Abstract
On colored light sources threshold illuminance was measured by laboratory and field experiments. In the laboratory a light source of the visual angle 50 seconds of are was observed at a distance of 7 ni against neutral background. Background luminances were 10.2, 2.0 and 0.1 cd/m2, which correspond to the luminance in the moonlight. By field experiments a Hight source of the visual angle less than 50 seconds of arc was observed at 500m. The background was natural objects. Its luminance and chromaticity were measured by a telephotometer. Background luminances were 2, 300 and 320 cd/m2.
For the light source three red filters, four yellow-red filters, one yellow filter and five green filters of various dominant wavelengths and purities were applied.
The experimental result shows that the threshold illuminance, εm for perceiving the brightness of the light source, has nearly the same value for different colors and depends on the background luminance. Author's data coincide approximately with the Tiffany Foundation's, Green's, and Knoll, Tousey and Hulbert's.
The threshold illuminance, εm, to perceive the chromaticity, increases when the chromaticity difference between source and background decreases, especially when the background luminance is high. It depends also on the hue of the source, and for the same chromaticity difference εme for yellow and yellow-red is higher than that for green. Representing the chromaticity difference by the distance d between the points for the chromaticities of the soure and the background on Breckenridge and Schaub's RUGS coordinates, an empirical formula d (εm) 1/2=k is obtained; k increases with background luminance and is greater for yellow and yellow-red than for green.
