Usually the visibility of a signal light is treated in terms of the illuminance at the observer's eye level when its visual angle is small, or in term of the luminance contrast when its visual angle is large. Namely the luminous intensity of a long distance signal or the luminance of a short range signal proides a measure in computing the visual range. In Japan the visual range of a road traffic signal is set at more than 150 m. A signal of effective diameter 250 mm (common in Japan) is recognized with a visual angle of only 5.7' from a Disfance of 150 m. However, the signal looks as a luminous surface with a shape to an observer stopping close to it. This means that a road traffic signal is recognized under the intermediate condition between the above two extreme cases. Then we studied the visibility of signal lights by field observations using practical devices.
The degree of easiness to recognize a signal was determined for signals with various sizes and luminances by the method of pair comparison and the method of limitation. Observations were made at distances of 50 m and 150 m by ten subjects. The results show that a higher degree of visibility is gained by a signal having a higher luminance than one signal having a lower luminance with a relatively large diameter though their luminous intensities are nearly equal. The degree of visibilitiey rises with luminance.
Next, the threshold and optimum values were measured against the background of 7, 000-10, 000 cd/m
2 at distances of 50 m (visual angles 13-20.5') and 150 m (visual angles 4.3-6.8'). The threshold illuminance varies with color, size and distance.
The threshold luminance does not depend on size of signals but varies with distance. Values of 160 cd/m
2 (at 50 m) and 300 cd/m
2 (at 150 m) for red light and 330 cd/m
2 (at 50 m) and 900 cd/m
2 (at 150 m) for green light were obtained. The optimum luminance is dependent on neither size nor dis-tance. Mean optimum values are 7, 000 cd/m
2 for red and 8, 000 cd/m
2 for green. These are nearly equal to those obtained by Adrian.
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