Abstract: The purpose of this study was to determine the optimum frequencies and light-dark ratio for two flash light signals which are supposed to he incorporated with a steady light signal. Experiments were performed both under low illumination and under sunshine. An equal discriminability scale for flash light frequencies has also been constructed. The results indicated that the optimum frequencies for the signal with high flash rate fall around 160 cycle per minute with light-dark ratio 2:1, and that for the signal with low flash rate fall within the range of 55—60 cycle per minute with light-dark ratio 1:1.

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