JP3526381B2 - Solid-state imaging device - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a solid-state image pickup device using a CCD image sensor (solid-state image pickup device) for preventing the generation of moire.

[0002]

2. Description of the Related Art In a solid-state image pickup device such as a television camera using a CCD image sensor, a subject image is projected on a light receiving surface of the image sensor by an optical mechanism including a single lens or a combination of a plurality of lenses. Then, the information charges corresponding to the projected image are respectively stored in the plurality of light receiving pixels arranged in a matrix on the light receiving surface.

FIG. 5 is a block diagram showing the structure of a solid-state image pickup device using a CCD image sensor, and FIG. 6 is a timing chart for explaining its operation. The CCD image sensor 1 has a light receiving surface in which a plurality of light receiving pixels are arranged in a matrix, and by receiving a subject image on this light receiving surface, each light receiving pixel generates and accumulates information charges corresponding to the subject image. . The image sensor 1 includes a vertical transfer section associated with each column of a plurality of light receiving pixels and a horizontal transfer section arranged on the output side of the vertical transfer section. The information charges accumulated in are continuously transferred and output on a row-by-row basis. When the image sensor 1 is of the frame transfer type, the vertical transfer unit is arranged in series in each column of the light receiving pixels, and the information charges of each light receiving pixel are serially transferred to the vertical transfer unit. When the image sensor 1 uses the interline transfer method, the vertical transfer units are arranged in parallel in the respective columns of the light receiving pixels, and the information charges of the respective light receiving pixels are transferred in parallel to the vertical transfer unit. Further, the image sensor 1 includes an output unit that receives the information charges transferred and output from the horizontal transfer unit in an electrically independent capacitance and converts the charge amount into a voltage value, and changes the information charge amount of each light receiving pixel. It is output as an image signal Y0 (t) represented by a change in voltage value.

The optical mechanism 2 includes one or a plurality of lenses for forming a subject image on the light receiving surface of the image sensor 1 and a lens barrel for supporting the lenses, and the image sensor 1 from the subject.
Is arranged on the optical path leading to the light receiving surface of. In this optical mechanism 2, as shown in FIG. 7, the light transmitted through the convex lens is converged to one point on the light receiving surface of the image sensor 1,
A subject image is formed on the light receiving surface of the image sensor 1.

The drive circuit 3 generates a multi-phase vertical transfer clock φv and a horizontal transfer clock φh in response to the vertical sync signal VT and the horizontal sync signal HT, and supplies them to the vertical transfer section and the horizontal transfer section of the image sensor 1. To do. That is, the vertical transfer unit and the horizontal transfer unit of the image sensor 1 are CCD shift registers, and the information charges of the respective light receiving pixels are transferred in a predetermined order by being pulse-driven by the vertical transfer clock φv and the horizontal transfer clock φh, respectively. Output. For example, the read clock φf is generated in synchronization with the vertical synchronizing signal VT to transfer the information charges of each light receiving pixel to the vertical transfer unit. After that, the vertical transfer clock φv is generated in synchronization with the horizontal synchronization signal HT to transfer the information charges of the vertical transfer unit to the horizontal transfer unit row by row, and the horizontal transfer clock φh is generated to generate the information charges of the horizontal transfer unit. Is output to the output unit. At the same time, the drive circuit 3 generates a reset clock φr synchronized with the horizontal transfer clock φh and supplies it to the output section of the image sensor 1. This reset clock φ
By r, the information charges transferred and output from the horizontal transfer unit to the output unit are discharged for each pixel, and the information charge amount is converted into a voltage value on a pixel-by-pixel basis.

The timing control circuit 4 determines the timing of vertical scanning and horizontal scanning of the image sensor based on the reference clock CLK having a constant cycle.
T and a horizontal synchronizing signal HT are generated. For example, NTSC
When the system is followed, the reference clock of 14.32MHz is 9
The frequency is divided by 10 to generate a horizontal synchronizing signal HT, and the horizontal synchronizing signal is divided by 525/2 to generate a vertical synchronizing signal VT. In order to keep the exposure state of the image sensor 1 optimal, the timing control circuit 4 corresponds to the amount of information charges generated in each light receiving pixel of the image sensor 1 in the light receiving pixel in the middle of the vertical scanning period. Shutter control for discharging information charges is performed. That is, if the timing of the shutter operation is advanced, the period from the start of the accumulation of the information charges to the start of the readout becomes long, and the information charges are accumulated in each light receiving pixel for a longer period.
On the contrary, if the timing of the shutter operation is delayed, the period from the start of accumulation of information charges to the start of reading becomes shorter,
In each light receiving pixel, information charges are accumulated in a short period. The shutter operation for discharging the information charges is executed by the action of the drive clock supplied from the drive circuit 3 to the image sensor 1.

