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 10min
2. It becomes gradually less complete as the area is more than 10min
2. As background luminance increases, the effect of the spatial summation decreases, and little summation occurs for an area of more than 103 min
2 at background luminance of 1cd/m
2 and 10cd/m
2.
(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.
View full abstract