JP2534051B2 - Gamma correction method for solid-state imaging device - Google Patents

Description

TECHNICAL FIELD The present invention relates to an electronic camera, a video camera or the like for performing photography while controlling the exposure amount to a solid-state image sensor while controlling an aperture and a shutter speed. In the solid-state imaging device,
The present invention relates to a gamma correction method for a solid-state imaging device, which corrects a decrease in sensitivity of the solid-state imaging device as the shutter speed increases even if the exposure amount is kept constant.

(Prior Art) A conventional solid-state image pickup device will be described with reference to an electronic still camera shown in FIG. In the figure, 1 is a taking lens provided in the optical system, 2 is a diaphragm, 3 is a shutter, and 4 is a CCD.
Is a MOS type solid-state image pickup device, and an optical image from a subject incident through the taking lens 1, diaphragm 2, and shutter 3 is photoelectrically converted by the solid-state image pickup device 4 to generate a video signal for each hue.

Reference numeral 5 is a synchronization signal generation circuit for generating various control signals for controlling the overall operation of the electronic still camera at a predetermined timing, and 6 is for outputting a signal generated at each pixel of the solid-state image pickup device 4 at a predetermined scanning timing. It is a drive circuit that generates a drive signal.

Reference numeral 7 denotes a signal processing circuit, which is a signal of various hues output from the solid-state image sensor 4, that is, R (blue), G (green), and B.
The (red) primary color signal or its complementary color signal is pre-processed, and a luminance signal and a color difference signal are formed by a matrix circuit provided inside.

A recording unit 8 so-called FM-modulates the luminance signal and the color difference signal and records them on a magnetic recording medium such as a magnetic tape or a magnetic disk.

Reference numeral 9 denotes a photometric circuit, which detects the exposure amount before photographing and transmits the detection data to the exposure amount control circuit 10.

A shutter / aperture drive circuit 11 drives the aperture 2 and the shutter 3. When the exposure amount control circuit 10 instructs the exposure amount to be constant based on the detection data, the aperture amount of the aperture 2 is instructed according to the instruction. And adjust the shutter speed of shutter 3.

Reference numeral 12 is a storage circuit composed of a semiconductor memory or the like, which is optimal in correspondence with data for enabling optimum photographing with the exposure amount always being constant, that is, the detection data value from the photometry circuit 9. The adjustment data that specifies the aperture and / or the shutter speed is stored.

That is, when a shooting instruction is issued by a shooting release, etc., first, the exposure amount control circuit 10 senses the exposure amount from the subject based on the detection data from the photometric circuit 9, and the aperture amount and / or shutter speed corresponding to this exposure amount. The adjustment data relating to the above is read from the storage circuit 12 and supplied to the shutter / aperture drive circuit 11 so that the optimum exposure can be used for photographing.

The structure of such an electronic still camera is generally used in those having a function such as aperture priority or shutter priority. Especially, in a solid-state image pickup device using a solid-state image pickup device having a narrow latitude, the exposure amount Control is extremely important.

(Problems to be Solved by the Invention) However, even if a constant exposure is obtained by controlling the aperture amount and the shutter speed in this way, it may not be possible to obtain the optimum shooting conditions due to the characteristics of the solid-state image sensor. As a result of this research, the inventor of the present application has developed a novel method capable of obtaining an extremely excellent image that has never been obtained.

According to the prior art, the constant exposure amount according to a predetermined latitude is defined as the case where the optimum photographing condition cannot be obtained due to the characteristics of the solid-state image sensor even if the constant exposure is obtained by controlling the aperture amount and the shutter speed. Although it was supposed that optimum shooting can be performed by setting to, in reality, even if the exposure amount is constant, there is a phenomenon that the deterioration of the reproduced image becomes remarkable as the shutter speed increases. .

This phenomenon will be described in detail with reference to FIG. 7 showing the characteristics of the solid-state image sensor in one hue. If the exposure amount is constant, it is output for each pixel of the solid-state image sensor regardless of whether the shutter speed is fast or slow. The signal level should be constant, but as shown in the figure, if the ratio of the incident light amount to the output signal level is 100% when the shutter speed is 1/60 seconds, the shutter speed will increase. Indeed, this output level decreases. As a result, the reproduced video from the video signal has a reduced gradation of brightness, which deteriorates the contrast and causes so-called "sleepiness".
It becomes an image.

Further, the characteristic of the signal level lowering depending on the shutter speed as shown in FIG. 7 changes differently for each hue, which causes a problem that the color balance of the reproduced image is lost. In particular, according to one experimental result of the inventor of the present application, as shown in the same figure, when the shutter speed is 1/1000 seconds or less, the output signal level from the solid-state image pickup device is significantly lowered, and the solid-state image pickup with a narrow latitude is performed. In the element, it is not possible to achieve high image quality of the image by the conventional technique only by controlling the exposure amount.

