Abstract:
An exposure controller comprises a discharge pulse calculation circuit for calculating a discharge pulse count to be output to a solid-state image pickup device within one field period, and a coring circuit for defining the quotient obtained from the discharge pulse count divided by a predetermined setting value and plus 1 as a coring value. In an electronic camera system incorporating an electronic iris, when one discharge pulse changes, the amount of change in the luminance level of an image signal becomes larger as an exposure time becomes shorter, whereby hunting is prevented from occurring at the convergent point of the luminance level, and an exposure controller which is compact and has excellent characteristics can be embodied.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to an exposure controller, more particularly to an exposure controller for a camera incorporating an electronic iris. 
     Instead of mechanical exposure control, electronic exposure control has recently been used to meet the needs for miniaturization of electronic cameras. 
     Conventional electronic exposure control has been disclosed in Japanese Laid-open Patent Application No. 5-48975, for example. 
     A conventional exposure controller is described below. FIG. 8 is a block diagram of this conventional exposure controller. Referring to FIG. 8, numeral  81  represents a rectifier circuit for receiving an image signal from an image pickup device at an input terminal and for rectifying the image signal, numerals  82  and  83  represent comparator circuits, numerals  84  and  85  represent reference voltage setting circuits, numeral  86  represents a counter circuit, and numeral  87  represents an exposure time control circuit for controlling exposure time. 
     The operation of the exposure controller having the above-mentioned configuration is described below. First, an image signal from the image pickup device is rectified by the rectifier circuit  81 . A reference voltage, that is, a potential difference corresponding to the amount of change in the output level of the image signal when the amount of input light entering the image pickup device is doubled, is generated by the reference voltage setting circuit  84 . Another reference voltage, that is, a potential difference not less than the above-mentioned potential difference, is generated by the reference voltage setting circuit  85 . The two reference voltages generated by the reference voltage setting circuits  84 ,  85  are compared with the output level of the rectified image signal by the comparator circuits  82 ,  83 , respectively. The count of the counter circuit  86  is incremented, decremented or stopped depending on the outputs of the comparator circuits  82 ,  83 , and the exposure time of the image pickup device is controlled by the exposure time control circuit  87  depending on the output of the counter circuit  86 . 
     However, in the above-mentioned conventional configuration, the level of the picked-up image signal at each exposure time control process changes larger as the exposure time becomes shorter, and the problem of hunting occurs at the convergent point of the image signal level. 
     Accordingly, an object of the present invention is to provide an exposure controller free from hunting by using a coring value adapted to the discharge pulse count of a solid-state image pickup device. 
     SUMMARY OF THE INVENTION 
     A first embodiment of the present invention comprises a lens, a solid-state image pickup device for picking up the image of light having passed through the lens, an AD converter for converting the image picked up by the solid-state image pickup device into a digital signal, a luminance level detector circuit for detecting the luminance level of the image signal digitized by the AD converter, a register having stored the target value of the luminance level, a subtracter for calculating the difference between the luminance level detected by the luminance level detector circuit and the target value of the luminance level, a discharge pulse calculation circuit for calculating a discharge pulse count (hereinafter referred to as sub) to be output to the solid-state image pickup device within one field period on the basis of the polarity of the difference and a discharge pulse hold signal to be output from a coring circuit described later without changing the discharge pulse count when the discharge pulse hold signal is H, or depending on the polarity of the difference between the luminance level and the target value of the luminance level to be output from the subtracter when the discharge pulse hold signal is L, and for outputting sub, an absolute value circuit for calculating the absolute value (hereinafter referred to as a luminance level error) of the difference between the luminance level and the target value of the luminance level to be output from the subtracter, a coring circuit for defining the quotient obtained from sub divided by a predetermined setting value and plus 1 as a coring value, for setting the discharge pulse hold signal at H when the coring value is larger than the luminance level error, or at L in other cases, and for outputting the discharge pulse hold signal, and a drive pulse generator circuit for converting sub into a discharge pulse signal and for outputting the discharge pulse signal to the solid-state image pickup device. 
     According to this embodiment, when light enters the solid-state image pickup device, the device performs photoelectric conversion, stores charges in a period during which no discharge pulse signal is input, and outputs the charges as an image signal. The luminance level detector circuit assigns weights to the screen center portion of the digitized image signal so as to average luminance levels on the screen, and outputs the average level as the luminance level of the image signal. The subtracter circuit calculates the difference between this luminance level and the target value of the luminance level, which has been stored in a register. The coring circuit defines the value of sub divided by a predetermined value and plus 1 as a coring value, and when 
     luminance level error&gt;coring value 
     the discharge pulse hold signal is set at L, or when 
     
