Light signal control circuit for image pick-up apparatus

The present invention relates to a light signal control circuit for an image pickup device, e.g. B., an iris control circuit with an automatic iris circuit for effecting an iris correction, which is effective when an image is taken in backlight or the like, which involves a large variation in brightness.

Conventional aperture control of an image pickup device has been accomplished by a method that combines a peak value system with an average value system. Such a conventional image pickup device is described in JP-A-58-38075. Fig. 1 is a block diagram showing this conventional image pickup device. In Fig. 1 Reference number 1 denotes an aperture, number 2 a lens, number 3 an image recording device such as a CCD, number 4 an amplifier circuit for amplifying a signal from the image recording device 3 and for generating an image recording signal of a predetermined level, number 5 an analog / digital converter circuit, number 6 shows a peak value detection circuit for detecting a peak value of the image pickup signal, Numeral 7 is an average value detection circuit for acquiring an average value of the image pickup signal, 8 is an aperture driver circuit for driving an aperture, 9 is a signal processing circuit for performing contrast processing or the like, and 10 is a digital / analog converter circuit.

In a conventional image pickup device constructed as described above, the aperture drive circuit 8 drives the aperture 1 in such a manner that a sum of a signal from the peak value detection circuit 6 and an output signal from the average value detection circuit 7 becomes equal to a reference value (shown in Fig. 1) ).

However, this conventional arrangement has a disadvantage in that when taking an image of an object in an environment with a large variation in brightness because it contains a light source or due to backlight, the output signal of the peak detection circuit becomes so large that the diaphragm is undesirably closed and the object goes dark.

An object of the present invention is to provide an iris control circuit for an image pickup device having a correction function for preventing an object from becoming dark by determining a backlight condition or the like with a large ratio or a large difference between lightness and darkness in a detection area and the aperture is opened.

In order to achieve the above and other objects, according to the present invention, there is provided an iris control circuit for an image pickup device including a first level detection circuit for detecting an essential peak value of an image pickup signal, a second detection circuit for detecting an essential average value of an image pickup signal, a decision circuit for determining a condition relating to brightness and darkness of a detection area, a weighting circuit for weighting an output signal of the first level detection circuit and an output signal of the second level detection circuit, and an aperture control circuit for controlling the aperture in accordance with an output signal of the weighting circuit. With the arrangement described above, the weighting circuit weights an output of the first level detection circuit to control the iris so that an unsaturated image pickup signal is obtained when the decision circuit decides that an image is taken from an object under an ordinary condition, i.e., an image is taken. that is, when the ratio or difference between the brightness and darkness of the object is small, whereas the weighting circuit weights an output of the second of the diaphragms so as to prevent the object from becoming dark under the influence of a light source or the like when the decision circuit decides that a Image of an object is taken in backlight.

While the novel features of this invention are presented in detail in the appended claims, the invention, in terms of both structure and content, along with the other objects and features thereof, will be better understood and appreciated from the following detailed description when taken in conjunction with the accompanying drawings.

Fig. 1 is a block diagram showing a conventional image pickup system.

Fig. 2 is a block diagram showing the principle of the present invention.

FIG. 3 is a characteristic diagram showing the characteristics of the weighting circuit 14 shown in FIG. 2.

Fig. 4 is another block diagram showing the principle of the present invention.

Fig. 5 is a block diagram showing first, second and third embodiments of the present invention.

FIG. 6 is an explanatory diagram showing the principle of the first level detection circuit 101 shown in FIG. 5.

FIG. 7 is a characteristic diagram showing the characteristics of the weighting circuit 104 shown in FIG. 5.

8 and 9 are explanatory diagrams showing the segmented blocks of an image pickup screen in the second and third embodiments.

Fig. 10 is a block diagram showing a fourth embodiment of the present invention.

Fig. 11 is a block diagram showing a fifth embodiment of the present invention.

12A and 12B are histograms of the image pickup signal in one field in the fifth embodiment of the present invention shown in FIG. 11.

Fig. 13 is a block diagram showing a sixth and a seventh embodiment of the present invention.

