Luminance change visibility of a Ganzfeld and circular target subtending 15° at the eye was measured for 12 subjects. Visibility of change increased from about 50 per cent at rate of change of 0·03 Log units/sec to 80 per cent at 0·17 Log units/sec. Visibility of change for any one rate and target was identical irrespective of the sign of luminance change. Visibility of luminance change in the Ganzfeld was generally 15 per cent lower than in the 15° target. The luminance change detecting mechanism must therefore depend less upon area signal than on contrast information.