JPH0668586B2 - Color solid-state imaging device - Google Patents

Description

Description: TECHNICAL FIELD OF THE INVENTION The present invention relates to a color solid-state image pickup device for obtaining a television image signal by using a CCD-type, MOS-type, or other solid-state image pickup device.

[Technical background of the invention]

Conventionally, a photoelectric conversion device for converting optical information into electrical information generally uses an image pickup tube, but it has drawbacks such as poor productivity, short life, and image sticking. Recently, image pickup devices have been receiving attention. However, in the image pickup tube, as compared with the case where an electron beam having a certain spread moves linearly to obtain electric information, in the solid-state image pickup element, the photosensitive parts having a fixed aperture are discretely arranged in space. Electrical information will be obtained by spatial sampling. In the case of spatial sampling, there is a drawback in that the frequency above the Nyquist frequency cannot be reproduced, the spatial frequency response (MTF) at that frequency is determined by the aperture of the photosensitive section, and the pass band contains many aliasing distortion components. In particular, in the case of the single plate type using a color mosaic filter adopted in a home video camera, in addition to aliasing distortion due to pixel pitch, aliasing distortion due to sampling of color information is present, and thus a complicated false signal is generated.

In the present invention, attention is paid to an imaging method that causes aliasing distortion due to a pixel pitch and aliasing distortion related to two color signals, and a method for optically reducing a false signal due to the aliasing distortion is considered.

FIG. 1 is a color filter array in an imaging method in which two color aliasing distortions are generated, and FIG. 2 is a spatial frequency domain that describes aliasing distortions in the color filter array, with spatial sampling frequency axes ω _x and ω _y as a reference. FIG. In FIG. 1, 2 × is used to enable interlaced scanning.
Four pixels are shown as a basic color array. Also W, G, Ye,
Each symbol of Cy is defined as follows.

W: All color light transmission = R + G + B G; Green light transmission = G Ye; Yellow light transmission = R + G Cy; Cyan light transmission = G + B Here, R, G, and B represent three primary colors of light: red light, green light, and blue light. . Therefore, when the color filter array 11 of FIG. 1 is decomposed into three primary color lights, it can be seen as separation filters 12, 13, and 14. The color filter array 11 is an application of a single tube frequency interleave method to a solid-state image pickup element, and is already known in Japanese Patent Laid-Open No. 57-203389. As can be seen from FIG. 1, the phase of the red light R is inverted between the horizontal lines, and the phase of the blue light B is the same. Therefore, the R and B signals are separated by adding and subtracting the signals using the comb filter using the 1 horizontal line H delay circuit.

Next, the folding distortion will be described with reference to FIG. If the pixel pitch in the horizontal direction is P _x and the pixel pitch in the vertical direction is P _y , the horizontal and vertical Nyquist limits due to the pixel repetition pitch are 1 / 2P _x and 1 / 2P _y indicated by the triangle, and more The spatial frequency of is turned back to the body frequency, becomes a zero beat at the points of ± 1 / P _x and ± 1 / P _{y in} the horizontal and vertical directions, and generates the strongest spurious signal. This point is indicated by a cross mark in FIG. This aliasing distortion appears as a false signal in the luminance signal on the reproduction screen. With respect to the color signal, the correlation processing of the signal obtained from the basic array by repeating the 2 × 2 pixel pitch in the same field is performed, so that the zero beat point is generated at a point lower than the luminance signal. Green light, the sampling pitch for 2P _x, generates a blue or complementary strong color false signal that the position in the horizontal direction ± 1 / 2P _x is the zero beat point. Since the two-line correlation is used in the vertical direction, the vertical resolution is 1/2 of the luminance signal, and the zero beat point is generated at the position of ± 1 / 2P _y . These points are indicated by marks in FIG. Next, the red signal has a checkerboard pattern of 2P _x and 2P _y periods, and although the horizontal resolution is improved, the diagonal resolution is reduced. The zero beat point for this red signal is the position of (± 1 / 2P _x , ± 1 / 4P _y ),
A false color signal of red or its complementary color is generated. The zero beat point in the vertical direction for both red and green signals is on the air-spatial frequency axis ω _y .
It occurs at the point 1 / 2P _y . This is because 2-line correlation is used for color signal processing, and 1/0 of the zero beat point for the luminance signal is used.
Position 2

