One of the most serious problems connected with tunnel lighting is the so-called “black hole effect” for long tunnel and the “black frame effect” for short tunnel. To eliminate these “black hole and frame effects”, a sufficient luminance has to be given to the road surface and walls in the tunnel.
Previous investigators, such as Waldram⁵⁾ , Brass⁶⁾ , de Boer⁹⁾ and Schreuder²⁾ revealed that the necessary values of luminance are at least 10 to 12.5 % for driver's adaptation luminance. Some authors of previous studies on the tunnel entrance lighting stated that the outdoor luminance near tunnel can be regarded as the adaptation luminance of the driver.
However, the adaptation luminance of human eye is not determined by the average luminance of the visual field, but strongly influenced by the luminace projected upon the fovea of retina, so the adaptation luminance of the driver approaching the tunnel mouth will be gradually reduced.
It is also clear from this consideration that high luminance of tunnel exit projected upon the driver's central vision will give a contrary effect on the adaptation. Therefore, the adaptation of the driver approaching the tunnel will differ for different distances from the tunnel and apparent lengths of the tunnel for the driver.
The visual condition of the driver on the motor vehicle is carefully analyzed. Previous investigators stated^{10) 16)} that the angular velocity of the object under 5 degrees per second gives a negligible influence on the ability of the eye. The angular velocity of objects, which is important for the driver, on the road some distance ahead and observed from a high speed vehicle is only several tens minutes per second, even the speed of the motor vehicle being 80 km/h or more. Thus, it is found that the influence of the angular velocity of the object is negligible, but the object must be perceived within a very short time, because the driver has to gather a lot of information at each instance.
With this born in mind, a simulator has been constructed which reproduces the visual condition of the driver approaching the tunnel as faithfully as possible (Fig. 4).
The observer looked at a simulated tunnel entrance which was extended perspectively with a synchronous motor driven on inclined rails (Fig. 5) reproducing the change of the apparent size of the tunnel mouth as looked from the driver who was driving a motor vehicle on the road leading to the tunnel. The observer pre-adapted a bright, uniform visual field having a luminance of 4000 cd/m² for 15 seconds. In this condition “the outside of the tunnel” and “the inside of the tunnel” of the simulator were-brightened. After 15 sec. of pre-adaptation, the simulated tunnel entrance began to be extended, and the lighting of only “the inside of the tunnel” was suddenly switched off, although luminance to be tested was kept in the tunnel. Eight differnt signals were sent to the observer, they were formed by different combinations of exposure position of the three rectangular objects with an apparent size of 15×15 cm² as viewed at a distance of 100 m and luminance contrast of 25 % with regard to the bac-kground, the objects being barker than the background. The observer perceives the “position” of the objects exposed, and operated push button switches corresponding to the positions of the object perceived.
The time of the object exposed was 0.5 sec. in one observation. All the combinations of the positions of the object exposed and the luminance in the tunnel tested were presented to the observer in a random order.
Two different observer groups took part in the experiments, one for long tunnels and the other for various lengths of short tunnels.

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