       
         luminance level error≦coring value  
       
     
     the discharge pulse hold signal is set at H, and the signal is output to the discharge pulse calculation circuit. 
     The discharge pulse calculation circuit determines the magnitude relationship between the luminance level and the target value depending on the polarity of the luminance input signal, and when 
     
       
         luminance level&gt;target value  
       
     
     the discharge pulse calculation circuit increments sub so as to shorten the charge time of the solid-state image pickup device and to lower the luminance level, and then outputs sub, or when 
     
       
         luminance level≦target value  
       
     
     the discharge pulse calculation circuit decrements sub so as to lengthen the charge time of the solid-state image pickup device and to raise the luminance level, and then outputs sub. In case the discharge pulse hold signal is H at this time, sub remains unchanged and is output. The drive pulse generator circuit outputs the discharge pulse signal having the same number of pulses as the value of sub to the solid-state image pickup device. 
     Since the exposure controller is provided with the coring circuit as described above, this exposure controller can be embodied as an exposure controller free from hunting by using a coring value adapted to the value of the discharge pulse count. 
     A second embodiment of the present invention, having a coring circuit different from that of the first embodiment, comprises a lens, a solid-state image pickup device for picking up the image of light having passed through the lens, an AD converter for converting the image picked up by the solid-state image pickup device into a digital signal, a luminance level detector circuit for detecting the luminance level of the image signal digitized by the AD converter, a register having stored the target value of the luminance level, a subtracter for calculating the difference between the luminance level detected by the luminance level detector circuit and the target value of the luminance level, a discharge pulse calculation circuit for calculating a discharge pulse count (hereinafter referred to as sub) to be output to the solid-state image pickup device within one field period on the basis of the polarity of the difference and a discharge pulse hold signal to be output from a coring circuit described later without changing sub when the discharge pulse hold signal is H, or depending on the polarity of the difference between the luminance level and the target value of the luminance level to be output by the subtracter when the discharge pulse hold signal is L, and for outputting sub, an absolute value circuit for calculating the absolute value (hereinafter referred to as a luminance level error) of the difference between the luminance level and the target value of the luminance level to be output from the subtracter, a coring circuit for defining the value of a second register as a coring value when sub is not more than the value stored in a first register or for defining the value of a third register as the coring value when sub is more than the value stored in the first register, for setting the discharge pulse hold signal at H when the coring value is larger than the luminance level error, or at L in other cases, and for outputting the discharge pulse hold signal, and a drive pulse generator circuit for converting sub into a discharge pulse signal and for outputting the discharge pulse signal to the solid-state image pickup device. 
     A third embodiment of the present invention, having a coring circuit different from that of the first embodiment, comprises a lens, a solid-state image pickup device for picking up the image of light having passed through the lens, an AD converter for converting the image picked up by the solid-state image pickup device into a digital signal, a luminance level detector circuit for detecting the luminance level of the image signal digitized by the AD converter, a register having stored the target value of the luminance level, a subtracter for calculating the difference between the luminance level detected by the luminance level detector circuit and the target value of the luminance level, a discharge pulse calculation circuit for calculating a discharge pulse count (hereinafter referred to as sub) to be output to the solid-state image pickup device within one field period on the basis of the polarity of the difference and a discharge pulse hold signal to be output from a coring circuit described later without changing sub when the discharge pulse hold signal is H, or depending on the polarity of the difference between the