JP2785160B2 - Imaging device - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image pickup device having an offset sampling structure suitable for a video camera, an electronic still camera, and the like, and a color image pickup device having an optical low-pass filter.

[Conventional technology]

In order to obtain a color signal in a single-plate color video camera, for example, a striped color filter array configuration as shown in FIG. 7 is used. At this time, for a specific color, the moiré of the color occurs because the spatial sampling position in the horizontal direction is once every three pixels.
In order to prevent this, in a color video camera, the same light beam is divided and incident at a position shifted horizontally by 1.5 pixels using an optical low-pass filter composed of one birefringent plate.

In this case, the horizontal frequency characteristic (transfer characteristic MTF) of the optical low-pass filter is C as shown in FIG.
It takes the form of an OS and becomes 0 at f _S / 3. On the other hand, since the color moiré turns from here, such an optical low-pass filter is effective in suppressing moiré. Here f _S is the sampling frequency per pixel is f _S = 1 / d. d is the pitch of one pixel.

[Problems to be solved by the invention]

On the other hand, each pixel of the solid-state image sensor has recently been configured to have an offset sampling structure, and each pixel has a structure shown in FIG.
An array of color filters of the colors shown in FIG.

In FIG. 3 (a), the solid-state imaging device has an offset sampling structure having a horizontal pixel pitch P _H , a vertical pixel pitch P _V , and a horizontal pixel offset amount P _H / 2. Green, cyan, and yellow light transmission filters Mg, Gr, Cy, and Ye are arranged at positions corresponding to the respective pixels. Each color filter has an offset sampling structure of horizontal 2P _H , vertical 2P _V , and horizontal offset amount P _H.

In FIG. 3 (b), the solid-state imaging device has an offset sampling structure with a horizontal pixel pitch P _H , a vertical pixel pitch P _V , and a horizontal pixel offset amount P _H / 2. Red, green, and blue light transmission filters R, G,
B is arranged at a position corresponding to each pixel. As shown in the figure, the color filter is horizontal 2P _H of R and B, the vertical 2P _V, the offset amount in the horizontal direction is offset sampling structure of P _H. On the other hand although the color filter of G has a stripe shape in an oblique direction, which is also horizontal 2P _H, vertical 2P _V, P _H / 2 offset sampling structure of horizontal offset amount P _H is the horizontal direction, the vertical direction it can be viewed as the overlapping shifted from each other by P _V in.

In this case, since the color filters are arranged not two-dimensionally, as in the case of the conventional stripe, but two-dimensionally, an optical low-pass filter that prevents color moiré also has a special two-dimensional structure. It must be designed to satisfy the conditions, and any conventional optical low-pass filter did not achieve the desired effect.

The present invention has been made under such circumstances,
An object of the present invention is to provide an image pickup device in which an image pickup device using a color filter array having an offset structure as shown in FIGS. 3 (a) and 3 (b) is combined with an optical low-pass filter effective for suppressing moiré. Things.

[Means for solving the problem]

According to the present invention, the sampling structure when the color filters are arranged as shown in FIGS. 3 (a) and 3 (b) is carefully analyzed two-dimensionally to determine the optimum conditions of the optical low-pass filter for preventing moiré. Made by derivation.

First, consider an offset sampling structure in which the horizontal and vertical pitches are P _H and P _V , respectively, and the horizontal offset amount is P _H / 2 as shown in FIG. The sampling structure in this case is the fifth on the two-dimensional frequency plane.
As shown in the figure, the horizontal and vertical pitches are 2 / P _H , 1 / P _V
In which it is known that offset amount in the horizontal direction is offset structure is 1 / P _H. This derivation is based on, for example, "Digital Signal Processing of Images" by Takahiko Fukisaka, Nikkan Kogyo Shimbun (198
5) As described on page 317.

Therefore, in an image pickup device using a color filter array as shown in FIGS. 3A and 3B, a luminance signal by a so-called switch-Y method, in which a color signal obtained by a color filter of each pixel is equivalently regarded as a luminance signal. Is obtained, the sampling structure of the luminance signal is as indicated by a mark in FIG. 6 (a). For the respective color signals, for example, in the case of Mg in FIG. 3 (a), the horizontal and vertical pitches are 2P _H ,
In 2P _V, it can be seen that the horizontal offset has a still offset sampling structure of P _H. Therefore,
The sampling structure of the color signal is as shown by the square in FIG. 6 (a).

