JP2568515B2 - Imaging device - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image pickup apparatus such as a television camera for automatically correcting a signal level.

2. Description of the Related Art As an automatic adjustment of the level of a conventional television camera, particularly a television camera using digital signal processing, there is one disclosed in, for example, JP-A-58-38075.

FIG. 8 shows a block diagram of a conventional imaging apparatus as described above. In FIG. 8, 1 is an aperture for controlling the amount of incident light, 2 is an image sensor (imaging element) (IS) for converting an optical image into an electric signal, 3 is a preamplifier (PA) for amplifying a signal level to a predetermined level, 4 is an A / D converter for converting an analog signal into a digital signal, 5 is an average level detection circuit for obtaining a value corresponding to the average level of the signal, 6 is a peak level detection circuit for obtaining the peak level of the signal, 7
Is an addition circuit for adding the output signals from the average level detection circuit 5 and the peak level detection circuit 6, 8 is an aperture control circuit for operating the aperture 1, 9 is a D / A conversion circuit for converting a digital signal into an analog signal, 10 Is a gamma correction circuit for converting a signal level, and 11 is a signal processing circuit for adding a synchronization signal or the like.

In the conventional imaging device configured as described above, the level of the output signal is controlled by the aperture 1. The level of the signal to be controlled is determined by the combined signal of the average level detection circuit 5 for calculating the average level from the frequency distribution of the signal and the peak level detection circuit 6 for detecting the peak level. Such control is to limit the incident light of the image sensor to within the dynamic range of the image sensor, and performs control to close the aperture 1 when the amount of incident light increases. In particular, when the imaging device is a solid-state imaging device such as a CCD, the dynamic range is small, so that the ratio of the peak level is increased to control the aperture.

However, in the above configuration, when the squeezing is controlled in accordance with the dynamic range of the image sensor, when a subject having a large difference in brightness and contrast and a large contrast ratio is imaged, the subject is focused on a bright subject. Has been controlled, and the dark part has been darkened in the captured image, and cannot be reproduced. In Japanese Patent Application Laid-Open No. 60-16072, a circuit for detecting an average value and a peak value is provided in an AGC loop, and when the level around an image is low, the AGC loop is controlled using peak detection. In this method, when a bright portion exists in the center of an image, a dark portion of the other portion is almost black and cannot be reproduced.

In view of the above, the present invention measures the frequency distribution for each signal level of a captured image signal, detects the ratio of the frequency of the entire signal to the frequency of the signal distributed at a certain level, and determines the conditions under which the image sinks, At this time, an object of the present invention is to provide an imaging device in which a gain of a signal processing circuit or a signal conversion characteristic, for example, a γ characteristic, is controlled so that a reproduced image does not sink in darkly.

Means for Solving the Problems In order to achieve the above object, the present invention provides an aperture which receives an output signal of an image sensor, controls the amount of incident light on the image sensor, and controls the output signal of the image sensor to a predetermined level. A control circuit detects a frequency distribution of signal levels from the image sensor, and determines a predetermined amount between a frequency distribution of all signal levels and a ratio of a frequency distribution of 20% or less or 80% or more, which is a specific part of the signal level, and a predetermined amount A distribution detection circuit that outputs a comparison result, and a gain control circuit that controls a gain of a circuit that processes an output signal of the imaging element based on an output signal of the distribution detection circuit or an output of the imaging element based on an output signal of the distribution detection circuit At least one of a characteristic control circuit for controlling a non-linear characteristic of a circuit for processing a signal, and a multiplication circuit for changing a gain of a circuit for processing an output signal of the image sensor Or at least one of characteristic conversion circuits for changing non-linear characteristics of a circuit that processes an output signal of the image sensor, under a condition where a specific portion of a signal level from the image sensor increases and black sun phenomenon occurs. The multiplication circuit or the characteristic conversion circuit is controlled so as to reduce black sun.

Function The present invention controls the incident light amount to a predetermined amount by the automatic aperture by the above-described configuration, measures the frequency distribution for each signal level in this state, and determines the ratio between the frequency of the entire signal and the frequency of the signal distributed at a certain level. To detect the conditions under which the captured image sinks darkly, and when this condition is detected, controls the gain of the circuit or the conversion characteristics of the circuit, performs a process of increasing a low signal level portion, and outputs the signal. Improve the blackened image.

