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 a maximum value permissible for {HEAD ~ the light control value} as {TAIL ~ the value} Kmin when the black area S is the threshold value S0 or less 
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 a maximum value permissible for { HEAD ~ the light control value } as { TAIL ~ the value } Kmin when the black area S is the threshold value S0 or less
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