The signal processing circuit 5 receives the vertical synchronizing signal VT and the horizontal synchronizing signal HT, operates in synchronization with the driving circuit 3, and samples and holds the image signal Y0 (t) output from the image sensor 1. Performs various signal processing such as gamma correction. For example, in the sample hold processing, the image signal Y0 (t) in which the reset level and the signal level are repeated
Only the signal level is extracted from the image signal, and in the gamma correction process, the shift of the human visual linearity with respect to the intensity of the image signal Y0 (t) is corrected. Further, in the signal processing circuit 5, a sync component according to the vertical sync signal VT and the horizontal sync signal HT is added to the image component of the image signal Y0 (t). As a result, the image signal Y1 (t) according to the predetermined format is generated and output to the circuit of the next stage.

[0008]

The resolution of the image sensor 1 is determined by the arrangement pitch of the light receiving pixels on the light receiving surface. In the image sensor 1 having a predetermined resolution,
When a regular pattern finer than the resolution, that is, a subject whose light and dark arrangement pitch is narrower than the arrangement pitch of the light receiving pixels is photographed, periodic beat noise (moiré) occurs on the reproduction screen.

The cause of the occurrence of the moire is as follows. As shown in FIG. 8, when the arrangement pitch d of the subject patterns is 3/4 with respect to the resolution of the image sensor 1, the light receiving surface of the image sensor 1 is 3/4 of the arrangement pitch p of the light receiving pixels. The pattern of the subject is projected at the doubled interval t. At this time, in each light receiving pixel on the light receiving surface, information charges are generated corresponding to the pattern of the subject, but since the number of corresponding patterns can be one light receiving pixel and two light receiving pixels, they are generated in each light receiving pixel. A difference also occurs in the amount of information charges to be applied. For example, when the signal intensity generated by one pattern is "3", the light receiving pixels having the signal intensity "6" are consecutive with two light receiving pixels having the signal intensity "3" placed therebetween. Therefore, since the strength of the image signal Y0 (t) is repeated three times as long as the original cycle t of the subject pattern, the reproduction screen displayed based on this image signal Y0 (t). Beat noise on top, ie
Moire occurs.

In order to prevent the occurrence of such moire,
In general, the optical mechanism 2 is provided with an optical low-pass filter that prevents light of a fine image exceeding the resolution of the image sensor 1 from passing through. A quartz birefringent plate, a diffraction grating, or the like is used as the optical low-pass filter. However, when the crystal birefringent plate is used, the crystal birefringent plate itself is expensive, which leads to a large increase in the cost of the device, and when the diffraction grating is used, the light transmittance of the diffraction grating is increased. However, there is a problem in that the light receiving sensitivity of the device is lowered. Further, providing the optical mechanism 2 with a complicated filter hinders downsizing of the apparatus.

Therefore, an object of the present invention is to prevent the occurrence of moire without significantly increasing the cost and reducing the light-receiving sensitivity and without hindering the miniaturization.

[0012]

The present invention has been made to solve the above-mentioned problems, and is characterized in that a plurality of light-receiving pixels are arranged in rows and columns, and each light-receiving pixel is formed in a subject image. A solid-state image sensor for accumulating information charges, an optical mechanism for capturing reflected light from the object and projecting a subject image on the light-receiving surface of the solid-state image sensor, and a plurality of light-receiving pixels for the solid-state image sensor. A solid-state imaging device comprising: a drive circuit for transferring and outputting information charges in a predetermined order to obtain an image signal, wherein the optical mechanism is a multifocal lens having different focal lengths at respective positions of a cross section perpendicular to the optical axis. To have.

According to the present invention, since the optical mechanism for projecting the subject image on the light-receiving surface of the solid-state image pickup device has multiple focal points, the subject image having a pattern finer than the resolution of the solid-state image pickup device is directly received by the solid-state image pickup device. It is no longer projected on the surface. At this time, since each pattern of the subject image is projected on the light receiving surface so as to spread over the plurality of light receiving pixels, information charges are generated almost uniformly on all the light receiving pixels. Therefore, a subject image having a pattern finer than the resolution of the solid-state image sensor is displayed in a halftone of the density according to the pattern arrangement rule.