luminance level and the target value of the luminance level to be output by the subtracter when the discharge pulse hold signal is L, and for outputting sub, an absolute value circuit for calculating the absolute value (hereinafter referred to as a luminance level error) of the difference between the luminance level and the target value of the luminance level to be output from the subtracter, a coring circuit for defining the value obtained by multiplying the target value by the reciprocal of the difference between the number of scanning lines in one field and sub and plus 1 as a coring value, for setting the discharge pulse hold signal at H when the coring value is larger than the luminance level error, or at L in other cases, and for outputting the discharge pulse hold signal, and a drive pulse generator circuit for converting sub into a discharge pulse signal and for outputting the discharge pulse signal to the solid-state image pickup device. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of an exposure controller in accordance with a first embodiment of the present invention; 
     FIG. 2 is a block diagram of a coring circuit in accordance with the first embodiment; 
     FIG. 3 is a block diagram of an exposure controller in accordance with a second embodiment; 
     FIG. 4 is a block diagram of a coring circuit in accordance with the second embodiment; 
     FIG. 5 is a block diagram of an exposure controller in accordance with a third embodiment; 
     FIG. 6 is a block diagram of a coring circuit in accordance with the third embodiment; 
     FIG. 7 is a schematic diagram showing the output timing of drive pulses at a drive pulse generator circuit; and 
     FIG. 8 is a block diagram of a conventional exposure controller. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A first embodiment of the present invention will be described below referring to the drawings. 
     FIG. 1 is a block diagram of an entire exposure controller in accordance with the first embodiment of the present invention. Referring to FIG. 1, numeral  10  represents a lens, numeral  11  represents a solid-state image pickup device for picking up the image of light s 10  having passed through the lens  10  and for outputting the image as an image signal s 11 . Numeral  12  represents an A/D converter for converting the image signal s 11  into a digital signal s 12 . Numeral  13  represents a luminance level detector circuit for detecting the luminance level s 13  of the digitized image signal s 12 . Numeral  14  represents a register having stored the target value s 14  of the luminance level s 13 . Numeral  15  represents a subtracter for calculating the difference s 15  between the luminance level s 13  and the target value s 14  and for outputting the difference s 15 . Numeral  16  represents a discharge pulse calculation circuit for outputting a discharge pulse count (hereinafter referred to as sub) s 16  to the solid-state image pickup device  11  within one field period on the basis of the polarity of the difference s 15  and a discharge pulse hold signal s 18  to be output from a coring circuit  18  described later. Numeral  17  represents an absolute value circuit for calculating the absolute value (hereinafter referred to as a luminance level error) s 17  of the difference s 15  between the luminance level s 13  and the target value s 14 . Numeral  18  represents a coring circuit for setting the discharge pulse hold signal s 18  on the basis of sub s 16  and the luminance level error s 17  and for outputting the discharge pulse hold signal s 18 . Numeral  19  represents a drive pulse generator circuit for converting sub s 16  into a discharge pulse signal s 19  and for outputting the discharge pulse signal s 19  to the solid-state image pickup device  11 . 
     The operation of the exposure controller of this embodiment having the above-mentioned configuration is described below. When the light s 10  enters the solid-state image pickup device  11 , the solid-state image pickup device  11  performs photoelectric conversion, stores charges in a period during which the discharge pulse signal sl 9  is not input, and outputs the charges as the image signal s 11 . The luminance level detector signal  13  obtains the average luminance of the digitized image signal s 12  and outputs the average luminance as the luminance level s 13  of the image signal s 12 . The subtracter  15  calculates the difference s 15  between the luminance level s 13  and the target value s 14  of the luminance level s 13 , which has been stored in the register  14 . The coring circuit  18  defines the value of sub s 16  divided by integer  64  and plus 1 as a coring value, and when 
     