14A and 14B and FIG. 15 are histograms of the image pickup signal in one field in the sixth and seventh embodiments of the present invention shown in FIG. 13, respectively.

Fig. 2 is a block diagram for explaining the principle of the present invention. The same sub-components of the device as those of the conventional device are given the same reference numerals and are not described in detail.

In Fig. 2 Number 11 denotes a first level detection circuit for detecting a signal with a large amplitude (namely an essential peak value) of image recording signals, number 12 denotes a second level detection circuit for detecting a medium level of image recording signals, number 13 denotes a decision circuit for determining a ratio (brightness ratio) of brightness and darkness a detection area, Numeral 14 is a weighting circuit for weighting a comparison output signal obtained by comparing an output signal from the first level detection circuit with a reference value 1 or a comparison output signal obtained by comparing an output signal from the second level detection circuit with a reference value 2.

The operation of an image pickup device according to the present invention constructed as described above is explained below.

The first level detection circuit 11 performs level detection substantially the same as the peak detection circuit 6 of the conventional pickup device shown in FIG. 1. By setting the reference value 1 to an appropriate value, unsaturation control of an image pickup signal is made possible by controlling an output signal of the first level detection circuit 11 so that it becomes equal to the reference value 1. The second level detection circuit 12, on the other hand, performs level detection which is substantially the same as that of the average value detection circuit 7 of the conventional image pickup device shown in FIG. 1. As a result, when the reference value 2 is appropriately set, by controlling an output of the second level detection circuit 12 so as to become the reference value 2, it becomes possible to perform open-aperture control because the device is less by a bright part of the detection area, such as a light source is affected compared to the conventional aperture control using the peak detection circuit 6. In the present invention, the decision circuit 13 is used to decide whether or not the system takes an image of an object containing a light source with a large light / dark ratio. When the decision circuit 13 determines a condition that an image of an ordinary object having a small light / dark ratio is taken, the weighting circuit 14 selects a comparison output signal obtained by a comparison between an output signal of the first level detection circuit 11 and the reference value 1, and supplies the comparison output signal to the aperture driver circuit 8, thereby making it possible that iris control is performed while preventing signal saturation. When the decision circuit 13 determines a condition that an image of an object with a large light / dark ratio, e.g. B. on the other hand, including a light source or the like, the weighting circuit 14 selects a comparison output signal obtained by a comparison between an output signal of the second level detection circuit 12 and the reference value 2, and supplies the comparison output signal to the diaphragm driver circuit 8. Thus, it becomes possible to perform open-aperture control, thereby preventing the object from becoming dark, which has become a problem in the prior art caused by the fact that the aperture is closed under the influence of a bright part such as a light source.

In order for the control explained above to be achieved, the operation described below is carried out by the weighting circuit 14.

Assuming that "peak" is an output signal of the first level detection circuit 11, "middle" is an output signal of the second level detection circuit 12, "R1" is the reference value 1 and "R2" is the reference value 2, then the weighting circuit 14 generates an output signal C. , which is defined by the equation shown below.

C = m · (R2 center) + (1-m) · (R1 peak) ... (1)

In equation (1), m is a weighting coefficient (or weighting factor). The operation of the weighting circuit 14 is indicated by a solid line in FIG. 3. In Fig. 3, the abscissa means an output signal of the decision circuit 13, that is, the size of a light / darkness ratio of an object or a detection area, and the ordinate means the weighting coefficient m. If the object's brightness / darkness ratio is small, m = 0. In this case, the weighting circuit 14 outputs a comparison output signal obtained by a comparison between an output signal of the first level detection circuit 11 and the reference value 1, thereby making it possible to perform control while preventing signal saturation. On the other hand, if the brightness / darkness ratio is large, m = 1, in which case the weighting circuit 14 outputs a comparison output signal obtained by a comparison between an output signal of the second level detection circuit 12 and the reference value 2, thereby making it possible to control the open aperture perform. In Fig. 3 the aperture can be operated smoothly without any sudden change in operation by changing the weighting coefficient m evenly in a range where the light / dark ratio of the object has an intermediate value. The broken line in FIG. 3 shows another example of an operation of the weighting circuit 14. The characteristic shown by the broken line represents a control mode in which the weighting coefficient m is simply switched according to the size of the light / dark ratio of an object.

