The properties of spatial summation by the eyes at suprathreshold levels are not clearly understood. An experiment on the brightness of a test stimulus as a function of its area and background luminance was carried out to clarify the properties. The luminous intensity of the test stimulus to match a point stimulus in brightness was measured.
The results are as follows:
(1) The spatial summation is complete regardless of the background luminance for an area of less than 10min². It becomes gradually less complete as the area is more than 10min². As background luminance increases, the effect of the spatial summation decreases, and little summation occurs for an area of more than 103 min² at background luminance of 1cd/m² and 10cd/m².
(2) The range of partial summation at suprathreshold levels is wider than that at threshold levels.
(3) Log (I_t/I_m) increases linearly with log (L_b).
(4) It is necessary to treat a light as a surface source to obtain the brightness, in the case that we observe a traffic signal light from a short distance.