Several factors affecting the visual range of colored point sources, i. e. recognition threshold illuminances, luminances and chromaticities of natural objects, and spectral extinction coefficients of light through the atmosphere, were measured and analysed. A graphical method to determine the visual range of colored point sources in the daytime is developed. The results show that red, yellow and green lights of adequate saturation have good visual ranges but a blue-purple light has poor visibility when it is viewed against the background of natural objects.
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