       
         luminance level error s 17 &gt;coring value  
       
     
     the discharge pulse hold signal s 18  is set at L, or when 
     
       
         luminance level error s 17 ≦coring value  
       
     
     the discharge pulse hold signal s 18  is set at H, and the signal is output to the discharge pulse calculation circuit  16 . 
     The discharge pulse calculation circuit  16  determines the magnitude relationship between the luminance level s 13  and the target value s 14  depending on the polarity of the luminance input signal s 15 , and when 
     
       
         luminance level s 13 &gt;target value s 14   
       
     
     the discharge pulse calculation circuit  16  increments sub s 16  so as to shorten the charge time of the solid-state image pickup device  11  and to lower the luminance level, and then outputs sub s 16 , or when 
     
       
         luminance level s 13 ≦target value s 14   
       
     
     the discharge pulse calculation circuit  16  decrements sub s 16  so as to lengthen the charge time of the solid-state image pickup device  11  and to raise the luminance level, and then outputs sub s 16 . In case the discharge pulse hold signal s 18  is H at this time, sub s 16  remains unchanged and is output. 
     The drive pulse generator circuit  19  outputs the discharge pulse signal s 19  having the same number of pulses as the value of sub s 16  to the solid-state image pickup device  11 . 
     FIG. 2 is a block diagram of the coring circuit  18  of the exposure controller of the present embodiment. Referring to FIG. 2, numeral  21  represents a bit shifter for shifting sub s 16  to its LSB side by 6 bits, numeral  22  represents an adder for adding 1 to the value s 21  bit-shifted by the bit shifter  21 , and numeral  23  represents a comparator for comparing the added value (hereinafter referred to as a coring value) s 22  with the luminance level error s 17 . 
     The operation of the coring circuit of the present embodiment having the above-mentioned configuration is described below. By the bit shifter  21 , sub s 16  is shifted by 6 bits to its LSB side, and 1 is added to s 21  by the adder  22 . As a result, the following equation is established: 
     
       
         coring value s 22 =sub s 16 / 64 +1.  
       
     
     The comparator  23  compares the coring value s 22  with the luminance level error s 17 , and when 
     
