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