Embodiment FIG. 1a shows a block diagram of an image pickup apparatus according to a first embodiment of the present invention. In FIG. 1a, L is a lens, 1 is an aperture for adjusting the amount of light, 2 is an image sensor for converting an optical image into an electric signal, 3 is a preamplifier for amplifying to a predetermined signal level, 11 is a process for adding a synchronization signal and the like. The circuit is the same as that of the conventional imaging device shown in FIG. Reference numeral 22 denotes a weighting circuit that allows the signal from the preamplifier to pass as necessary.Reference numeral 23 measures the frequency distribution of each level of the captured image signal, and calculates the ratio of the frequency of the entire signal to the frequency of the signal distributed at a certain level. A distribution detection circuit for determining the state of dark sinking, 24 is a gain control circuit that sets the gain of the circuit according to the ratio of the frequency of the signal level to the frequency of the signal distributed at a constant level, and 20 is a multiplication that varies the gain of the circuit. Reference numeral 21 denotes a gamma correction circuit for performing non-linear processing, and reference numeral 26 denotes an aperture control circuit for controlling the aperture 1 so that the light amount becomes a predetermined amount.

The operation of the imaging apparatus according to the present embodiment configured as described above will be described below.

The aperture control circuit 26 controls the aperture 1 so that the signal of the image sensor 2 becomes a predetermined level. The characteristics of the aperture control may be controlled by mixing at an arbitrary ratio from the peak value control to the average value control as in the conventional imaging apparatus, or may be switched between the peak value control and the average value control according to the imaging conditions. good. The weighting circuit 22 weights the image signal thus controlled to a predetermined level. Weighting passes only the signal at the center of the captured image. In the case where the distribution of the peripheral portion is used, the light can be passed at an appropriate ratio and the weight can be changed. In this manner, the center part of the image is weighted, and the frequency distribution for each signal level of the image signal captured by the distribution detection circuit 23 is measured, and the ratio between the frequency of the entire signal and the frequency of the signal distributed at a certain level is calculated. Perform detection. FIG. 2 shows an example of a frequency distribution for each signal level. FIG. 7A shows a case where the contrast ratio of the subject is small and the signal distribution is concentrated in the center. FIG. 6B shows a case where the subject has a large contrast ratio and bright illumination or the like enters the subject. At this time, the signal distribution is separated into two parts, a bright part and a dark part. The captured image in this state has many portions close to saturation and black portions, which results in an unnatural image. In this embodiment, the ratio of the frequency distribution in the portion where the signal level is low is obtained from the distribution detection circuit 23, and the gain of the signal processing circuit is controlled to improve blackening of the image. The distribution detection circuit 23 measures the frequency distribution for each signal level, and calculates the ratio of the frequency of the entire signal to the frequency of the signal distributed at a certain level, for example,
A frequency distribution of 20% or less (hereinafter referred to as low level distribution) is obtained. The ratio between the frequency distribution of the entire signal shown in FIG. 2 and the frequency distribution of the hatched portion is obtained and output. The gain of the circuit is controlled according to the ratio of the low level distribution. Gain control circuit 24
Determines this gain.

FIG. 3 shows the characteristic of changing the gain. The horizontal axis indicates the ratio of the low-level distribution, and the vertical axis indicates the gain of the multiplier 20. The gain is set to 1 when the low level distribution is 0 to 20% of the frequency distribution of all signal levels, the gain is gradually increased from 20% to 50%, and the gain is set to 2 when the low level distribution is 50% or more (FIG. 3). solid line). By providing such characteristics, the signal level can be increased as the number of states in which the image is darkened black is increased, and the darkening is improved. Although the maximum value of the circuit gain is set to 2, it may be determined according to the relationship between the S / N ratios of the image pickup device. If the S / N ratio is good, the gain can be further increased.

The dashed line in FIG. 3 changes the gain up to 3.
The alternate long and short dash line indicates that the black sun correction is increased in a portion where the ratio of the low level distribution is small. Thus, the gain of the circuit is controlled and input to the multiplier 20. Increasing the gain further increases the signal level in a portion where the signal level is high. This is dealt with by the characteristic of the γ (gamma) correction circuit 21 which is a nonlinear process. FIG. 1B shows the characteristics of the gamma correction circuit 21 when the gain of the multiplier 20 is 2. Large compression is applied to inputs whose input level is 100% or more, and the output level is output within 100%. The gain for each signal level is changed by the multiplier 20 and the γ correction circuit 21 which is a non-linear process, and characteristic conversion is performed to selectively increase the gain of a low-level signal in which an image sinks black. The process circuit 11 performs blanking processing, pedestal processing, addition of a synchronization signal, and the like, and outputs the result.

