JPH04170885A - Image pickup device - Google Patents

Abstract

PURPOSE:To suppress a moire in an image pickup element by setting specified offset sampling structure where an image element pitch in a horizontal direction is PH, an image element pitch in a vertical direction is PV and an image element offset quantity in the horizontal direction is PH/2. CONSTITUTION:The image pickup element 2 having offset sampling structure where the image element pitch in the horizontal direction is PH, the image element pitch in the vertical direction is PV and the image element offset quantity is PH/2 is given. Then, an optical low pass filter 1 which has more than three kinds of color filters, in which pitch in the horizontal direction is 2PH, the pitch in the vertical direction is 2PV and the offset quantity in the horizontal direction is PH in respective color filters, which has the color filter array 2a of offset structure provided for the image pickup element, and the three specified optical members 3-5, which is provided for an image pickup optical system and which satisfies the condition of an inequality is provided. Thus, a carrier generated by the color filter array 2a is removed by the optical low pass filter 1 and the occurrence of the moire can be suppressed.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a color imaging device having an image sensor with an offset sampling structure and an optical low-pass filter, suitable for video cameras, electronic still cameras, and the like.

[Conventional technology]

In order to obtain a color signal in a single-chip color video camera, a striped color filter arrangement as shown in FIG. 7, for example, is used. At this time, for a particular color, color moiré occurs because the spatial sampling position in the horizontal direction is once every three pixels.

To prevent this, color video cameras use an optical low-pass filter made up of a single birefringent plate so that the same light beam is split and incident on positions horizontally shifted by 1.5 pixels.

In this way, the horizontal frequency characteristic (transfer characteristic MTF) of the optical low-pass filter takes the form of CO5, as shown in FIG. 8, and becomes 0 at fs/3. On the other hand, since color moiré returns from this point, such an optical low-pass filter is effective in suppressing moiré. Here, f3 is the sampling frequency per pixel, and fs
=1/d. d is the pitch of one pixel.

[Problem to be solved by the invention]

On the other hand, in recent years, each pixel of a solid-state image sensor has been configured to have an offset sampling structure, and each pixel has a
The arrangement of color filters shown in the image is attracting attention.

In FIG. 3(a), the solid-state image sensor has a pixel pitch P in the horizontal direction, 4. It has an offset sampling structure with a vertical pixel pitch PV and a horizontal pixel offset NPH/2, and magenta, green, cyan, and yellow light transmission filters Mg.

Gr, Cy, and Ye are arranged at positions corresponding to each pixel. Each color filter is horizontal 2PH.

The offset sampling structure is 2 PV in the vertical direction and PH in the horizontal direction.

Further, in FIG. 3(b), the solid-state image sensor has a pixel pitch P in the horizontal direction. , vertical pixel pitch PV, and horizontal pixel offset amount P. /2 offset sampling structure, and red, green, and blue light transmission filters R, G, and B are arranged at positions corresponding to each pixel. As shown in the figure, the R and B color filters are horizontal 2P.
H.

The offset sampling structure is 2 PV in the vertical direction and PM in the horizontal direction. The color filter in direction G has a stripe shape in the diagonal direction, and this also has an offset sampling structure of 2 PH horizontally, 2 PV vertically, and an offset amount PH in the horizontal direction.
, can be seen as overlapping objects that are shifted from each other by PV in the vertical direction.

In this case, the color filters are arranged two-dimensionally, rather than simply one-dimensionally as in the case of conventional stripes, so the optical low-pass filter that prevents color moiré is also
It must be designed to meet certain special two-dimensional conditions, and any conventional optical low-pass filter could not achieve the desired effect.

The present invention was made under these circumstances, and is effective in suppressing moiré in an image sensor using a color filter array with an offset structure as shown in FIGS. 3(a) and 3(b). It is an object of the present invention to provide an imaging device that combines an optical low-pass filter.

[Means to solve the problem]

The present invention carefully analyzes the sampling structure two-dimensionally when the color filter arrangement as shown in FIGS. This was done by deriving.

First, as shown in Fig. 4, the horizontal and vertical pitches are p, , pv, respectively, and the horizontal offset amount is PH.
Consider an offset sampling structure such that /2. In this case, the sampling structure has a horizontal and vertical pitch of 27 on a two-dimensional frequency plane, as shown in Figure 5.
P.H.

It is known that the offset structure is 1/Pν and the offset amount in the horizontal direction is 1/P□.

This derivation is described, for example, in "Digital Signal Processing of Images" by Takahiko Fukisaka, Nikkan Kogyo Shimbunsha (1985), p.

317.

Therefore, in an image sensor using a color filter array as shown in FIGS. 3(a) and 3(b), the luminance signal is processed using the so-called switch-Y method, in which the color signal from the color filter of each pixel is equivalently regarded as a luminance signal. Assuming that the sampling structure of the luminance signal is obtained, the sampling structure of the luminance signal is as shown in FIG. 6(a). Also, regarding each color signal, for example, Fig. 3 (a
), the horizontal and vertical pitches are 2PH each.
, 2PV, and the horizontal offset is P, 4, which also has an offset sampling structure.