As explained above, the present invention can realize the characteristic of an optimal shutter operation according to the light / dark ratio of an object or a detection area.

The aperture control circuit of the present invention may alternatively be constructed as shown in FIG. 4. The arrangement shown in Fig. 4 is different from that of Fig. 2 in that in Fig. 4, a comparison output signal obtained by comparing an output signal of the weighting circuit 14 with a reference value is given to the diaphragm driver circuit 8, to thereby operate the diaphragm 1 Taxes. In this case, assuming that peak 'represents an output signal of the first level detection circuit 11, "middle" represents an output signal of the second level detection circuit 12, and "m" represents the weighting coefficient, by comparing that obtained from the following equation (2) Output signal D of the weighting circuit 14 with a reference value possible to perform the aperture control similar to that which is achieved by the arrangement shown in FIG. 2.

D = mMedium + (1-m) Peak ... (2)

In particular, equation (1) can be modified to give the following:

C = R & sub1; + m · (R 2 -R 1) - (m · average + (1-m) · peak)

In Fig. 4, on the other hand, when an input signal of the shutter driver circuit 8 is designated E, the following equation results.

E = R- (m · middle + (1-m) · peak),

where R is a reference value shown in FIG. 4. Therefore, when the reference value 1 is substantially the reference value 2, an equivalent characteristic can be obtained by making the reference value of FIG. 4 equal to the reference value 1 or 2. When the reference value of Fig. 4 is made equal to the reference value 1 (R = R 1), and when the weighting circuit 14 generates an output signal of the value F given by the following equation (3),

F = m · (middle (R 2 -R 1)) - (1-m) · peak ... (3)

then the value G of the input signal of the diaphragm driver circuit 8 is given by

G = R 1 -m · (Central (R 2 -R 1)) - (1-m) · tip.

Since the modification of this equation results in an equation like equation (1) explained above, it is possible to achieve that the arrangement of FIG. 4 has the same characteristics as that of the arrangement of FIG. 2. As described above, at In the present invention, the arrangement of either Fig. 2 or Fig. 4 can be used with the same effect. Therefore, the embodiments of the present invention described hereinafter are shown to have the arrangement of FIG. 4. However, it is clear that the same effect can be obtained using the arrangement of Fig. 2.

The principle of determining the lightness / darkness ratio of an object or a detection area by the decision circuit 13 of the present invention will be explained in detail with reference to the embodiments of the present invention described hereinafter. The decision circuit 13 can determine the brightness / darkness ratio of an object by inputting a signal from the first level detection circuit 11, the second level detection circuit 12, or any other level detection circuit, which will also be explained in detail hereinafter with reference to the embodiments of the present invention.

In the above-mentioned principle of this invention, an aperture has been used as the means for controlling a light signal. However, a light signal control device is not limited to a conventional diaphragm.

For those skilled in the art, it is obvious that a liquid crystal can also be used as a control device for controlling a light signal. Furthermore, this can also be accomplished by controlling the size of charges stored in a photosensitive element of an image capture device, such as a CCD.

5 is a block diagram showing an image pickup system according to a first embodiment of the present invention. In FIG. 5, the first level detection circuit 101, the second level detection circuit 102, the decision circuit 103 and the weighting circuit 104 have the same functions as the first level detection circuit 11, the second level detection circuit 12, the decision circuit 13 and the weighting circuit 14, each of which is shown in FIG. 4 are shown. The same arrangement as that of the conventional device is to be denoted by the same reference numerals as those used in the latter, and will not be explained in detail.

The operation of the image pickup system according to this embodiment will be described below.

