs. Image deterioration can be maintained to an acceptable level.

1, 6al <b, <2. Oaj (4-
1) 0.3al <C+ <1.3a+ (4
-2>0,3<TI/TO<0,8
(4-3) Here, each of the above-mentioned expressions will be explained as follows.

(11 Regarding equation (4-1), the length of pixel 1- in the scanning direction Y is 1.6a1
If it is smaller, the value of the transfer function MTF(ν) becomes high at a spatial frequency slightly higher than the Nyquist frequency ν, and aliasing increases.

■1-Note b When 1 becomes larger than 2,6a, the transfer function 1 at a spatial frequency twice the Nyquist frequency νN. I
As the value of TF(v) increases, aliasing increases.

(Regarding Equation 21 (4-2) ■ When the length C1 of the center part 1a of pixel 1 in the scanning direction Y becomes smaller than 0.3al, the value of the transfer function MTF(ν) at frequency 2νN becomes high and aliasing occurs. To increase.

(2) When C4 of L is larger than 1.3a+, the transfer function MTF(ν) in a spatial frequency band from 1.5 times to twice the Nyquist frequency νN becomes negative, and aliasing increases.

+31 Regarding equation (4-3) ■ Sensitivity ratio T. between peripheral area 1b and central area 1a of pixel 1. /
T. becomes smaller than 0.3, the Nyquist frequency ν,
The transfer function MTF (
The value of ν) increases in the negative direction, and aliasing increases.

(2) When T I/T o becomes larger than 0.8, the value of the transfer function MTF (ν) near the Nyquist frequency νN becomes large on the negative side, and aliasing increases.

By satisfying the conditions shown in each of the above equations (4-1) to (4-3), the conventional image sensor can be
) - CMTF at the Nyquist frequency νN as shown in
While the value was 0.64, it can be reduced to about 0.35 or less. Furthermore, in the comparative example shown in Fig. 3(a), the 14TF value near 3νN/'2 is -0.
, 21, it is now possible to reduce the absolute value of +4TF to 0.1 or less in the vicinity of 3SN/2 to SIN, and a sufficient effect for practical use can be obtained.

By the way, as shown in FIG. 1(C), the sensitivity distribution of pixel 1 can be changed by controlling the photoelectric power incident on pixel J. Specifically, this can be achieved by the following method. Figure 11(a) is a cross-sectional view of a conventional image sensor, in which photodiodes 3 are separated by a light-shielding plate 4 formed of A, Q, etc. to form each pixel 1. FIG. 11(b) is a sectional view showing an embodiment of the image sensor of the present invention. Transmissive films 5 for controlling incident light are provided on both sides of the light shielding plate 4. The induced current M5 is composed of a thin film such as a gold E absorption film made of Aρ, Cr, or a dielectric absorption film.

In other words, all of the light weight is incident on the part where the transparent film 5 is not formed, but the amount of light transmitted through the transparent film 5 changes depending on the transmittance of the transparent film 5, and as a result, the pixel 1
It is possible to control the sensitivity distribution to a desired value.

Now, in the one-dimensional image sensor 10 as shown in FIG. 1(a), since the pixels 1 are arranged closely together, the length of the pixels 1 in the arrangement direction X cannot be changed. While it is possible to change the length of the pixels 1 in the array direction X and the straight scanning direction Y, in the staggered array one-dimensional image sensor 20 shown in FIG. Each pixel 2 for either direction Y
The length can be changed.

That is, as shown in FIG. 12(a), the staggered array 1
The dimensional image sensor 20 has pixels 2 arranged in an array and a gate 2a2.
The second pixel column is composed of a first pixel column arranged in the same manner as shown in FIG. In addition, al is the arrangement pitch of the first pixel row and the second pixel row (same value as the scanning pitch al), bl is the length of the pixel 2 in the arrangement direction X, and C2 is the length of the central part of the pixel 2. length, the sensitivity of the central part 2a is ro, the sensitivity of the peripheral part 2b is T,
shall be. Further, a sensitivity distribution diagram in the arrangement direction X of the pixels 2 shown in FIG. 1-2(a) is shown in FIG. 12(b).

At this time, the transfer function MTF (v) in the array direction X due to the opening ['' of each pixel 2 is as shown in equation (5).