Therefore, an optical low-pass filter that is optimal for preventing color moiré has a diamond shape such that the frequency characteristic becomes zero or minimum around the solid line in FIG. 6A.

According to the present invention, based on the above consideration, the imaging apparatus is configured as in the following (1) in order to achieve the above object.

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

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

b.Has three or more color filters,
A color filter array having an offset structure provided in the image sensor, wherein a horizontal pitch is 2P _H , a vertical pitch is 2P _V , and a horizontal offset amount is P _H.

c. a first optical member for splitting the incident light into two light beams spaced by 45 ° or -45 ° direction counterclockwise to the direction of horizontal scanning by a distance P ₁ of the image pickup device, the incident a second optical member for dividing the light rays into two beams of light at a distance P ₂ in a direction parallel to the direction of horizontal scanning, 90 ° a light beam incident to the beam splitting direction of the first optical member third and an optical member, satisfy optical low-pass filter follows provided in the imaging optical system is divided into two light beams spaced in the direction by a distance P _3.

[Operation] According to the configuration (1), the color carriers generated by the color filter array are removed by the optical low-pass filter, and the occurrence of color moire is suppressed.

〔Example〕

Hereinafter, the present invention will be described in more detail with reference to Examples. In the description, “direction” refers to two directions that differ by 180 °, and “direction” refers to one direction.

As described above, the diamond characteristic is ideally good. However, in order to actually configure an optical low-pass filter using a birefringent plate or the like, as shown by a solid line in FIG. closest color of the carrier frequency _{(± 1 / 2P H, ±} 1 / 4P V) through the four straight lines which form a 45 ° angle ± a f _X axis _{l 1, l 1 ', l} 3, l
_A simple configuration is to make the frequency characteristic zero on _two straight lines l ₂ and l ₂ ′ parallel to the ₃ ′ and f _y axes.
To that end, a first optical member for splitting the two beams at the incident counterclockwise 45 ° or -45 ° direction with respect to the direction of the horizontal scanning light beam at intervals of P _1, the light beam incident a first optical member for dividing a direction parallel to the direction of horizontal scanning two light beams at intervals of P _2, the P ₃ rays incident at 90 ° direction with respect to dividing direction of the first optical member An optical low-pass filter may be constituted by the third optical member that divides the light into two light beams at intervals. 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 causes the occurrence of the chrominance signal carrier. Traps the neighborhood, does not degrade the horizontal resolution, and generates a luminance signal carrier The vicinity can be trapped, and all of the color signal and luminance signal carriers closest to the origin can be suppressed.

Further, in order for this optical low-pass filter to exhibit sufficient performance, straight lines l ₁ , l ₁ ', shown in FIG.
Since the distance from the origin of l ₃ , l ₃ ′ is l ₁ , l ₁ ′: 1 / 2P ₁ l ₃ , l ₃ ′: 1 / 2P ₃ , this value is Within ± 25% around the center and l ₂ , l from the origin
Distance 1 / 2P ₂ to ₂ 'to be within 25% plus or minus around the 1 / P _H.
Wherein a straight line l _1, l ₁ value four the h ₀ is ', l _3, l _3' This is the distance from the origin to the straight line when passing through.

In order to satisfy the above conditions, the separation (division) widths P ₁ , P ₂ ,
P ₃ conditions to be satisfied are, be a thing of the following.

In any case, when the value exceeds the upper limit, the amount of moiré due to luminance and chrominance signal carriers increases.

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

The optical low-pass filter 1 includes first, second, and third optical members 3, 4, and 5 including birefringent plates 6, 7, and 8.
The separation directions of the birefringent plates 6, 7, and 8 are 45 °, 0 °, and −45 ° counterclockwise with respect to the horizontal scanning direction, respectively. It is.

