an acquisition unit adapted to acquire a brightness of {TAIL ~ the image} displayed on the display surface by {HEAD ~ said display unit}; and 
an acquisition unit adapted to acquire a brightness of { TAIL ~ the image } displayed on the display surface by { HEAD ~ said display unit } ; and
OTH OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH