JPH04170884A - Image pickup device - Google Patents

Abstract

PURPOSE:To suppress a color 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 offset quantity in a horizontal direction is PH/2. CONSTITUTION:The image pickup element 2 of 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 in the horizontal direction is PH/2 is given. An optical low pass filter 1 which has more than three kinds of color filters, in which the 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 filters, which has a color filter array 2a provided for the image pickup element and two specified optical members, which is provided for an image pickup optical system and which satisfies the condition of an inequality I is provided. Thus, a color carrier generated by the color filter array 2a is removed by the optical low pass filter 1 and the occurrence of the color 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.

(Prior Art) In order to obtain a color signal in a single-chip color video camera, a stripe-like color filter array configuration 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 O 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, fs is the sampling frequency per pixel, and f,
=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
An arrangement of colored filters as shown in the figure is attracting attention.

In FIG. 3(a), the solid-state image sensor has an offset sunblink structure with a horizontal pixel pitch PH1, a vertical pixel pitch PV, and a horizontal pixel offset jlpH/2, and has magenta, green, cyan, Yellow light transmission filter Mg.

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

The vertical offset amount is 2 PV, and the horizontal offset amount is PI (offset sampling structure).

Further, in FIG. 3(b), the solid-state image sensor has a horizontal pixel pitch of PI4. It has an offset sampling structure of vertical pixel pitch PV+horizontal pixel offset amount PM/2, and red, green, and blue light transmission filters R1G and B are arranged at positions corresponding to each pixel. As shown in the figure, the R and B color filters are
It has an offset sampling structure of P8° vertical 2 PV and horizontal offset amount P□. On the other hand, the G color filter has a diagonal stripe shape, which also has horizontal 2P, 4. Offset sampling structure with vertical 2 PV and horizontal offset amount PH
/2. It can be seen that they overlap each other by being shifted 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 even conventional optical low-pass filters could not achieve the desired effect.

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

[Means for solving Problem B]

The present invention carefully analyzes the sunblink structure two-dimensionally when the color filters are arranged as shown in FIGS. This was done by deriving the conditions.

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 /Z. In this case, the sampling structure has a horizontal and vertical pitch of 2/2 on a two-dimensional frequency plane, as shown in Figure 5.
P.H.

It is known that the offset structure has an offset amount of 1/P in the horizontal direction at 1/PV.

This derivation is described, for example, in "Digital Signal Processing of Images" by Takahiko Fukisaka, Nikkan Kogyo Shimbunsha (1985) PH317.

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 becomes as shown in FIG. 6(a). Also, if we look at each color signal, for example, Fig. 3 (a
), the horizontal and vertical pitches are 2P each)
1.2Pν, it can be seen that the horizontal offset also affects the offset sampling structure of the PM.

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 following components a, b, and c.

a, horizontal pixel pitch is P, 4. The vertical pixel pitch is PV and the horizontal pixel offset is PM /2
An image sensor having an offset sampling structure.

b. There are three types of similar color filters, and each color filter has a
Horizontal pitch is 2PH, 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 PH at v.

C1 The incident light ray is measured at a distance PI in the 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 ray into two light rays separated by a distance of 11P2, and a second optical member that splits the incident light ray into two light rays separated by a distance 11P2 in a direction 90° to the splitting direction of the first optical member. An optical low-pass filter having an optical member provided in an imaging optical system and satisfying the following conditions.

2.12PHPV 3.54PHPV-<p,
< - PH+2PV PH◆2PV 2, l2P) IPV 3.54P, PV-<P
2<-Ps'' 2PV PHφ2PH (Function) With the configuration (1) above, the color carriers generated by the color filter array are removed by the optical low-pass filter, and the occurrence of color 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, a 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 (±x/2PI4.±1/4P
Four straight lines passing through V) and forming an angle of ±45° with the fX axis 1. .

The simplest method is to make the frequency characteristic O on x,', x□, 1□'. To do this, a first optical member that splits the incident light beam into two light beams with an interval of Pl in a direction of 45° or -45° counterclockwise with respect to the horizontal scanning direction, and An optical low-pass filter may be configured with a second optical member that divides the first optical member into two light beams with an interval of P2 in a direction 90° with respect to the dividing direction of the first optical member. At this time, the two-dimensional MTF value, which is the frequency characteristic of the optical low-pass filter, is as follows.

From this, the MTF value changes in the form of a cosine function in the direction making ±45 degrees from the horizontal direction, and in the form of the square of the cosine, which is the product of both in the horizontal and vertical directions. The aliasing distortion from the diagonal direction is well suppressed.

Furthermore, in order for this optical low-pass filter to exhibit sufficient performance, the four straight lines shown in Figure 6(b) must be
Can it pass near 1/2PH, ±1/4 Pv)? Delicious. To do this, the distances of these four straight lines from the origin are it +, 11 I': 1/2 P I4
22, j! □': 1/2P2, so this value should be within plus or minus 25% around . Here, h is four straight lines f, , j2 . '.

u2. I12° is (±1/2P, 4.±t 74P
V ) is the distance from the origin to the straight line when it is turned off.

