Study on Adaptive Illumination for Sudden Change of Brightness

Abstract

In Japan, since 1958 lighting problems on the autoway-tunnels have occurred concerning the constructions of the express-ways, such as the Tokyo express highway or the Meishin express way. Tosolve the problems of tunnel lighting some studies have been carried out in Japan by committees in the I. E. I. of Jap., and some solutions of the problems had been reported in the Journal of I. E. I. of Japan from 1960 to 1963, these solutions were adopted in some new tunnels as shown in Fig. 1.These experimental studies on the instantaneous adaptation of the eye for rapid changes of brightness, were carried out as supplementary studies to the committee reports of tunnel-lighting from the physiological view point.The first experiments were carried out for the purpose of measuring the recovering time of the visual acuity after the sudden change of brightness. For the light-adaptation, the adapting field was illuminated with a maximum 30, 000 lx using a projector with 1k. w. incandescent lamp shown in Fig. 2.With the brightness on the adapting field changed momentarily from light to dark, or from dark to light, recovering times of the visual acuities were measured using the Snellen's test-target in the middle of the adapting field. The results of the experiments by three observers with normal visual acuities, are shown in Fig. 4 to Fig. 8.Fig. 4 shows the relation between the recovering times of visual acuities and the terminal luminanceson the adapting field which was switched off momentarily from 7, 500 cd/m2, and Fig. 5 shows the process of the visual-acuities after the darkening of the adapting field derived from same data of Fig. 4. In Fig. 6 a case of the 750cd/m2 pre-adaptation is compared with that of the 7, 500cd/m2 pre-adaptation using a test target of 0.75.Fig. 7 shows the recovering time of the visual acuity in the brightness change from 2.5-25cd/m2 to 7, 500cd/m2 and it was compared in Fig. 8 with that of from 2.5-25cd/m2 to 750cd/m2.Those data show that the human eye needs about 10 seconds to recover the practical visual acuity in the dark-adaptation, on the other hand it needs only about a second in the light-adaptation. This fact indicates the necessity of the adaptive illumination for sudden drops of brightness, such as at the tunnel-entrance.The second experiments were carried out for the purpose of finding such brightness reduction as keeps the visual acuity at practical degree.A ladder-shaped target, as shown in Fig 11, was used in this experiment for keeping the visualacuity within the limit of over 0.75 under 1, 0. If the reduction of brightness were too rapid, the target would be seen as a black dot shown (a) in Fig. 11, and if the reduction were too slow it would be seen as a ladder shape as shown (c) in Fig. 11. Only at the proper speed of brightness-reduction the target could be seen as an H shape shown (b) in Fig. 11, because the two horizontal bars of the target make in visual angle of I minute and the two vertical bars make a visual angle of 1/0.75 minute from observer's eyeOn many levels of pre-adapting brightness from 25cd/m2 to 1, 500cd/m2, the proper decreasing speeds of brightness were measured using three observers with a visual acuity of 1, 5The results of experiments were shown in Fig. 14, which indicates the proper reduction of brightness keeping the practical visual acuity for sudden drops of brightness. In Fig. 13 author's data on the 1, 500 cd/m2 pre-adaptation which has adopted in the tunnel-lighting of the Meishin express-way, was compared with the De Boer's data on the 8, 000cd/m2 pre-adaptation found by another method. It was noticed that shapes of two courves are similar to each other in spite of differences of searching methods.