FIG. 2 shows how light rays are separated by the optical low-pass filter of FIG. FIG.
As shown in the figure, the incident light is split by the birefringent plate 6 in a direction of 45 ° counterclockwise with respect to the horizontal scanning direction, and an ordinary ray 9 having the same intensity and having a polarization component in the direction indicated by the solid line arrow in FIG. And the extraordinary ray 10 are emitted as two linearly polarized lights. Next, the light beam emitted from the birefringent plate 6 as shown in FIG.
The light beams 9 and 10 are split by the birefringent plate 7 in parallel in the horizontal scanning direction, and become four light beams 9, 10, 11, and 12. The polarization direction is the direction indicated by the solid line arrow in FIG. Next, as shown in FIG. 3C, the light beams 9, 10, 11, and 12 emitted from the birefringent plate 7 are split by the birefringent plate 8 in a direction of −45 ° counterclockwise with respect to the horizontal scanning direction. As a result, eight light beams 9, 10, 11, 12, 13, 14, 15, and 16 are obtained, and these eight light beams are eventually 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 Thus, as shown in FIG. 6 (b), the suppression of the moire by the luminance carrier and the chrominance signal carrier is performed more favorably.

FIG. 9 shows a main configuration 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 including a birefringent plate 94 and a phase plate 95 that converts linearly polarized light into circularly polarized light, a second optical member 92 including a birefringent plate 96 and a phase plate 97, Birefringent plate
And a third optical member 93 made up of 98. Birefringent plate 94, the light incident to the direction of horizontal scanning by dividing by 45 ° direction counterclockwise by a distance P _1, 2 pieces of rays of the injected ordinary ray and extraordinary ray by the phase plate 95 is converted from linearly polarized light into circularly polarized light, which light rays emitted by are separated by a distance P ₂ parallel to the direction of horizontal scanning by the birefringent plate 96, it is converted from linearly polarized light into circularly polarized light by the phase plate 97, thereby injection the light rays are divided by a distance P ₃ at -45 ° direction with respect to the direction of the horizontal scanning in a counterclockwise direction by the birefringent plate 98, light rays eventually emitted from the optical low-pass filter 90 shown in FIG. 2 (c) Eight light rays similar to the above are obtained. For this reason, 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.

In the above embodiment, the first, second, and third optical members do not necessarily need to be arranged in this order from the object side, and the frequency characteristics of the optical low-pass filter are those of the formula (1). Any order may be used. Further, in each of the embodiments, the optical member is constituted by a single birefringent plate or a combination with a phase plate. However, the present invention is not limited to this. May be used as long as it is divided into two directions.

〔The invention's effect〕

As described above, according to the present invention, moire can be effectively prevented by using a color single-chip image sensor having an offset sampling structure.

[Brief description of the drawings]

FIG. 1 is a diagram showing a main part of a first embodiment of the present invention, FIG. 2 is a diagram for explaining the operation of the optical low-pass filter 1, FIG. 3 is a diagram showing an example of an offset structure color filter array, FIG. Figure,
FIG. 5 is a diagram illustrating an offset sampling structure, FIG. 6 (a) is a diagram illustrating a two-dimensional frequency characteristic of an ideal optical low-pass filter, and FIG. 6 (b) is an optical diagram used in the first embodiment. FIG. 7 shows a two-dimensional frequency characteristic of a low-pass filter, FIG. 7 shows a conventional color filter array, FIG. 8 shows a frequency characteristic of the color filter array of FIG. 7, and FIG. It is a principal part block diagram of 2 Example. DESCRIPTION OF SYMBOLS 1 ... Optical low-pass filter 2 ... Color image sensor 2a ... Color filter array 3 ... 1st optical member 4 ... 2nd optical member 5 ... 3rd optical member

Claims (1)

(57) [Claims] 1. An image pickup apparatus comprising the following components a, b, and c. a. The horizontal pixel pitch is P _H and the vertical pixel pitch is P
An image sensor having an offset sampling structure in which the pixel offset amount in the horizontal direction at _V is P _H / 2. b.Having three or more color filters, the horizontal pitch of each color filter is 2P _H , the vertical pitch is 2P _V and the horizontal offset is P _H , provided in the image sensor. Color filter array with offset structure. c. a first optical member for splitting the incident light into two light beams spaced by 45 ° or -45 ° direction counterclockwise to the direction of horizontal scanning by a distance P ₁ of the image pickup device, the incident a second optical member for dividing the light rays into two beams of light at a distance P ₂ in a direction parallel to the direction of horizontal scanning, 90 ° a light beam incident to the beam splitting direction of the first optical member third and an optical member, satisfy optical low-pass filter follows provided in the imaging optical system is divided into two light beams spaced in the direction by a distance P _3.