A study of light signals in aviation and navigation

TLDR: In this paper, the time it takes to locate a visible beacon was studied as a function of the beacon intensity and frequency of flashing, and the threshold candle power required for visibility of a point source at distance D (cm) against a background of brightness H (candles×cm−2) was given by the empirical equation C/D2=3.5×10−9 H1/2.

Abstract: Visibility of point sources.—Laboratory experiments have been devised to make measurements of the visibility of light signals under conditions essentially similar to those encountered by the aviator or the navigator. Data have been collected on the direct visibility of flashing point sources of light of different colors, flash lengths and intervals, against different backgrounds; the time it takes to locate a visible beacon was studied as a function of the beacon intensity and frequency of flashing. The threshold candle power C required for visibility of a point source at distance D (cm) against a background of brightness H (candles×cm−2) is given by the empirical equation C/D2=3.5×10−9 H1/2. Colored point sources were not found to be useful except in the case of red lights with background intensities above moonlight. For an airplane approaching a beacon it is advantageous to use frequencies of flashing as high as 12 to 30 per minute, although with exceptionally clear atmosphere, lower frequencies may be ...