T1b2+(Tg Tl)C2KvC2 Further, Nyquis 1 to frequency νN in the arrangement direction X is expressed as equation (6).

vpt = 1/ (2az)
f6) In a general staggered array one-dimensional image sensor, each constant of pixel 2 is as follows.

bz = (2=az (7-1
) To = 1 (7
-2)'r2=0
(7-3) The above-mentioned one-dimensional image sensor] If 0, the length of the pixel in the scanning direction Y and the pixel 1
The characteristics of the transfer function MTF(ν) were changed by changing the sensitivity distribution of
Instead, by changing the length and sensitivity distribution of the sensor in the arrangement direction X, the characteristics of the transfer number MTF (v) can be changed to improve aliasing. +5]
As a result of determining the characteristics of the transfer function MTF(v) based on the equation and equation (6), it was concluded that it is desirable to control each constant of the sensor within the following range.

1.6az <bz <2.6az (8-1
)0.3a2 <C2<1.3a2 (8-2
)0, 3<T2/TO<0.8 (8-3
>In the case of this staggered arrangement, the method for controlling the sensitivity distribution of pixel 2 may be the same as the method explained in FIG. 11, and as shown in FIGS. 13(a) and (b)
) shows the details. The pixel 2 is formed by separating a photodiode 3 with a light shielding plate 4, and a transmission film 5 can be formed at a desired position to control a 15 degree distribution.

By the way, if pixel 2 is staggered and 1~, -E
As mentioned above, not only can the length and sensitivity distribution of pixels 2 in the arrangement direction X be changed, but also the length and sensitivity distribution of pixels 2 in the arrangement direction
As in the case of the one-dimensional array image sensor 10 shown in , the length and sensitivity distribution of the pixels 2 can be changed in each G direction and in the scanning direction Y as well.

FIG. 14 is a plan view of one pixel 2 constituting a staggered one-dimensional image sensor. The length of the pixel 2 in the arrangement direction X is b1, the length of the central part 2a is C2, and
The length of the pixel 2 is b1, and the length of the central portion 2a is C4. Further, the sensitivity of the central portion 2a is T0, and the sensitivity of the peripheral portion 2b is T1.

In such a pixel 2, the transfer function 14TF, (v) in the scanning direction Y perpendicular to the arrangement direction is shown in equation (9), and the transfer function 14TF2(v) in the arrangement direction X is shown in equation 00).

MTFl(ν) T1h1b21(C2TO-CzTt)Ct
rvc1MTF2(p) Tlb,b2+(c,To-clTl)C2rvc2 Based on the above equation (9) and 001, transfer function 14T
As a result of determining the characteristics of F1(ν) and transfer function MTF2(v), in order to suppress aliases, pixel 2
It was concluded that it is desirable to control each constant in the following range.

1, 6al <b, <2.6aL (ill)0
．． 3a(<ct<1.3al (11,-2
>1,6 az<bz<2,6 C3(11-
3>0.3a,,<C2<1.3a2 (11-
4) (To-Tt)e t +Tt b tThe staggered array shown above is a combination of two pixel rows, but in this invention, the pixel rows are combined in three or four rows.
It goes without saying that a staggered arrangement in which rows are combined is also possible.

[Effects of the Invention] As explained above, according to the image of the present invention, by controlling the sensitivity distribution on the pixels constituting the image sensor, the transfer function 141 (ν) in the image sensor is reduced. It is possible to change the characteristics of Therefore, by controlling the sensitivity distribution on the pixels constituting the 5-image sensor to a predetermined value, the transfer function 1 (TF(ν)) in the spatial frequency band exceeding the Nyquist frequency of the image sensor is
By suppressing the value of , aliasing of image information captured by the image sensor can be reduced.

As a result, aliasing can be reduced without using an optical low-pass filter, which has the effect of simplifying the configuration of the optical system compared to the prior art and eliminating the need for adjustment of the optical system.

[Brief explanation of the drawing]