       
         luminance level error s 17 &gt;coring value s 22   
       
     
     the comparator  23  outputs 0, or when 
     
       
         luminance level error s 17 ≦coring value s 22   
       
     
     the comparator  23  outputs 1, and this output value is output as the discharge pulse hold signal s 18 . 
     As described above, the electronic-iris type exposure controller of the first embodiment, having the drive pulse generator circuit  19  which outputs the discharge pulse signal to the solid-state image pickup device  11  at a rate of an effective scanning period divided by 16 in a vertical blanking period, is provided with the coring circuit  18  which defines sub s 16  divided by integer  16  and plus 1 as the coring value s 22 . This exposure controller can be embodied as an exposure controller free from hunting by using a coring value adapted to the value of sub s 16 . 
     FIG. 3 is a block diagram of an entire exposure controller in accordance with a second embodiment of the present invention. Referring to FIG. 3, numeral  10  represents a lens, numeral  11  represents a solid-state image pickup device for picking up the image of light s 10  having passed through the lens  10  and for outputting the image as an image signal s 11 . Numeral  12  represents an A/D converter for converting the image signal s 11  into a digital signal s 12 . Numeral  13  represents a luminance level detector circuit for detecting the luminance level s 13  of the digitized image signal s 12 . Numeral  14  represents a register having stored the target value s 14  of the luminance level s 13 . Numeral  15  represents a subtracter for calculating the difference s 15  between the luminance level s 13  and the target value s 14  and for outputting the difference s 15 . Numeral  36  represents a discharge pulse calculation circuit for outputting a discharge pulse count (hereinafter referred to as sub) s 16  to the solid-state image pickup device  11  within one field period on the basis of the polarity of the difference s 15  and a discharge pulse hold signal s 18  to be output from a coring circuit  38  described later. Numeral  17  represents an absolute value circuit for calculating the absolute value (hereinafter referred to as a luminance level error) s 17  of the difference s 15  between the luminance level s 13  and the target value s 14 . The configuration described above is similar to that shown in FIG.  1 . The configuration of the second embodiment only differs from that shown in FIG. 1 in the following two points: the process by the coring circuit  38  differs from the process by the coring circuit  18 , and a drive pulse generator circuit  39  converts sub s 16  into a discharge pulse signal s 19  and outputs the discharge pulse signal s 19  to the solid-state image pickup device  11 . 
     FIG. 4 is a block diagram of the coring circuit  38  of the exposure controller in accordance with the second embodiment. Referring to FIG. 4, numerals  40 ,  42  and  43  represent registers, numeral  41  represents a comparator for comparing sub s 16  with the value s 41  of the register  40 , numeral  44  represents a selector for outputting the value s 43  of the register  42  or the value s 44  of the register  43  depending on the output value s 42  of the comparator  41 , and numeral  45  represents a comparator for comparing the output value (hereinafter referred to as a coring value) s 45  of the selector  44  with the luminance level error s 17 . 
     The operation of the coring circuit  38  having the above-mentioned configuration is described below. The comparator  41  compares sub s 16  with the value s 41  of the register  40 , and when the result is represented as follows: 
     
       
         sub s 16 &gt;value s 41  of the register  40 ,  
       
     
     the comparator  41  outputs 1, and in other cases, the comparator  41  outputs 0. If the output value s 42  of the comparator  41  is 1, the selector  44  outputs the value s 43  of the register  42  as the coring value s 45 . If the output value s 42  is 0, the selector  44  outputs the value s 44  of the register  43  as the coring value s 45 . The comparator  45  compares the luminance level error s 17  with the coring value  45 , and when 
     
       
         luminance level error s 17 &gt;coring value s 45   
       
     
     the comparator  45  outputs 0, or when 
     
       
         luminance level error s 17 ≦coring value s 45   
       
     
     the comparator  45  outputs 1. This output value is output as the discharge pulse hold signal s 18 . 
     As described above, the electronic-iris type exposure controller of the second embodiment is provided with the coring circuit  38  which changes the coring value s 45  depending on whether the value of sub s 16  is larger than the value s 41  of the register  40  or not. This exposure controller can be embodied as an exposure controller free from hunting by using a coring value adapted to the value of sub s 16 . 
     FIG. 5 is a block diagram of an entire exposure controller in accordance with a third embodiment of the present invention. Referring to FIG. 5, numeral  10  represents a lens, numeral  11  represents a solid-state image pickup device for picking up the image of light s 10  having passed through the lens  10  and for outputting the image as an image signal s 11 . Numeral  12  represents an A/D converter for converting the image signal s 11  into a digital signal s 12 . Numeral  13  represents a luminance level detector circuit for detecting the luminance level s 13  of the digitized image signal s 12 . Numeral  14  represents a register having stored the target value s 14  of the luminance level s 13 . Numeral  15  represents a subtracter for calculating the difference s 15  between the luminance level s 13  and the target value s 14  and for outputting the difference s 15 . Numeral  56  represents a discharge pulse calculation circuit for outputting a discharge pulse count (hereinafter referred to as sub) s 16  to the solid-state image pickup device  11  within one field period on the basis of the polarity of the difference s 15  and a discharge pulse hold signal s 18  to be output from a coring circuit  58  described later. Numeral  17  represents an absolute value circuit for calculating the absolute value (hereinafter referred to as a luminance level error) s 17  of the difference s 15  between the luminance level s 13  and the target value s 14 . Numeral  39  represents a drive pulse generator circuit for converting sub s 16  into a discharge pulse signal s 19  and for outputting the discharge pulse signal s 19  to the solid-state image pickup device  11 . The configuration described above is similar to that shown in FIG. 1 or  3 . The configuration of the third embodiment only differs from that shown in FIG. 1 or  3  in that the process by the coring circuit  58  differs from the process by the coring circuit  18  or  38 . 
     FIG. 6 is a block diagram of the coring circuit  58  of the exposure controller in accordance with the third embodiment. Referring to FIG. 6, numeral  61  represents a register, numeral  62  represents a subtracter for subtracting sub s 16  from the value of the register  61 , numeral  63  represents a divider for dividing the target value s 14  by the output s 62  of the subtracter  62 , numeral  64  represents an adder for adding 1 to the value s 63  obtained by the divider, and numeral  65  represents a comparator for comparing this added value (hereinafter referred to as a coring value) s 64  with the luminance level error s 17 . 
     The operation of the coring circuit of the third embodiment having the above-mentioned configuration is described below. The register  61  has stored the number of scanning lines per field. The subtracter  62  subtracts sub s 16  from the value s 61  of the register  61 . By dividing the target value s 14  by the result s 62  of the subtraction, the following equation can be established: 
     