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a block diagram of a main part of a solid-state image pickup device of the present invention, showing a light receiving surface of an image sensor and a cross section of a lens as a part of an optical mechanism. Image sensor 1
0 has a light receiving surface 11 in which a plurality of light receiving pixels are arranged in a matrix, and is pulse-driven by various transfer clocks supplied from a drive circuit. A drive circuit, a timing control circuit, and a signal processing circuit (not shown) for the image sensor 10 are the same as those in FIG.

The lens 20 forming the optical mechanism is arranged so that its center substantially coincides with the center of the light receiving surface 11 and faces the light receiving surface 11. This lens 20 is an aspherical lens formed so that its focal length becomes shorter as it goes away from the center of the optical axis. That is, the light transmitted through the vicinity of the center of the lens 20 is converged at a position deeper than the light receiving surface 11, and the light transmitted through the side portion is converged before the light receiving surface 11, so that the curved surface of the lens 20 is an aspherical surface. Has been formed. The lens 20 may have an aspherical shape such that the focal length near the center is shorter than the focal length at the side portion.

The difference between the focal length in the vicinity of the center of the lens 20 and the focal length in the side portion is increased as the difference is increased. However, if the difference is too large, the light receiving surface 1
Therefore, it becomes impossible to project a clear subject image on the first image, and the resolution deteriorates. Therefore, the difference in focal length between the vicinity of the center and the side portion can be set so that the diameter a of the converged light on the light receiving surface 11 is about 1/2 to 3 times the arrangement pitch of the light receiving pixels. preferable. The relationship between the difference in the focal length from the center to the side portion and the diameter a of the converged light on the light receiving surface 11 depends on the diameter of the lens, the refractive index of the lens, the distance from the lens to the light receiving surface 11 (focal length Of the average) etc.

Here, the diameter a of the convergent light on the light receiving surface 11
The state of light reception when the light receiving pixel array pitch d is set to 1/2 will be described. The pattern arrangement pitch d of the subject is set to 3/4 of the resolution of the image sensor 10 as in the case of FIG. Diameter a of convergent light of lens 20
Is 1/2 of the arrangement pitch p of the light receiving pixels, the subject pattern is projected on both sides of the light receiving surface by p / 4 as shown in FIG. At this time, the arrangement pitch t itself of the subject patterns on the light receiving surface remains 3/4 of the arrangement pitch p of the light receiving pixels, as in the case of FIG. Then, on the light-receiving surface, each of the two patterns
A light receiving pixel corresponding to / 3 and a light receiving pixel corresponding to one pattern and one third of one pattern can be formed. Each of these light-receiving pixels has a pattern of 4 /
3 corresponds, and the information charges generated corresponding to the pattern of the subject become uniform in each light receiving pixel. For example, when the signal intensity generated by one pattern is "3", the pattern extending over two light receiving pixels has a signal intensity of "1".
And “2”, the signal intensity is “4” in all the light receiving pixels. Therefore, the image sensor 1
For a subject pattern finer than 0 resolution, it is possible to obtain an image signal having a constant intermediate level according to the ratio of the shade of the subject pattern. Then, in the reproduction screen displayed by this image signal, the image sensor 1
A subject pattern finer than the resolution of 0 is pseudo-displayed in a halftone according to the shade ratio of the subject pattern, and moire does not occur.

FIG. 3 is a block diagram showing a second embodiment of the present invention, showing a cross section of a light receiving surface of an image sensor which is a main part of a solid-state image pickup device and a lens as a part of an optical mechanism. In the second embodiment, the concave lens 31 having an aspherical shape is combined with the general lens 30 having a single focus, so that the light transmitted near the center of the lens 30 is less than the light receiving surface 11. The light that converges at a deep position and that has passed through the side portions converges before the light receiving surface 11. That is, similar to FIG. 7, by arranging the concave lens 31 at the position where the light transmitted through the lens 30 should converge at one point on the light receiving surface 11, the same effect as in FIG. 1 can be obtained. . Here, the difference in the convergence distance between the light passing through the vicinity of the center of the lens 30 and the light passing through the side portion is that the diameter a of the converged light on the light receiving surface 11 is 1/2 times the arrangement pitch of the light receiving pixels. It is preferable to set it so that it is about 3 times.