The first level detection circuit 101 divides a detection area into a number of small areas, and averages the levels in each of the small areas, and then detects a maximum value of the averages. Fig. 6 is an explanatory diagram showing the principle of the operation of the first level detection circuit 101. It is assumed that each small area is e.g. B. covers an area defined by eight picture elements in the horizontal direction and four lines in the vertical direction of an image pickup device 3. This area is approximately 3% of the effective area of ​​the detection area (for an image pickup device with 420 picture elements in the horizontal direction and 500 lines in the vertical direction). In Fig. 6 the largest value of arithmetic mean values ​​of data of the eight successive picture elements is determined for each of the lines (D1, D2, D3,... DN,..., D250). Then, successive four of the largest values ​​in the vertical direction are added together to obtain the data (X1, X2, X3,..., XN,..., X250), and a maximum value of the data is determined. In this way, a maximum value of the respective mean values ​​of the data of the small areas defined in each case by eight picture elements in the horizontal direction and four lines in the vertical direction is obtained. The second level detection circuit 102 detects an average of the entire screen or detection area.

The operation of the decision circuit 103 and the weighting circuit 104 will now be explained.

The decision circuit 103 determines and outputs a value representing a relationship between an output signal of the first level detection circuit 101 and an output signal of the second level detection circuit 102 (such a value is denoted by K). Assuming that "peak" denotes an output signal of the first level detection circuit and "medium" denotes an output signal of the second level detection circuit, K = peak / medium ... (4)

In general, when taking an image of an object with a large brightness / darkness ratio including a light source or the like, the level (peak) corresponding to a bright part becomes higher and therefore the value of K becomes larger, which makes it possible to base on the basis of the Size of the value K to determine whether the object has a large light / dark ratio because it contains a light source or the like.

The weighting circuit 104 generates a value T obtained by the following equation (5).

T = mediumm + peak (1-m) (0≤m≤1) ... (5)

In this equation, the weighting coefficient m is determined depending on the output signal of the decision circuit 103. The operation of the weighting circuit 104 is illustrated by a solid line in FIG. 7. In Fig. 7, the weighting circuit 104 makes the value of m equal to 0 when K is less than 4, and thereby uses the output signal of the first level detection circuit 101 to control the iris so as to preclude signal saturation. On the other hand, the weighting circuit 104 makes m equal to 1 when K is larger than 6, and thereby selects the output of the second level detection circuit 102 to control the iris so as to reduce the effect of the bright part such as a light source. Another example of the operation of weighting circuit 104 is indicated by the broken line in FIG. 7. In Fig. 7 the aperture when K is less than 4 is controlled by a level indicative of a combination of an output of the first level detection circuit 101 and an output of the second level detection circuit 102. In this case, an optional aperture characteristic can be set by changing the weighting coefficient m. For example, if the weighting coefficient m is increased, where K is less than 4, the iris control approaches that using an average. The aperture characteristic varies slightly when K is close to 5. Furthermore, the operation of the weighting circuit 104 is not limited to the examples described above.

As explained above, according to this embodiment, the decision circuit 103 determines the condition of lightness or darkness of an object or a detection area. When an image is taken of an object with a small brightness / darkness ratio, the iris is controlled so that signal saturation is excluded, whereas when an image is taken of an object with a large brightness / darkness ratio, the aperture control is carried out so that a Preventing the object from becoming dark. As a result, it becomes possible to reproduce an excellent picture. Further, in the above embodiment, the first level detection circuit 101 operates to obtain an average for each of the small areas of the screen, and then outputs a maximum value of the average thus obtained. However, an average of a number of larger ones can be generated from the average for the small areas instead. Furthermore, the decision circuit 103 can alternatively determine and generate a difference between the output signal "peak" of the first level detection circuit 101 and the output signal "middle" of the second level detection circuit 102 in order to achieve essentially the same effect.

A second embodiment has the same block arrangement as the first embodiment, with a variation in the function of the second level detection circuit 102.

The second level detection circuit 102, as shown in FIG. 8, divides the screen into 25 blocks (B1 to B25) and determines an average within each of the blocks B1 to B25. Then the average values ​​of the respective blocks are input to a computing circuit such as a microcomputer, and processed in the manner described below.

First, the mean values ​​of the blocks (B1 to B25) are rearranged in order from the largest mean value in descending order. 25 data records arranged in such a descending order are represented by S1, S2, S3,. , ., S25 designated.