As described above, in the present embodiment, the weighting circuit 22, the distribution detection circuit 23, the gain control circuit 24, the multiplication circuit 20, and the gamma correction circuit 21 are provided, and the state in which the captured image sinks black is represented by the frequency distribution of the signal level of the imaging signal. To increase the gain of the circuit and compress and output the high signal level portion to convert the signal level characteristics of the captured image signal, and to improve the image from sinking black due to backlight or the like. be able to. In particular, since the frequency of each level of the imaging signal is used compared to the method of controlling using simply the peak level and average level of the imaging signal, it correctly responds to the state where the frequency of the signal level of the imaging signal is locally distributed It is possible to do. In addition, since it is an open loop, there is no fear of oscillation even if any control characteristics are given. Here, the characteristics of the gamma correction circuit 21 need not be limited to those shown in FIG.

FIG. 4a is a block diagram of an imaging apparatus showing a second embodiment of the present invention. In FIG. 1, reference numeral 1 denotes an aperture for controlling the amount of light;
Reference numeral 3 denotes a preamplifier for amplifying a signal to a predetermined level, reference numeral 11 denotes a process circuit for adding a synchronization signal or the like, and reference numeral 25 denotes an aperture control circuit for controlling the light amount to a predetermined value. a). The difference from the configuration of FIG. 1A is that a distribution detection circuit 30, a characteristic control circuit 31, and a signal characteristic conversion circuit 32 are provided.

The operation of the imaging apparatus according to the second embodiment configured as described above will be described below.

The distribution detection circuit 30 obtains a frequency distribution of each signal level of the captured image signal with respect to the weighted signal. Examples of distribution are shown in FIGS. 2a and 2b. FIG. 3A shows a case where the contrast ratio of the subject is small and the frequency distribution of each signal level of the captured image signal is concentrated at the center, and FIG. 4B is a case where the contrast ratio is large and the frequency distribution of each signal level of the captured image signal is up and down. Are separated into two levels. The distribution detection circuit 30 outputs a distribution (hereinafter referred to as a high level distribution) in which the signal level is 80% or more, and outputs the ratio of the dot portions in FIG. The nonlinear signal characteristic conversion circuit 32 of the signal processing circuit is controlled by the ratio of the high level distribution. FIG. 4B shows the configuration of the signal characteristic conversion circuit 32. In FIG. 4b, reference numerals 33 and 34 denote non-linear γ (gamma) circuits, 35 and 36 denote multiplication circuits, 37 denotes an addition circuit, and 38 denotes a ratio calculation circuit. FIG. 5 shows the characteristics of the nonlinear γ circuits 33 and 34. γ ₁ is the characteristic of the nonlinear γ circuit 33, and γ ₂ is the characteristic of the nonlinear circuit 34. The nonlinear conversion characteristics are controlled by changing the multiplication values m and n of the multipliers 35 and 36. Becomes gamma ₁ characteristic at m = 0, n = 1, as shown in FIG. 5, m = 1, n =
At 0, the characteristic becomes γ ₂ , and the intermediate value can be selected by the values of m and n. The value of n and m is n + m = 1 and the non-arithmetic circuit
Ask at 38. The control input of the non-arithmetic circuit 38 is the characteristic control circuit 31
Ask for. FIG. 6 shows the characteristics of the characteristic control circuit 31. The horizontal axis in the figure is the distribution where the signal level is 80% or more (high level distribution)
The vertical axis indicates m which is a control input of the non-arithmetic circuit 38.
Is the value of

With the configuration as described above, the state where the signal has a high level distribution, that is, the state where the signal is simultaneously concentrated and distributed at a low level is detected, and the characteristic of the nonlinear signal characteristic conversion circuit 32 is controlled. In addition, a signal in which the frequency distribution of each signal level of the captured image signal is distributed at a low level can be reproduced at a high level, and it is possible to improve the captured image from sinking in black. In the second embodiment, even when the characteristics (shown in FIG. 5) of the signal characteristic conversion circuit 32 are changed, the output signal level is 0 to 100%.
There is a feature that can be used effectively up to.

Note that the signal conversion characteristics (fifth
Figure) is an example, and it is natural that other characteristics can be used depending on the S / N ratio of the image sensor. Further, the relationship between the ratio of the high level distribution and the control signal m shown in FIG. 6 is also an example, and it is not necessary to limit to this characteristic.