       
         target value s 14 /(the number of scanning lines per field−sub s 16 )  (Equation 61)  
       
     
     Equation 61 indicates the amount of change in the luminance level s 13  when sub s 16  is changed by 1, and 1 is added to the amount by the adder  64 , and then the following equation can be established: 
     
       
         target value s 14 /(the number of scanning lines per field−sub s 16 )+1  (Equation 62)  
       
     
     the value represented by this equation is defined as a coring value s 64 . The comparator  65  compares the luminance level error s 17  with the coring value s 64 , and when 
     
       
         luminance level error s 17 &gt;coring value s 64   
       
     
     the comparator  65  outputs 0, or when 
     
       
         luminance level error s 17 ≦coring value s 64   
       
     
     the comparator  65  outputs 1, and this output value is output as the discharge pulse hold signal s 18 . 
     As described above, the electronic-iris type exposure controller of the third embodiment is provided with the coring circuit  64  which obtains the amount of change in the luminance level s 13  when sub s 16  is changed by 1, adds 1 to this amount and defines the result of the addition as the coring value s 64 . This exposure controller can be embodied as an exposure controller free from hunting by using a coring value adapted to the value of sub s 16 . 
     FIG. 7 is a schematic diagram showing the output timing of drive pulses at the drive pulse generator circuits  19 ,  39  of the exposure controller in accordance with the first, second and third embodiments. As shown in FIG. 7, outside a vertical blanking period, the drive pulse generator circuits  19 ,  39  output one discharge pulse during each horizontal blanking period. Within the vertical blanking period, the drive pulse generator circuits  19 ,  39  output discharge pulses during effective scanning periods as well as during the horizontal blanking periods. Because of these characteristics, even when exposure is controlled in a very high electronic shutter speed condition wherein discharge pulses are output within the vertical blanking period, the change rate of the luminance level of the pickup image in accordance with the change in the number of the discharge pulses can be decreased, whereby exposure can be controlled by using fewer coring values. 
     FIG. 1 
       11  solid-state image pickup device 
       12  ad converter 
       13  luminance level detector circuit 
       14  target value 
       15  subtracter 
       17  absolute value circuit 
       18  core ring circuit 
       16  discharge pulse calculation circuit 
       19  drive pulse generator circuit 
     FIG. 2 
       21   6- bit shifter 
       22  adder 
       23  comparator 
     FIG. 4 
       40  register 
       44  selector 
     FIG. 7 
       1  vertical blanking 
       2  horizontal blanking 
       3  discharge pulse 
       4  vertical blanking period 
       5  time 
     FIG. 8 
       1  input terminal 
       2  output terminal 
       81  rectifier circuit 
       82  comparator circuit a 
       86  counter circuit 
       87  exposure time control circuit