FIG. 4 is a block diagram showing a third embodiment of the present invention, showing a light receiving surface of an image sensor which is a main part of a solid-state image pickup device and a cross section of a lens as a part of an optical mechanism. In the third embodiment, by combining a general lens 30 having a single focus with a convex lens 32 having an aspherical shape, light transmitted near the center of the lens 30 is in front of the light receiving surface 11. The light that has converged at and is transmitted through the side portion is converged at a position deeper than the light receiving surface 11. Also in the third embodiment, similarly to FIG. 7, by arranging the convex lens 32 at the place where the light transmitted through the lens 30 should be converged at one point on the light receiving surface 11, the light receiving surface 11 can be obtained. The convergent light is spread over a predetermined range having a diameter a. However, in this case, contrary to FIG. 7, the light transmitted near the center of the lens 30 converges at a short distance, and the light transmitted at the side portion converges at a long distance.

In any of the second and third embodiments, even if a pattern of an object finer than the resolution of the image sensor 10 is projected on the light receiving surface 11, as shown in FIG. The signal strength becomes uniform at. Therefore, in any case, it is possible to obtain an image signal that is constant at an intermediate level according to the contrast ratio of the subject pattern, and moire does not occur on the reproduction screen displayed by this image signal.

In the above embodiment, the case where the convergence distance (focal length) of the light transmitted through the lens is uniformly shortened or lengthened from the center of the lens to the side portion is illustrated. The convergence distance of light at each position may be changed randomly. In this case, it can be realized by arranging a filter for light dispersion having concentric circular irregularities or random irregularities in place of the concave lens 31 (convex lens 32) in the second embodiment (third embodiment).

The concave lens 31 and the convex lens 32 in the second and third embodiments are arranged on the object side of the lens 30, and the lens 30 and the image sensor 10 are arranged.
The same effect can be obtained even if it is arranged between and. Then, the concave lens 31 and the convex lens 32 may be combined and arranged together with the lens 30.

[0023]

According to the present invention, it is possible to prevent a fine pattern from being projected on the light receiving surface of an image sensor without using an expensive low-pass filter such as an expensive crystal birefringent plate or a diffraction grating having a low light transmittance. You can Therefore, it is possible to reduce the cost of the device while preventing moire caused by a subject having a pattern finer than the resolution of the image sensor.

[Brief description of drawings]

FIG. 1 is a configuration diagram showing a first embodiment of the present invention.

FIG. 2 is a schematic diagram illustrating how a solid-state imaging device of the present invention images a subject.

FIG. 3 is a configuration diagram showing a second embodiment of the present invention.

FIG. 4 is a configuration diagram showing a third embodiment of the present invention.

FIG. 5 is a block diagram showing a configuration of a conventional solid-state imaging device.

FIG. 6 is a timing diagram illustrating an operation of the conventional solid-state imaging device.

FIG. 7 is a diagram showing how transmitted light of a lens is converged on a light receiving surface of an image sensor.

FIG. 8 is a schematic diagram illustrating how a subject is imaged by a conventional solid-state imaging device.

[Explanation of symbols]

1, 10 image sensor 2 Optical mechanism 3 drive circuit 4 Timing control circuit 5 Signal processing circuit 11 Light receiving surface 20, 30 lens 31 concave lens 32 convex lens

Claims (3)

(57) [Claims] 1. A solid-state image sensor in which a plurality of light-receiving pixels are arranged in rows and columns and information charges corresponding to a subject image are accumulated in each light-receiving pixel, and reflected light from the subject is taken in to the light-receiving surface of the solid-state image sensor. An optical mechanism for projecting an image, a drive circuit for transferring and outputting information charges accumulated in a plurality of light receiving pixels of the solid-state image sensor in a predetermined order to obtain an image signal,
A solid-state imaging device comprising: the optical mechanism, wherein the optical mechanism has multiple focal points having different focal lengths at respective positions of a cross section perpendicular to the optical axis. 2. The optical mechanism according to claim 1, wherein the size of the converged light on the light receiving surface of the solid-state image sensor is larger than 1/2 of the arrangement pitch of the light receiving pixels. Solid-state imaging device. 3. The solid-state imaging device according to claim 2, wherein the optical mechanism includes an aspherical lens whose focal length becomes longer or shorter depending on the distance from the center of the optical axis.