Therefore, the sampling structure of the color signal is as indicated by the 0 mark in FIG. 6(a).

Therefore, the optimal optical low-pass filter for preventing color moiré is a diamond-shaped filter in which the frequency characteristic becomes 0 or minimum near the solid line in FIG. 6(a).

Based on the above considerations, the present invention configures an imaging device as follows (1) in order to achieve the above object.

(1) An imaging device including the components a, b, and c.

a, An image sensor having an offset sampling structure in which the horizontal pixel pitch is PH1, the vertical pixel pitch is PV, and the horizontal pixel offset amount is P, 4/2.

b. It has color filters of three or more colors, and each color filter has,
Horizontal pitch is 2PH, vertical pitch is 2P
A color filter array having an offset structure and having an offset amount of P and 4 in the horizontal direction at v and provided in the image sensor.

C3 The incident light beam is moved at a distance P1 in a direction of 45° or 145° counterclockwise with respect to the horizontal scanning direction of the image sensor.
A first optical member that splits the incident light beam into two light beams separated by a distance of P in a direction parallel to the horizontal scanning direction.
a second optical member that splits into two light beams separated by 2;
The incident light ray is 9
an optical low-pass filter that is provided in an imaging optical system and satisfies the following conditions, the third optical member splitting into two light beams separated by a distance P3 in the 0° direction.

0.375PH<P2<0.825PH (Function) With the configuration (1) above, carriers generated by the color filter array are removed by the optical low-pass filter,
The occurrence of moiré is suppressed.

〔Example〕

The present invention will be explained in detail below with reference to Examples. In the description, "direction" refers to two directions that differ by 180 degrees, and "direction" refers to one direction.

As mentioned above, ideally, the diamond-shaped characteristic is good, but in order to actually construct an optical low-pass filter using a birefringent plate, etc., it is necessary to Carrier frequency of the closest color (±1/2PH, ±1/4P
Four straight lines II pass through V) and make an angle of ±45° with the 8th axis.

An easy configuration method is to make the frequency characteristic zero on two straight lines n, , u,' parallel to the axes 11°, Jls, j23' and f. To do this, the incident light beam must be rotated 45° counterclockwise with respect to the horizontal scanning direction.
Alternatively, there is a first optical member that splits the incident light beam into two light beams at an interval of Pl in the -45° direction, and a second optical member that splits the incident light beam into two light beams at an interval of P2 in a direction parallel to the horizontal scanning direction. 2
If an optical low-pass filter is constructed by the optical member and a third optical member that splits the incident light beam into two light beams at an interval of P3 in a direction 90° with respect to the dividing direction of the first optical member. good. At this time, the two-dimensional MTF value, which is the frequency characteristic of the optical low-pass filter, is as follows.

MTF (fx, fy) = 1cosπP2f, -c
osπP, f-!゛! 'f ding full-f.

-cosycP° ----(1) As a result, the MTF value is set close to (±[, ±17) where the luminance signal carrier occurs, without degrading the horizontal resolution even when the chrominance signal carrier occurs. can be trapped, and all of the color signal and luminance signal carriers closest to the origin can be suppressed.

Furthermore, in order for this optical low-pass filter to exhibit sufficient performance, the straight lines u+ and j shown in FIG. 6(b) must be
The distance from the origin of Qr”, Il*, Its” is il
, JLr': 1/2P+ It 3, 113': 1/2P
3, so this value is within plus or minus 25% around h・=-'T (2Ps ``4P,), from the origin to i
2. Distance 1/2P to IL2°.

It is sufficient if it is within plus or minus 25% around 1/PH. Here, the above values are the values of the four straight lines J2. ．． f,
“,f,,i,' is (±□,±□)2P, 4
It is the distance from the origin to the straight line when passing through PV.

In order to satisfy the above conditions, the separation (split) congestion p,,p
,,p, should satisfy the following conditions.

In the above formula, when the upper limit is exceeded, the amount of moiré due to the luminance two-color 1'H'5-carrier increases, and when the lower limit is exceeded, the resolution of the entire system consisting of the optical low-pass filter and the image sensor decreases. Undesirable.

FIG. 1 shows the main part configuration of an "imaging device" which is a first embodiment of the present invention. In the figure, 1 is an optical low-pass filter, and 2 is a color solid-state image sensor having an offset sampling structure provided with a color filter array 2a having the color filter arrangement shown in FIGS. 3(a) and 3(b).

The optical low-pass filter 1 includes a first filter consisting of a birefringent plate of 6° 7.8°. Second. It consists of a third optical member 3.4.5. The separation direction of the birefringent plate 6°7.8 shows how the light rays are separated depending on the direction of horizontal scanning. As shown in the figure (a), the incident light is divided by the birefringent plate 6 at an angle of 45 degrees counterclockwise with respect to the horizontal scanning direction, and the incident light is divided into equal parts having polarization components in the direction indicated by the solid line arrows in the figure. The light is emitted as two linearly polarized lights, an ordinary ray 9 and an extraordinary ray 10 of high intensity.