It is conventionally known to use an optical filter means in order to reduce the false signal at the zero beat point due to the aliasing distortion. An optical band-limiting filter having a zero on a vertical line passing through ± 1 / 2P _x on a spatial frequency axis ω _y , an optical band-limiting filter having a zero on a vertical line passing through ± 1 / P _x , and a spatial frequency axis ω _y False filters are suppressed by using three filters, an optical band limiting filter having a zero on a horizontal line passing through the above ± 1/2 P _y . The restricted position by these filters is shown by the dotted line in FIG.

The optical band limiting filter utilizes the interference effect of birefringent light and ordinary light of a substance having a birefringent effect such as quartz, and its spatial frequency characteristic is known to be a cosine wave shape. The spatial frequency characteristics in the horizontal and vertical directions of the above three optical band limiting filters are as shown in FIG. That is, the characteristic of FIG. 3A is a characteristic for suppressing the false signal of the luminance signal, and the characteristic of FIG. 3B is a characteristic for suppressing the color false signal of the red and blue signals.

When these spatial frequencies are associated with television signals, horizontal 380 pixels and vertical 4 pixels, which are commonly used at present, are used.
The solid-state image sensor with about 80 pixels is as follows.

1 / 2P _x ; color carrier frequency in frequency interleave system 3.56 MHz 1 / P _x ; horizontal pixel transfer frequency 7.16 MHz 1 / 2P _x ; vertical limit resolution 480 TV lines Therefore, as shown in FIG. The aliasing distortion at the position marked with ∘ is within the band of the television signal, which gives a visually very deteriorated image quality.

[Problems of background technology]

As described above, in the spatial sampling by the photosensitive portion of the solid-state image sensor, the false signal at the zero beat point of the aliasing distortion component at the spatial frequency is reduced by the optical band limiting filter having the cosine wave characteristic. However, since this band-limiting filter has a cosine-wave characteristic, it can completely cancel the signal at the zero beat point, but it can completely suppress the components at the frequency positions slightly apart from the zero beat point. has no effect. In particular, in the case of color false signals, the development that colors are added to the non-colored parts appears on the screen,
There has been a strong demand for improvement in this respect.

[Object of the Invention]

The present invention has been made in view of the above circumstances, and by significantly reducing aliasing distortion caused by spatial sampling by pixels of a solid-state image sensor, a color solid image that can obtain a high-quality color reproduction image without false signals is obtained. An object is to provide an imaging device.

[Outline of Invention]

This invention is based on the attenuation frequency line 4a shown by the broken line in FIG.
The object described above is achieved by providing an optical filter that realizes 4f and has a spatial frequency characteristic represented by the square of the cosine.

Example of Invention

 The embodiments of the present invention will be described with reference to the drawings.

First, the direction in which the band is optically limited in the spatial frequency band and its characteristics are shown. Next, a configuration example of an optical band limiting filter using a birefringent plate for realizing this is shown.

FIG. 4 is a diagram showing the direction of band limitation according to the present invention. That is, the direction indicated by the dotted line is the optical band limiting direction, and the spatial frequency positions (0, ± 1 / 2P _y ), (± 1 / 2P _x , 0)
Stop the conventional optical band limitation that passes through (0, ± 1/2
P _y ), (± 1/2 P _x , ± 1/4 P _y ), (± 1 / P _x , 0), and band limitation on a rhombic line.

Here, the formulas of the band limiting characteristic of the conventional filter and the band limiting characteristic of the present invention are compared and shown as follows.

