{TAIL ~ A minimum light control value output section} determines a minimum light control value Kmin based on comparison between the measured black area S and a black area threshold S0 and outputs {HEAD ~ a maximum value} permissible for the light control value as the value Kmin when the black area S is the threshold value S0 or less 
{ TAIL ~ A minimum light control value output section } determines a minimum light control value Kmin based on comparison between the measured black area S and a black area threshold S0 and outputs { HEAD ~ a maximum value } permissible for the light control value as the value Kmin when the black area S is the threshold value S0 or less
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