(Means for Solving Problems) The present invention has been made in view of such problems, and provides a compensating method for preventing deterioration of image quality that occurs as the shutter speed increases even when the exposure amount is constant. The purpose is to do.

To achieve this object, the present invention comprises an optical system for controlling an aperture and a shutter speed, and a solid-state image sensor for photoelectrically converting an optical image for each hue rather than an object incident through the optical system, and an exposure amount. By adjusting the variation of gamma of the solid-state image sensor for each hue according to the change of the shutter speed under a certain condition, the level change is compensated for each hue, and the image quality is improved. The technical point is to be able to achieve.

The signals output from the solid-state imaging device for each hue are red, blue, and green primary color signals or their complementary color signals. Gamma compensation is performed for each signal according to the shutter speed. Providing a circuit is preferred in achieving this method.

(Embodiment) An embodiment of the present invention will be described below with reference to the drawings.
FIG. 1 is a block diagram showing the configuration, and the same or corresponding parts as in FIG. 6 are designated by the same reference numerals.

First, the configuration will be described. In the figure, 1 is a taking lens, 2 is a diaphragm, 3 is a shutter, 4 is a solid-state imaging device,
Is a sync signal generation circuit for generating a sync signal for controlling the operation of the camera system and a signal necessary for forming a video signal, and 6 is a drive circuit for controlling signal reading of the solid-state imaging device 4.

Reference numeral 21 is a signal separation / sample and hold circuit, which separates the signal output from the solid-state image pickup device 4 into signals corresponding to each pixel and samples and holds them.

Reference numeral 22 is an automatic gain control circuit, and 23 is a color separation circuit. The signal sampled by the signal separation / sample hold circuit 21 is amplified to a predetermined amplitude by the automatic gain control circuit 22, and the color separation circuit 23
Are divided into R (red) and B (blue) color signals. still,
In this embodiment, the case of processing the R, G, B primary color signals will be described, and the method of the present invention can be similarly applied to the case of processing the complementary color signals, and therefore the description thereof will be omitted.

24,25 are white balance adjustment circuits, 26,27,28 are clamp circuits, 29,30,31 are blanking mixing circuits for adding blanking pulses in the blanking period, 32,33,34
Is a gamma correction circuit.

35 is a matrix circuit, and each gamma correction circuit 32,3
Luminance signal Y based on R, G, B color signals corrected in 3, 34
And color difference signals RY and BY.

Reference numeral 36 is a luminance signal compensating circuit, which automatically changes the amplification factor of the luminance signal Y according to the shutter speed described later.

Reference numeral 37 denotes an encoder circuit, which is a luminance signal Y from the luminance signal compensating circuit 36 and a color difference signal R− from the matrix circuit 35.
Based on Y, BY, a standardized video signal of, for example, the NTSC system is generated.

Reference numeral 38 denotes a recording unit, which records the video signal from the encoder circuit 38 on the magnetic recording medium after performing FM modulation and the like.

The circuit in the dotted line indicated by reference numeral 39 in the figure is an exposure amount control unit, and is a central processing circuit using a microcomputer or the like.
40, input / output interface circuits 41, 42, an A / D converter 43, and a memory circuit 44.

The A / D converter 43 includes a signal separation / sample hold circuit 31
The G (green) color signal is converted into a digital signal and transferred to the central processing circuit 40.

The central processing circuit 40 measures (meters) the amount of light incident on the solid-state image sensor 4 based on the signal from the A / D converter 43.
That is, this camera system constitutes an internal photometry system, and this photometry is performed before the actual photographing.

The memory circuit 44 stores adjustment data for adjusting the aperture 2 and the shutter 3 based on the detection result obtained by photometry, and the central processing circuit 40 converts the signal from the A / D converter 43 into a signal. Corresponding adjustment data is read and supplied to a drive circuit (not shown) for driving the diaphragm 2 and the shutter 3 via the input / output interface circuit 41, and shooting is performed under optimum preset exposure conditions. To set.

In addition to the adjustment data for controlling the aperture amount and shutter speed described above, the memory circuit 44 also stores gamma correction circuits for controlling the correction values of the gamma correction circuits 32, 33, 34 according to the shutter speed. Data and luminance signal compensation circuit 36
Similarly, the brightness compensation data for controlling the compensation value of 1 according to the shutter speed is stored.

That is, as shown in FIG. 2, the configuration corresponds to an adjustment data storage area 44a for storing diaphragm adjustment data in an address area corresponding to the above-described photometric value detected by photometry, and the above-mentioned photometric value. An adjustment data storage area 44b for storing shutter speed adjustment data in the address area, and a memory for storing brightness compensation data in the address area corresponding to the shutter speed read from the adjustment data storage device 44b. An area 44c and a storage area 44d that stores gamma correction data in an address area corresponding to the shutter speed are provided.