Spatial frequency axis (ω _x ) direction Conventional; MTF (ω _x ) = cos (πP _x ω _x ) ・ cos (π / 2P _x ω _x ) The present invention; MTF (ω _x ) = cos (πP _x ω _x ) ・cos ² (π / 2
P _x ω _x ) Spatial frequency axis (ω _y ) direction Conventional; MTF (ω _y ) = cos (πP _y ω _y ) The present invention; MTF (ω _y ) = cos ² (π / 2P _y ω _y ) Spatial frequency axis (Ω _s ) direction As seen in the above equation, the zero beat of the luminance signal generated at (1 / P _x , 0) and (0, 1 / 2P _y ), (1 / 2P _x , 1 / 4P
_The color signal generated in _y ) has a cosine square characteristic in its vicinity. That is, the characteristic of each band difference limiting filter in the directions of the axis ω _y , the axis ω _s , and the axis ω _x is as shown by the dotted line in FIGS. 5 (a), (b), and (c). The solid line is the characteristic of the conventional one. As can be seen from this characteristic diagram, the aliasing distortion component in the vicinity of the zero beat point can be remarkably suppressed by the cosine square characteristic as compared with the conventional case. The zero beat occurring at the position of (1 / 2P _x , 0) has almost no change inside, but the outside can be reduced to about 1/2 at the peak due to the cosine square characteristic. Since the inside is within the band of the luminance signal, it is desirable that it is not attenuated, and this method of attenuating only unnecessary aliasing distortion on the outside is a very effective means. As described above, according to the present invention, aliasing distortion that occurs when spatially sampling pixels that are discretely arranged can be significantly reduced as a whole.

FIG. 6 is a structural explanatory view for obtaining an optical filter having the above-mentioned characteristics. S _{1 to} S ₅ are optical comb filters using a birefringent material such as quartz. Further, R _h is a depolarizing plate and is provided so that the characteristics of the filter parts on both sides do not interfere with each other.

In FIG. 6 (b), the light separating directions of the respective optical comb filters S _{1 to} S ₅ are indicated by arrows. Then, S ₁ + S ₂ indicates the light separation direction due to the combined characteristics of the optical comb filters S ₁ and S ₂ . Further, S ₃ + S ₄ indicates a light separating direction due to the combined characteristic of the optical comb filters S ₃ and S ₄ .

The optical comb filters S ₁ and S ₃ are arranged so that the horizontal sampling frequency of the image sensor is the first attenuation frequency, and the optical comb filters S ₂ and S ₄ are arranged such that the vertical sampling frequency is the first attenuation frequency. Furthermore, in the optical comb filter S ₅ , the thickness of each optical comb filter S ₅ is selected such that the half of the horizontal sampling frequency becomes the first attenuation frequency.

As described above, by directly bonding the filters having the axes of the orthogonal light separation directions, the characteristic of the vector sum of the respective filter characteristics can be obtained. By combining the optical comb filters S ₁ and S ₂ , the oblique optical comb filter characteristics shown in FIG. 4 are obtained. This filter characteristic is shown by broken line 4
The spatial frequencies indicated by a and 4b have a first attenuation frequency. The combined characteristics of the optical comb filters S ₃ and S ₄ have the first attenuation frequency at the spatial frequencies indicated by the broken lines 4c and 4d. At the attenuation frequency positions shown by the broken lines 4a, 4b, 4c, and 4d, aliasing distortion appears with respect to the color signal.
Pass through the marked position.

Further, the optical comb filter S ₅ has the first attenuation frequency at the spatial frequencies indicated by the broken lines 4e and 4f. The attenuation frequency of this filter S ₅ is the horizontal sampling frequency.
It has a frequency of 1/2 and passes through the zero beat points marked with ○ and △, including the position of ± 1 / 2P _x .

FIG. 7 shows another configuration example of the optical band limiting filter. The optical comb films S ₁₁ and S ₁₃ are 45
A filter having an axis of an angle of °, which is a combination of the above optical comb filters S ₁ and S ₂ and the optical comb filter S.
_It corresponds to the composite of ₃ and S ₄ . The optical comb filter S ₁₂ corresponds to the optical comb filter S _{5 described} above. As a result, the convolution characteristic of each filter, that is, the characteristic shown in FIG. 4 can be obtained. In the case of this embodiment, the optical band limiting filter according to the present invention can be realized with a smaller number of filter plates than the filter of FIG.