Second, an arithmetic mean of the rearranged data (S1, S2, S3,..., S25) except the data of the extremely bright part corresponding to a light source or the like and the data of the extremely dark part corresponding to a shadow or the like is obtained and output. If the Hth through Lth sets of the reordered data are used to form an arithmetic mean thereof and the resulting arithmetic mean output is denoted by "mean", then

Medium =

For example, if H is 5 and L is 21, the influence of the light or dark parts of the screen can be removed 16%. With an increase in the value H, the influence of a larger area of ​​the bright part such as a light source can be removed.

The second level detection circuit 102 may alternatively generate an arithmetic mean of the data except for the bright part of the screen including a light source or the like. As an example, H and L can be set to 5 and 25, respectively, to thereby remove the influence of the extremely bright part by 16%. Also, only the influence of the extremely dark part corresponding to a shadow or the like can be removed. As an example, the influence of the extremely dark part can be removed 16% by setting H and L to 1 and 21, respectively.

As can be seen from the above description, according to this embodiment, the level of a main object is detected more precisely by using data different from the data of the extremely bright part corresponding to a light source or the like and / or the extremely dark part corresponding to a shadow or the like are. As a result, it is possible to control correct exposure to a main subject, thereby creating high image quality.

A third embodiment has the same block arrangement as the second embodiment with a further variation in the function of the second level detection circuit 102.

As in the second embodiment, the second level detection circuit 102 divides the screen into 25 blocks and determines mean values ​​B1 to B25 for the respective blocks. Then the mean values ​​for the respective blocks are input to a computing circuit such as a microcomputer.

From a selection of the mean values ​​B1 to B25 for the respective blocks, the arithmetic circuit generates an arithmetic mean of the data from B7, B8, B9, B12, B13, B14, B17, B18, B19, B22, B23 and B24, as in FIG. 9 shown. If the arithmetic mean is called "mean", then

Medium = (B7 + B8 + B9 + B12 + B13 + B14 + B17 + B18 + B19 + B22 + B23 + B24) / 12 ... (7)

In this method, the second level detection circuit 102 uses the data in a central part of the screen. Thus, when an image is taken of an object including a bright part located in a central part of the screen, corresponding to a light source or the like, the output value "middle" of the second level detection circuit 102 obtained by this method becomes higher than the output value of the second level detection circuit 102 of the first embodiment, with the result that the output signal K of the decision circuit 103 determined by the equation (4) becomes smaller. Therefore, the iris control is changed to have a characteristic that excludes signal saturation depending on the size of a bright part in a central part of the screen. On the other hand, when a bright part such as a light source or the like is contained in a peripheral area of ​​the screen, the level of the output signal "middle" of the second level detection circuit 102 becomes lower than that of the first embodiment, and therefore the output signal K of the decision circuit 103 takes a larger value so that the iris control is changed in the sense of a characteristic that ignores a bright part such as a light source or the like.

As described above, according to this embodiment, it is possible to realize an iris control that emphasizes a central part of a screen and to prevent the iris from being closed under the influence of a bright part such as a light source or the like located in an edge area of ​​the screen which makes it possible to get an excellent picture.

In the second and third embodiments, the screen can be divided into blocks of 25 different numbers. In the third embodiment, the configuration of the block division is not limited to that explained, but it can be designed in a different way, as will be obvious to those skilled in the art.

10 is a block diagram showing an image pickup system according to a fourth embodiment of the present invention. The subcomponents of the arrangement shown in FIG. 10, which are the same as the corresponding subcomponents of the first exemplary embodiment, are designated by the same reference numerals and are not explained in detail.

In Fig. 10, numeral 140 designates a third level detection circuit which operates to obtain an average for each of the areas into which the screen is divided and each of which is smaller in size than the level detection circuit 101, and which has a maximum value of Averages are recorded.

