Visual Efficiency in Coloured Light

Abstract

The visual efficiencies of observers in a street scene which was illuminated in turn with sodium, mercury, and white light have been compared. The scene was a brightness replica of a selected street-lighting installation reproduced on a screen by means of a cinematograph film. The street lamps were introduced into the picture from behind the screen.

Two films were used for the tests. In the first film, objects in the street scene were suddenly presented to the observer, while in the second, the objects were allowed gradually to attain the contrast which they presented in the actual installation. The number of objects detected by each observer was noted and the time taken to detect each object was automatically recorded.

The comparison of visual efficiency was carried out at three brightness levels, and for each comparison the street scene had the same distribution of photometric brightness whatever the illuminant, i.e., the comparison was based on sources of the same lumen output.

The results obtained at the three brightness levels, and with both methods of object presentation, show no significant differences in the number of objects detected with the three illuminants. An analysis of the detection times has also failed to reveal grounds for attributing a superiority to any one of the three illuminants.

The investigation gives no support to the claim that the visibility of objects in a street lighting installation will be improved by using sources which emit light of a particular spectral quality.