Abstract

Optimum intensities for a red road-traffic signal light have been determined for observers with normal and protanopic color vision. The term optimum intensity is used to mean the intensity necessary to minimize the chance of not seeing the signal and also the time of response. The experimental conditions simulated an 8-in.-diam signal light viewed from 100 m against a sky with a luminance of about 1500 ft-L. Under these conditions the optimum intensity was shown to be at least 83 cd and preferably 133 cd. Protanopes required about four times this intensity for the particular red filter used. For a sky of 3×104 ft-L luminance a red signal should have intensity of 160–260 cd. Protanopic drivers would require at least a 600 cd intensity. Surround screens were shown to improve the visibility of a red signal for normal observers only when the intensity of the signal was less than optimum.

© 1966 Optical Society of America

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