In the image pickup device according to this embodiment, the decision circuit 103, assuming that "peak2" means an output signal of the third level detection circuit 140 and "medium" denotes the output signal of the second level detection circuit 102, determines the brightness / darkness ratio of an object or a detection area using one value of K obtained from the following equation:

K = Peak2 / Medium ... (8)

The other operations are similar to those of the first embodiment.

In this embodiment, the third level detection circuit 140 detects a maximum value for smaller areas than in the previous embodiments, and therefore the output "peak2" of the third level detection circuit 140 takes a large value, resulting in a larger brightness / darkness ratio K even if an image is captured by a detection area that contains a small point light source. As a result, the device of this embodiment is able to determine a lightness / darkness ratio more sensitively compared to the device of the first embodiment, thereby making it possible to perform open-aperture control even when a small light source is involved.

11 is a block diagram showing an image pickup device according to a fifth embodiment of the present invention. The subcomponents shown in FIG. 11, which are the same as those of the first exemplary embodiment shown in FIG. 5, are designated by the same reference numerals as those used in FIG. 5 and will not be described again in detail.

In Fig. 11, numeral 151 denotes a distribution detection circuit for determining a frequency distribution representative of the frequency of the amplitude of image pickup signals, and numeral 152 denotes a decision circuit for determining whether a frequency distribution determined by the distribution detection circuit 151 in the part of a small amplitude (i.e., a part for a dark object) or in part of a large amplitude (i.e. That is, a part for a light object) accumulates weight.

The operation of the decision circuit 152 according to this embodiment with the arrangement described above will be explained below.

Figures 12A and 12B are histograms, each of which shows a frequency distribution obtained by sampling image pickup signals in a field. The decision circuit 152 divides a frequency distribution by a level S obtained by the distribution detection circuit 151 into two parts, and then determines and outputs a ratio between an average of signals below the level S and that of signals above the level S. The level S is determined by the following equation (9):

S = (MAX + MIN) / 2 ... (9)

where MAX is a maximum value of the level (amplitude) and MIN is a minimum value of the same. Fig. 12A shows a frequency distribution obtained when an image of an ordinary object is captured. In Fig. 12A, L2 is an average of signals below level S and H1 is an average of signals above level S. A ratio k1 between the two average values ​​is obtained by k1 = H1 / L1. FIG. 12 B shows a frequency distribution obtained when an image of an object is taken in backlight. In Fig. 12B, L2 is an average of signals below level S, and H2 is an average of signals above level S. A relationship between the two average values ​​is obtained by k2 = H2 / L2. When an image of an object is taken under back light, the brightness / darkness ratio becomes large and the frequency distribution experiences an accumulation of weight in two parts of large or small amplitudes and therefore k1 becomes smaller than k2. It is thus possible to decide whether or not there is backlight based on the size of an output signal of the decision circuit 152.

As described above, according to this embodiment, the brightness / darkness ratio of an object can be precisely determined if the frequency distribution for the object determined by the distribution detection circuit 151 is clearly divided into two respective parts of large and small amplitudes.

Furthermore, it is possible to obtain substantially the same effect as in the above-described embodiment by having the decision circuit 152 determine a difference between the average of signals above the level S and the average of signals below the level S, and by control the weighting circuit 104 so that the weight of an output signal of the second level detection circuit 102 increases when the difference is large, whereas the weight of an output signal of the first level detection circuit 101 increases when the difference is small.

13 is a block diagram showing an image pickup system according to a sixth embodiment of the present invention. In FIG. 13, the subcomponents that are the same as those of the fifth exemplary embodiment shown in FIG. 11 are denoted by the same reference numerals as those used in FIG. 11 and are not described again in detail.

In Fig. 13, numeral 163 denotes a first level detection circuit for detecting a signal having a large amplitude, namely a substantial peak value from a frequency distribution obtained from the distribution detection circuit 151, and numeral 164 denotes a second level detection circuit for detecting an average of an image pickup signal from that of the Distribution detection circuit 151 obtained frequency distribution.

The operation of an image pickup device according to the sixth embodiment constructed as described above is explained below.