For this purpose, the separation (or division) widths p, , p2 should satisfy the following conditions.

2.12Pru'<p2<”−■j飄 −=(3)PH+
2PV PH'' 2PVFor both formulas, if the upper limit is exceeded, the amount of color moiré increases, and if the lower limit is exceeded, the resolution of the entire system consisting of the optical low-pass filter and the image sensor decreases, which is not preferable.

FIG. 1 shows the main part configuration of an "imaging device" which is a first embodiment of the present invention. In the figure, reference numeral 1 denotes an optical low-noise filter, and reference numeral 2 denotes a color 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 is composed of a first optical member 3 consisting of a birefringent plate 5.6 and a second optical member 4 consisting of a birefringent plate 7. Birefringent plates 5, 6.7 in the figure
The separation direction is 90° counterclockwise with respect to the horizontal scanning direction.

0°, 45°, and the size of the separation width is p+/, /7.
P, /-r, P2. FIG. 2 shows how the optical low-pass filter 1 shown in FIG. 1 separates light rays. As shown in Figure (a), the incident light is emitted by the birefringent plate 5 as two linearly polarized lights, an ordinary ray 8 and an extraordinary ray 9, of equal intensity and having polarization components in the direction shown by the solid arrow in the figure. Ru. Next, the ray 9 enters the birefringent plate 6 whose optical axis is tilted by 90 degrees with respect to the birefringent plate 5, but since the ray 9 becomes an ordinary ray with respect to the birefringent plate 6, it continues straight and exits.
Since the light ray 8 becomes an extraordinary ray, it is refracted and emitted to the position 10. This situation is shown in FIG. 4(b). In the end, from the optical member 3 consisting of the birefringent plate 5.6, two light rays 9.10 are obtained which are separated in the direction of -45° counterclockwise with respect to the horizontal scanning direction by a separation width P as shown in the figure. . Each of these is 90° counterclockwise relative to the horizontal scanning direction.
Since the light is polarized at 0°, the birefringent plate 7 separates the light into four beams of equal intensity, as shown in FIG. 2(C). The separation width by the birefringent plate 7 is P2. And p...
The desired performance is obtained by setting the value of P2 to satisfy equations (2) and (3).

FIG. 9 is a diagram showing the main part configuration of 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 is a phase plate 9 that converts linearly polarized light into circularly polarized light.
4 and a birefringent plate 95, and a second optical member 91 consisting of a birefringent plate 93.

The first optical member 92 and the second optical member 91 do not necessarily have to be arranged in this order from the image sensor 2 side, and may be arranged in the reverse order. The separation width of the birefringent plate 93 is P2, and the two linearly polarized lights, the ordinary ray and the extraordinary ray, are separated by the phase plate 94.
The light is converted into circularly polarized light by a birefringent plate 95 with a separation width of P1, and is separated into four light beams of equal intensity as shown in FIG. 2(c).

FIG. 10 is a diagram showing the main structure of a third embodiment of the present invention. Here, an optical low-pass filter 100 is used instead of the phase plate 94 in the optical low-pass filter 90 of FIG. A birefringent plate 96 having a separation direction in the 0° direction is used. This separation width is d3
Then, as shown in FIG. 11, the light rays emitted from the optical low-pass filter 100 are a total of eight light rays, in which the light rays in FIG. 2(c) are shifted by d3 in the 45° direction and overlapped. , d3 are sufficiently small, the frequency characteristics are almost the same as those in FIG. 2(C). Furthermore, this is preferable because the frequency characteristics have less wavelength dependence than when the phase plate 94 is used, and the overall thickness can be made thinner and more compact.

In each of the above embodiments, each optical member is configured by one or more birefringent plates, or a combination of these and a phase plate, but the present invention is not limited to these. For example, any device that splits light into two directions, such as a prism placed in the optical system, may be used.

〔Effect of the invention〕

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

[Brief explanation of the drawing]

FIG. 1 is a block diagram of the main parts of the first embodiment of the present invention, FIG. 2 is a diagram explaining the action of the optical low-pass filter 1, FIG. 3 is a diagram showing an example of color filter arrangement, FIGS. Figure 5 is a diagram 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 a diagram showing the optical low-pass used in the first embodiment. FIG. 7 is a diagram showing the two-dimensional frequency characteristics of the filter. FIG. 7 is a diagram showing the conventional color filter array. FIG. 8 is a diagram showing the frequency characteristics of the color filter array of FIG. FIG. 10 is a block diagram of main parts of a third embodiment of the present invention. 1------Optical low pass filter 2------Color image sensor 2a---Color filter array 3---First optical member 4---Second optical member

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 image sensor having a Z offset sampling structure. b. There are three types of similar color filters, and each color filter has a
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 ray into two light rays separated by a distance P_2 in a direction of 90 degrees with respect to the dividing direction of the first optical member, and is provided in the imaging optical system, and Optical low-pass filter that meets the requirements. (2.12P_HP_V)/(P_H+2P_V)<P
_1<(3.54P_HP_V)/(P_H+2P_V
)(2.12P_HP_V)/(P_H+2P_V)<
P_2<(3.54P_HP_V)/(P_H+2P_
V)