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Abstract: Several variables, i.e. colour, visual angle and brightness contrast that affect theefficiency of railway light signal display were investigated. About 1500 observers se-lected from various districts and nationalities participated in a colour naming experi-ment in which colour zones on the chromaticity diagram had been determined foruse as railway light signals. Two series of identification experiments were carried out at a railway yard. Inone series, visual angle was kept constant and brightness contrast was a variable. Inanother series brightness was kept coustant and visual angle was a. variable. In bothseries red, green and yellow signals were presented randomly. In the preliminary experiments it was found that while the red and yellow signalswere kept in favourable display conditions the observers showed higher probability ofcorrect identification for the green signal even if it was under unfavourable coudition,because if the signals were not red or yellow, it should have been green. After addinga white signal and a non-signal as interference to eliminated the guess effect, twosets of psychophysical functions ——correct identification rate vs. visual angle curveand correct identification rate vs. brightness contrast curve had been obtained.

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