Next, FIG. 7 shows a third embodiment in which the first embodiment (the configuration of FIG. 1A) is slightly modified. 1A differs from FIG. 1A in the input signal of the weighting circuit 22 and the gain control circuit 25. An input signal of the weighting circuit 22 is that an output signal of the gamma correction circuit 21 which is a signal after gain correction of the circuit is used (the same effect is obtained with an output signal of the multiplier 20). Further, the gain control circuit 25 controls the frequency distribution (low level distribution) in which the signal level of the captured image signal is 20% or less, that is, the closed-loop control so that the portion indicated by the hatched portion in FIG. The point is to do. Other configurations are the same as those in FIG. In the configuration of FIG. 7, the control of the ratio of the low-level distribution is a closed loop, and the gain control circuit 25 increases the gain of the multiplier 20 when the ratio of the low-level distribution is smaller than the target value of the control, / 4.
When the number is large, control is performed to reduce the gain, and the nonlinear γ circuit 21
The conversion of circuit characteristics is performed by integrating the above. The upper limit of the gain is set to about two to three times depending on the S / N of the image sensor. This value can be further increased if the S / N is good. Therefore, as compared with the configuration shown in FIG. 1A, the low-level distribution can be accurately controlled.

In the first embodiment, the distribution detection circuit 23 detects and controls the low-level frequency distribution of the captured image signal, but this is the same as the distribution detection circuit 30 of the second embodiment. The same effect can be obtained by detecting and controlling the frequency distribution of the level. Similarly, in the second embodiment, the method of the distribution detection circuit 23 shown in the first embodiment using the low-level frequency distribution of the captured image signal can be used.

Effects of the Invention As described above, according to the present invention, a state in which a captured image sinks black is detected from the ratio of the entire frequency distribution of the captured image signal level and the partial frequency distribution of each signal level,
In accordance with this state, a process of converting the characteristics of a captured image signal that reproduces a high signal level is performed. As a result, a portion where the captured image sinks in black can be improved and output. Therefore, even when the image is captured in a backlight state, the captured image can be reproduced brightly, and its practical effect is great.

[Brief description of the drawings]

FIG. 1a is a block diagram of an image pickup apparatus according to a first embodiment of the present invention, FIG. 1b is a characteristic diagram of a gamma correction circuit, FIG.
a and b are characteristic diagrams showing a state of a signal level distribution, FIG. 3 is a characteristic diagram of a gain control circuit, FIGS. 4a and 4b are block diagrams of an image pickup apparatus according to a second embodiment of the present invention, and FIG. FIG. 6 is a characteristic diagram of a signal characteristic conversion circuit, FIG. 6 is a characteristic diagram of a characteristic control circuit, FIG. 7 is a block diagram of an image pickup apparatus according to a third embodiment of the present invention, and FIG. is there. 1 ... Aperture, 2 ... Image sensor, 3 ... Preamplifier, 22 ... Weighting circuit, 23,30 ... Distribution detection circuit, 24,
25: gain control circuit, 20: multiplication circuit, 21: gamma correction circuit, 11: process circuit, 31: characteristic control circuit,
32: Signal characteristic conversion circuit, 26: Express control circuit.

Continuation of the front page (72) Inventor Hiroki Matsuoka 1006 Kadoma, Kadoma Matsushita Electric Industrial Co., Ltd. (56) References JP-A-60-16072 (JP, A)

Claims (3)

(57) [Claims] An aperture control circuit that receives an output signal of an image sensor as an input, controls an amount of incident light on the image sensor, and controls an output signal of the image sensor to a predetermined level, and a frequency of a signal level from the image sensor. Detect distribution and frequency distribution of all signal levels and specific parts of signal level
A distribution detection circuit that outputs a comparison result between a frequency distribution ratio of 20% or less or 80% or more and a predetermined amount, and a gain of a circuit that processes an output signal of the image sensor based on an output signal of the distribution detection circuit. A gain control circuit or at least one of a characteristic control circuit that controls a non-linear characteristic of a circuit that processes an output signal of the image sensor by an output signal of the distribution detection circuit; and a circuit that processes an output signal of the image sensor. At least one of a multiplication circuit for changing a gain or a characteristic conversion circuit for changing a non-linear characteristic of a circuit for processing an output signal of the image sensor, wherein a specific portion of a signal level from the image sensor increases and blacksink occurs. An imaging device for controlling the multiplication circuit or the characteristic conversion circuit so as to reduce black sun under a condition in which the blackout occurs. 2. The image pickup apparatus according to claim 1, wherein the distribution detection circuit includes a weighting circuit for changing a detection weight according to a scanning position of a signal of the image pickup device. 3. The imaging apparatus according to claim 1, wherein the distribution detection circuit uses a signal from a circuit after the multiplication circuit or the characteristic conversion circuit as an input signal.