| Definition: | visual photometer in which the observer sees either an undivided field illuminated successively, or two adjacent fields illuminated alternately, by two sources to be compared, the frequency of alteration being conveniently chosen so that it is above the critical flicker frequency for colours but below the fusion frequency for brightness Note 1 to entry: This entry was numbered 845-05-19 in IEC 60050-845:1987.
|