The first level detection circuit 163 and the second level detection circuit 164 receive respective levels in the manner described below. 14A and 14B are histograms, each showing a frequency distribution obtained in the same manner as in Figs. 12A and 12B. Fig. 14A shows a frequency distribution obtained when taking an image of an ordinary object. The first level detection circuit 163 obtains and outputs an average value A of signals M shown by oblique hatching, which corresponds to 20% of the total frequency in the distribution range of larger amplitudes. 14B shows a frequency distribution which is obtained when taking an image of an object in backlight. The second level detection circuit 164 obtains an average value B of signals of a number M shown by oblique hatching, corresponding to 20% of the total frequency in the distribution range of larger amplitudes, signals of a number N corresponding to 10% of the total frequency of a signal of the greatest amplitude shown by the distribution region by vertical hatching have been counted out and issues it. It is possible to detect a true level of a recording area at a high level regardless of a distribution caused by a light source or the like, and when the iris is controlled by an output of the second level detection circuit 164, the detection level of the level detection circuit is reduced compared to a conventional iris control circuit , thereby making it possible to perform open-aperture control.

As described above, according to this embodiment, it is possible to obtain an effect similar to that of the third embodiment using the distribution detection circuit 151.

A seventh embodiment has the same block arrangement as the sixth embodiment with a variation in the function of the second level detection circuit 164.

15 is a histogram showing a frequency distribution obtained when an object is backlit in the same manner as FIG. 12B. The decision circuit 152, as explained with reference to the fifth exemplary embodiment, divides the frequency distribution with respect to a level S into two parts and determines an average value L of signals below the level S and an average value H of signals above the level S and then determines one Ratio K between the two mean values ​​by the equation K = H / L and outputs this. The second level detection circuit 164 obtains your level L 'by multiplying the mean of signals below the level S by, e.g. B. five and then wins an average of signals shown by oblique hatching corresponding to a number M, z. B. 20 % of the total frequency in the distribution range of larger amplitudes, signals from above the level L ', which are included in the region shown by vertical hatching, being excluded from the distribution range and output this.

As explained above, the second level detection circuit 164 according to this seventh embodiment disregards signals whose level exceeds a level several times as high as the signal level L, which signals represent a dark part of an image of a backlit object. Therefore, it is possible to detect the level of an object other than a light source or the like without being affected by the light source or the like even if the light source or the like occupies a large part of the screen. As a result, it becomes possible to obtain an excellent image by preventing an image from an object from becoming dark in the same manner as in the fifth embodiment.

In the sixth exemplary embodiment, the second level detection circuit 164 obtains a frequency distribution, from which signals of 10% of the total frequency have been excluded from a signal with the greatest amplitude in a corresponding number N. Here, if the value of N increases, a larger light source is ignored, whereas if the value of N decreases, a smaller light source is ignored. Thus, the value of N is not limited to the number specified above.

In the sixth and seventh embodiments, the second level detection circuit 164 and the first level detection circuit 163 operate to obtain an average of signals of a number M corresponding to 20% of the total frequency. When the value of M is increased, the iris control approaches an average control, whereas when the value of M is reduced, the iris control approaches a peak control. The aperture characteristic can therefore be freely changed by changing the value of M. Further, in the sixth embodiment, when the second level detection circuit 164 acquires and outputs an average of signals corresponding to 100% of the total frequency without excluding signals of N number, the output level of the second level detection circuit 164 becomes lower than that of the first level detection circuit 163. Therefore, when the iris is controlled by an output of the second level detection circuit 164, it becomes possible to perform an open iris control and thus obtain the same effect as that obtained in the first embodiment.

Further, in the fifth, sixth and seventh embodiments, the decision circuit 152 divides a frequency distribution into two parts with respect to the level S obtained by the equation (9). The level S is not necessarily determined by the equation (9). In the seventh embodiment, the level L 'is determined by multiplying the mean L of signals below the level S by the multiplier 5. However, if a smaller multiplier is used, it becomes possible to perform control with the aperture more open than that of the sixth embodiment, and vice versa.

Although specific embodiments of the invention have been shown and described herein, it is apparent that modifications and changes to the embodiments can be achieved by those skilled in the art.