Next, as shown in FIG. 6(b), the light rays 9 and 10 emitted from the birefringence plate 6 are divided by the birefringence plate 7 in parallel to the horizontal scanning direction, resulting in four light rays 9 and 10°11.12. Become. The polarization direction is the direction shown by the solid arrow in the figure. Next, the same figure (
As shown in C), the light rays 9 and 1 emitted from the birefringent plate 7
0,11°12 is divided by the birefringent plate 8 in a direction of 145° counterclockwise with respect to the horizontal scanning direction, resulting in eight light beams 9,10, 11, 12, 13, 14, 15°16. , these eight light beams are ultimately emitted from the optical low-pass filter 1. The MTF value of the transfer characteristic of the optical low-pass filter at this time is: . syrμmudankendan1 ---(
5) PM"2PV, and as shown in FIG. 6(b), moiré caused by the luminance carrier and the color signal carrier is suppressed better.

FIG. 9 shows the main structure of an "imaging device" according to a second embodiment of the present invention. The color image sensor 2 and the color filter array 2a provided therein have the same configuration as in the first embodiment. The optical low-pass filter 90 includes a first optical member 91 consisting of a birefringent plate 94 and a phase plate 95 that converts linearly polarized light into circularly polarized light, and a second optical member 9 consisting of a birefringent plate 96 and a phase plate 97.
2, and a third optical member 93 made of a birefringent plate 98. The birefringent plate 94 divides the incident light beam by a distance Pl in a direction of 45° counterclockwise with respect to the horizontal scanning direction, and the two emitted light beams, the ordinary ray and the extraordinary ray, are divided into straight lines by the phase plate 95. The polarized light is converted to circularly polarized light, and the emitted light ray is divided by a distance P2 in parallel to the horizontal scanning direction by the birefringent plate 96. The linearly polarized light is converted to circularly polarized light by the phase plate 97, and the emitted light ray is is divided by a distance P3 in a direction of 145° counterclockwise with respect to the horizontal scanning direction by a birefringent plate 98, and the light rays finally exiting from the optical low-pass filter 9o are shown in FIG. 2(c).
There are 68 rays as shown in . Therefore, the MTF value of the optical low-pass filter 90 is as shown in equation (1), and the same frequency characteristics as in the first embodiment can be obtained.

Note that in the above embodiments, the first. Second. The third optical member does not necessarily have to be arranged in this order from the subject side, and the frequency characteristic of the optical low-pass filter is (1).
Any order is acceptable as long as it satisfies the formula.

In each example, the optical member includes one birefringent plate,
Alternatively, the present invention is configured by combining it with a phase plate, but the present invention is not limited to a rigid structure, and any device may be used as long as it splits the light into two directions, such as placing a prism in the optical system.

〔Effect of the invention〕

As described above, according to the present invention, moiré can be effectively prevented when used in a color single-chip image sensor having an offset sampling structure.

[Brief explanation of drawings]

FIG. 1 is a diagram showing the main part of the first embodiment of the present invention, FIG. 2 is a diagram explaining the function of the optical low-pass filter 1, and FIG. 3 is a diagram showing an example of an offset structure color filter arrangement.'! J
Figures 4 and 5 are diagrams explaining the offset sampling structure, Figure 6 (a) is a diagram showing the two-dimensional frequency characteristics of an ideal optical low-pass filter, and Figure 6 (b) is the diagram of the first embodiment. Figure 7 shows the two-dimensional frequency characteristics of the optical low-pass filter used; Figure 7 shows the conventional color filter array; Figure 8 shows the conventional color filter array;
This figure is a diagram showing the frequency characteristics of the color filter array of FIG. 7, and FIG. 9 is a diagram showing the main part configuration of a second embodiment of the present invention. 1.--One optical low-pass filter 2----Color image sensor 2a---=Color filter array 3---First optical member 4----1g2 optical member 5----Third optical components

Claims (1)

[Claims] (1) An imaging device characterized by comprising the following components a, b, and c. a, the horizontal pixel pitch is P_H, the vertical pixel pitch is P_V, and the horizontal pixel offset amount is P_H/
An imaging device having an offset sampling structure of 2. b. It has color filters of three or more colors, and each color filter has,
Horizontal pitch is 2P_H, vertical pitch is 2P
A color filter array having an offset structure provided in the image sensor, wherein the offset amount in the horizontal direction is P_H at _V. c. Direct the incident light beam at a distance P_ of 45° or -45° counterclockwise with respect to the horizontal scanning direction of the image sensor.
a first optical member that splits into two light beams separated by 1;
a second optical member that splits the incident light beam into two light beams separated by a distance P_2 in a direction parallel to the horizontal scanning direction; and a second optical member that splits the incident light beam into two light beams separated by a distance P_2; an optical low-pass filter that is provided in an imaging optical system and satisfies the following conditions. (2.12P_HP_V)/(P_H+2P_V)<P
_1<(3.54P_HP_V)/(P_H+2P_V
)0.375P_H<P_2<0.625P_H(2.
12P_HP_V)/(P_H+2P_V)<P_3<
(3.54P_HP_V)/(P_H+2P_V)