In this study visual brightness discrimination was investigated for three centrally fixated incremental areas (1° 20′, 5°, and 19°) each superimposed upon a 26° surround. Surround luminances from logI=3.48 to logI=−3.02 (millilamberts) were used. The objectives of the experiment were to find: (a) the shape of the transition between the two theoretical conditions It=constant and I=constant as the duration of incremental exposure was increased, and (b) the magnitudes of the critical durations at which this transition occurred.
The results indicated that the transition between conditions It=constant and I=constant for central fixation was relatively sharp, regardless of field size or luminance level. This was true for both differential and absolute threshold determinations. The critical durations, depending upon field size, ranged from 0.02–0.06 second at high luminances to as long as 0.20–0.35 second at low scotopic luminance levels. The curves relating critical duration to log luminance showed irregularities in the mesopic range for the two larger fields. These irregularities were interpreted as indicating a possible transition between rod and cone functioning in this luminance range.
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