Address signal AD corresponding to the photometric amount from the central processing circuit 44
When i and BDi are supplied to the respective storage areas 44a and 44b, the adjustment data ACi and BCi are read out, and these data are passed through the input / output interface circuit 41 to the diaphragm 2 and the shutter 3.
Transferred to Further, since the data ACi for setting the shutter speed is supplied to the storage areas 44c and 44d as address signals, the luminance compensation data YCi and the gamma correction data γCi (R), γC corresponding to the shutter speed are supplied.
i (G) and γCi (B) are output. The subscripts R, G, and B of the gamma correction data are predetermined gamma correction circuits 32,
Shall be supplied to 33, 34. Therefore, gamma correction is independently performed for each hue.

The gamma correction circuits 32, 33, 34 are configured so as to change the gain by folding line approximation. For example, as shown in FIG. 3, a semiconductor switch for turning on / off the current from the current source circuit according to the gamma correction data. By controlling with r1, r2, ..., Rn, the gain adjusting current of the gamma correction circuit is changed to obtain the gain change according to the polygonal line approximation.

The change in gain based on this approximation has a characteristic of increasing with an increase in shutter speed in order to correct the level decrease for each hue shown in FIG.

FIG. 4 shows a specific example of the luminance signal compensation circuit 36. That is,
An amplifier AMP is connected to a signal line for luminance signal transfer connecting between the matrix circuit 35 and the encoder circuit 37, and a plurality of feedback resistance elements R1, R2, ..., Rn having mutually different resistance values are provided in parallel, Luminance compensation data YC from the central processing unit 40
, Rn are provided so that the amplification factor can be changed by selectively connecting these feedback resistance elements R1, R2, ..., Rn to the amplifier AMP based on i. That is, as shown in FIG. 5, when the shutter speed increases, the amplification factor increases to compensate for the level decrease in the luminance signal.

As described above, according to this embodiment, even if the exposure amount is constant, the signal level output from the solid-state image sensor is lowered with the increase of the shutter speed, so that the gamma correction circuit corrects it. The deterioration of the gradation of the reproduced video can be prevented, and since the same compensation is performed in the luminance signal compensating circuit as well, it is possible to perform accurate and appropriate adjustment. Incidentally, in practice, the compensation by the luminance signal compensating circuit can be omitted.

In this embodiment, the internal photometry is explained,
The present invention can also be applied to external photometry.

Furthermore, the so-called
The camera may have an EE mechanism, or may be applied to a camera with aperture priority or shutter speed priority.

(Effect of the Invention) As described above, according to the present invention, the decrease in the output signal level of the solid-state image sensor as the shutter speed becomes optimal is corrected by performing the gamma correction at least for each hue. It is possible to provide a reproduced image with extremely excellent gradation. Further, by performing compensation according to the shutter speed for the luminance signal synthesized from the color signal thus corrected, more accurate compensation can be performed.

Therefore, it is possible to provide a solid-state imaging device capable of obtaining an excellent image that is not affected by the shutter speed.

[Brief description of drawings]

1 is a block diagram showing an embodiment of an electronic still camera constructed based on the gamma correction method for a solid-state image pickup device according to the present invention, and FIG. 2 is a block diagram showing the configuration of the memory circuit shown in FIG. FIG. 3 is a circuit diagram showing the configuration of each gamma correction circuit shown in FIG. 1, FIG. 4 is a circuit diagram showing a concrete example of the luminance signal compensation circuit shown in FIG. 1, and FIG. 5 is a luminance signal compensation circuit. 6 is a characteristic curve diagram showing a gain characteristic of FIG. 6, FIG. 6 is a block diagram showing a configuration of a conventional electronic still camera, and FIG. 7 is a schematic diagram showing a change for each hue with respect to a change in shutter speed for one of the hues. It is a characteristic curve figure shown in FIG. 2: Aperture 3: Shutter 4: Solid-state image sensor 32, 33, 34: Gamma correction circuit 36: Luminance signal compensation circuit 40: Central processing circuit 41, 42: Input / output interface circuit 43: A / D converter 44: Memory circuit 44a, 44b, 44c, 44d: storage area

Claims (3)

(57) [Claims] 1. An optical system for controlling an aperture and a shutter speed, and a solid-state image pickup device for photoelectrically converting an optical image of a subject incident through the optical system for each hue, wherein the exposure amount is constant. A gamma correction method for a solid-state image pickup device, comprising adjusting a change amount of gamma of the solid-state image pickup element for each hue according to a change in shutter speed. 2. The signals output from the solid-state image sensor for each hue are red, blue, and green primary color signals or their complementary color signals, and gamma correction is performed for each signal according to the shutter speed. The gamma correction method for a solid-state image pickup device according to claim 1, further comprising a gamma correction circuit for performing the gamma correction. 3. A compensating means for forming a luminance signal based on a signal outputted for each hue outputted from the gamma correction circuit and compensating the luminance signal with a change in an amplification factor corresponding to a shutter speed. The gamma correction method for a solid-state image pickup device according to claim 1 or 2, wherein: