PHOTOMETRIC REQUIREMENTS FOR SIGNAL LAMPS USING INNOVATIVE LIGHT SOURCES: UPDATING REQUIREMENTS BASED ON LIGHTED SECTIONS

Current photometric requirements for some signal lamps in the U.S. are based on the number of "lighted sections" that comprise the lamp. Intensity requirements are somewhat higher for lamps with more sections, partially adjusting for the presumably larger lighted area of a lamp with more sections, and thereby establishing some control of luminance. The number of sections is used as a proxy for area, based on the assumptions that lamps will be constructed with optical units of a certain approximate size, each with a single incandescent bulb of relatively high candle power. However, recently there has been growing interest in, and in some cases use of, a greater variety of sources for signal lamps, including LEDs, neon tubes, miniature halogen bulbs, and distributive lighting sources. The relationship between number of lighted sections and surface area that could be assumed for lamps with relatively large incandescent bulbs cannot be extended to the new sources. Therefore the way in which photometric standards address the areas or luminances of signal lamps must be reassessed. This document reviews the existing research that is relevant to the roles of lighted sections and area in signal-lamp performance, presents the findings from a new experiment, and offers tentative recommendations about how to update the lighted-section photometric requirements for new light sources. The available evidence suggests that, although references to lighted sections should be replaced by something more broadly applicable to a variety of sources, something similar to the spirit of the current requirements should be retained (i.e., primary emphasis on control of intensity but with some recognition of area and luminance). One major alternative would be to abandon control of area and luminance entirely, regulating lamps only in terms of intensity. This would be an extremely flexible approach that would allow maximum innovation in the use of new technologies, and it would continue to insure control of the most important aspect of signal lamps (intensity). However, the current evidence does not support a complete elimination of area and luminance control. The authors recommend that the concept of a lighted section in the current regulations be replaced by a standard area, with a tentative value being 225 cm squared.