New, stannous halide-mercury lamps have been developed based on a new principle. Continuous spectrum caused by the molecular emission of stannous bromide and stannous iodide is used in the discharge tube, and it constisutes more than 80per cent of the light output. The CIE general color rendering index Ra of the lamp is 94, which is comparable with the value for super de luxe fluorescent Iamps. The color temperature is 5300°K±200°K and the chromaticity lies on the Iocus of natural daylight. The efficiency is about 55lm/W for the lamp rated 400W. Lamps rated 250W and 125W are also available.
Another characteristic feature of the lamp is small fluctuation of light output with a. c. operation. The flicker ratio (Imax-min) / (Imax+Imin) of the Tamp is 18%, Which is about one fourth as compared with the conventional mercury lamps.

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The lighting inside vehicular tunnels should provide the drivers passing through them with a safe and agreeable visual environment. In the quest of lighting conditions that will permit the drivers to maintain an adequate visual distance in front of their vehicles to ensure safe traffic, the authors carried out a series of tests and experiments, model tests and field driving tests in actual tunnels.
From the results of these experiments, the authors have derived the following conclusions in the interest of the best lighting inside a tunnel and around its entrance. With regard to the lighting inside tunnels in the presence of exhaust fumes, sodium lamps have ensured a far better visibility than fl uorescent lamps and other types of lamps, for the same degree of road luminance.
There is a fixed relationship between the average road luminance inside the tunnel and the car speed. Hence, if the tunnel is designed for a car speed of 100 km/h, the average road luminance should be 90 cd/m², and in the same way a car speed of 60 km/h calls for a luminance of 25 cd/m². Moreover the road luminance varies depending on the type of road surface and each car speed has its own permissible transmittance of exhaust fumes.

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