FIG. 1(a) is a plan view of a one-dimensional array image sensor showing an embodiment of the present invention, FIG. 1(b) is a plan view of one pixel of the same one-dimensional array image sensor, and FIG. ) is a sensitivity distribution diagram on a pixel, in Figures 2 to 10, (a) is a transfer function characteristic diagram on each side, (b) is a plan view of a sensor corresponding to the same example, and (C) is a diagram of the same implementation. FIG. 11(a) is a sectional view of a conventional image sensor, FIG. 11(b) is a sectional view showing the configuration of an image sensor according to an embodiment of the present invention, and FIG. 12(a) is a sensitivity distribution diagram corresponding to the example. a) is a plan view of an image sensor with a one-dimensional array in a staggered array; FIG. 12 (b)
) is a sensitivity distribution diagram corresponding to one pixel configuring the same image sensor, FIG. 13(a) is a plan view showing the configuration of one pixel configuring the staggered array image sensor, and FIG. 13(b) is the same. FIG. 14 is a cross-sectional view showing the configuration of a pixel, and FIG. 14 is a plan view showing the same pixel. 1.2--pixel, la, 2a--center, 1b, 2b
... Peripheral part, 5... Transmissive absorption film, 1.0... One-dimensional array image sensor, 20... Staggered array one-dimensional array image sensor, al... Scanning pitch in the direction perpendicular to the array direction , bl...Pixel length in the scanning direction that is perpendicular to the arrangement direction, C1...Length of the central part of the pixel in the scanning direction, al...Pixel arrangement pitch (array interval), bz.・Pixel length in the array direction, C2
...The length of the pixel center in the arrangement direction, 'ro...
・Sensitivity at the center of the pixel, Tl...Sensitivity at the periphery of the pixel, X
... Array direction, Y... Scanning direction perpendicular to the arrangement direction.

Claims (1)

[Scope of Claims] (1) An image sensor comprising a plurality of pixels regularly arranged, each pixel having a peripheral region with lower sensitivity in a predetermined direction relative to the center thereof. image sensor. (2) In an image sensor consisting of a plurality of pixels arranged regularly, a thin film filter is provided whose transmittance decreases in the periphery in a predetermined direction relative to the center of each pixel. An image sensor characterized by having a configuration having a peripheral portion where sensitivity is substantially reduced. (3) In claim 1 or 2, the image sensor is a one-dimensional image sensor in which each pixel is arranged one-dimensionally, and the direction in which the transmittance changes in the pixel is perpendicular to the direction in which the pixels are arranged. Featured image sensor. (4) In claim 3, each pixel constituting the image sensor has a center part with a substantial sensitivity T_0 and a peripheral part with a substantial sensitivity T_1 sandwiching this center part, and the pixel of each of the pixels has an array direction. Let the scanning pitch interval in the direction perpendicular to a_
1. When the length of the pixel in the vertical direction is b_1 and the length of the center of the pixel in the vertical direction is c_1, 1.6a_1<b_1<2.6a_1 0.3a_1<c_1<1.3a_1 0. An image sensor characterized by satisfying 3<T_1/T_0<0.8. (5) The image sensor according to claim 1 or 2, wherein the image sensor is a staggered one-dimensional image sensor, and the direction in which the substantial sensitivity of each pixel decreases coincides with the direction in which the pixels are arranged. (6) In claim 5, the pixels constituting the image sensor are composed of a center part with a substantial sensitivity T_0 and a peripheral part with a substantial sensitivity T_1 surrounding this center part, and the pixel arrangement pitch is a_2, and each pixel is When the length in the arrangement direction is b_2 and the length at the center is c_2, 1.6a_2<b_1<
2.6a_2 An image sensor that satisfies 0.3a_2<c_1<1.3a_2 0.3<T_1/T_0<0.8. (7) A first pixel column consisting of a plurality of pixels arranged at an arrangement pitch of 2a_2, and a first pixel column consisting of a plurality of pixels arranged at an arrangement pitch of 2a_2 and arranged in parallel with the first pixel column shifted by a_2 in the pixel arrangement direction. In a staggered array one-dimensional image sensor consisting of two pixel rows, each pixel has a peripheral portion in which sensitivity is substantially lower in the pixel arrangement direction and in a direction perpendicular to the pixel arrangement direction with respect to the center thereof. , the distance between the first pixel column and the second pixel column is a_1, the length of each pixel in the direction perpendicular to the arrangement direction is b_1, and the length of the central part of each pixel in the direction perpendicular to the arrangement direction is c_1, the length of each pixel in the array direction is b
_2, when the length of the central part of each pixel in the arrangement direction is c_2, the substantial sensitivity of the central part of each pixel is T_0, and the substantial sensitivity of the peripheral part is T_1, 1.6a_1<b_1<2.6a_1 0.3a_1<c_1<1.3a_1 1.6a_2<b_2<2.6a_2 0.3a_2<c_2<1.3a_2 0.3<(T_1b_1)/((T_0-T_1)c_
1+T_1b_1)<0.80.3<(T_1b_2)
/((T_0-T_1)c_2+T_1b_2)<0.
An image sensor characterized by satisfying 8.