In the above-mentioned embodiment, the inclination of the polarization axis of the optical comb filters S ₁₁ and S ₁₃ with respect to the horizontal direction is set to 45 °, which is most effective in view of the polarization characteristics of the crystal filter. Because it is easy. In this case, considering that the false signal of the color is visually more conspicuous than the false signal of the luminance, it is preferable to configure the optical filter so that the zero point of the filter characteristic passes through the zero beat point in the oblique direction for red. . In the case of a 380 × 480 pixel solid-state image sensor which is generally used at present, there is no problem in practical use at the inside of the zero beat point related to the luminance signal in the horizontal direction, but there is no problem in practical use. This is due to the signal side, resulting in the attenuation of the color artifacts that are visually noticeable,
Show better effect.

The present invention is not limited to the above embodiment, and various modifications can be made to the number of filter plates, the axial direction, and the like.

Further, the filter array can also be applied to the color filter arrays as shown in FIGS. In a solid-state image pickup device using these color filter arrays 8a or 9a, horizontal scanning line signals of upper and lower two lines are added and read out, and the output signals are color signals by the signal processing method of the frequency interleave method as in the previous embodiment. Separation takes place.

In the case of the color filter arrays 8a and 9a, equivalently, red,
The spatial sampling points for blue are the same as in the previous example. Therefore, the zero beat generation point of the false signal is the same as that of the color filter array 11 described above, and the false signal can be significantly suppressed by the optical band limiting filter described above.

〔The invention's effect〕

As described above, according to the present invention, the aliasing distortion of the luminance signal and the chrominance signal, which is caused by the spatial sampling performed by the spatially discrete photosensitive portions, is changed by changing the direction of the optical band limitation. By taking the cosine squared characteristic,
It is possible to greatly reduce the number, and it is possible to obtain a high-quality color reproduction image without false signals.

[Brief description of drawings]

FIG. 1 is a diagram showing an example of a color filter array used in a color solid-state imaging device using a frequency interleave method,
FIG. 2 is a characteristic diagram for explaining the conventional optical band limiting characteristic,
FIG. 3 is also a characteristic diagram showing conventional optical band limiting characteristics divided for each spatial frequency axis, FIG. 4 is a characteristic diagram showing optical band limiting characteristics according to the present invention, and FIG. FIG. 6 is a characteristic diagram showing the optical band limiting characteristics divided for each spatial frequency axis, and FIG. 6 is an explanatory diagram showing an embodiment of an optical band limiting filter for obtaining the characteristics of FIGS. 4 and 5. FIG. 7 is an explanatory view showing another example of the optical band limiting filter shown in FIG. 6, and FIGS. 8 and 9 are views showing other examples of color filter arrays to which the present invention can be applied. . S _{1 to} S ₅ , S _{11 to} S ₁₃ ... Optical comb filter.

 ─────────────────────────────────────────────────── --Continued from the front page (56) References JP-A-57-203389 (JP, A) JP-A-59-75222 (JP, A)

Claims (1)

[Claims] 1. A basic color array of 2 × 2 pixels in the vertical and horizontal directions within the same field of a solid-state image sensor in which photosensitive units are arranged two-dimensionally in the vertical and horizontal directions, and at least the first
In a color solid-state imaging device for obtaining a color video signal by arranging pixels for obtaining information on the color of a checkerboard pattern and pixels for obtaining information of a second color on a stripe pattern, Two sets of optical band-limiting filter members are provided so that the zero-points of the optical band-limiting filter are rhombic so that the zero-points of the optical band-limiting filter pass through four zero-beat points diagonally generated by spatial sampling of one color. An optical band limit is provided in front of the image pickup element, and connects a zero beat point generated by the spatial sampling of the first color and two zero beat points generated in the horizontal direction by the spatial sampling of the second color. 1 so that the zero of the filter passes at a right angle to the horizontal axis
A color solid-state image pickup device comprising a pair of optical band-limiting filter members provided in front of the solid-state image pickup device.