Physiological basis for the lighting levels in the transition zone of tunnels. Comparison of CIE and DIN with the IES recommendations.

The Illuminating Engineering Society of North America (IES) has adopted recommendations for the luminance transition in a tunnel that deviate greatly from the curves proposed by the international Commission on Illumination (CIE) and contained in the DIN Standards (German Standards). The IES has based its guidelines on practical considerations but they have neither scientific nor experimental foundation. In order to clarify the discrepancy, the physiological processes of adaptation of the eye during a change in luminance have been modeled, and their impact on the required luminance in the transition necessary to ensure visibility have been derived. Using Fry's model for the kinetics of the eye's response, and Adrian's delta L model, the course of the luminance transition has been calculated. The results are compared with the IES and the CIE standards. A comparison of the resulting curve with that suggested by the CIE reveals small differences only. In general the comparison indicates that the experimentally determined CIE curve agrees with results derived from the fundamentals of dark adaptation. However, the IES suggestion falls short. An eye requires about twice as long to adapt to the luminance transition as is considered in the IES proposal.