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distributes 60-90% down, with the remainder upward. Designing the lighting system should incorporate the different light distributions of different fixtures to maximize comfort and visual quality.

Fixture mounting height and light distribution are presented together since they are interactive. HID systems are preferred for high mounting heights since the lamps are physically small, and reflectors can direct light downward with a high degree of control. Fluorescent lamps are physically long and diffuse sources, with less ability to control light at high mounting heights. Thus fluorescent systems are better for low mounting heights and/or areas that require diffuse light with minimal shadows.

Generally, "high-bay" HID fixtures are designed for mounting heights greater than 20 feet high. "High-bay" fixtures usually have reflectors and focus most of their light downward. "Low-bay" fixtures are designed for mounting heights less than 20 feet and use lenses to direct more light horizontally.

HID sources are potential sources of direct glare since they produce large quantities of light from physically small lamps. The probability of excessive direct glare may be minimized by mounting fixtures at sufficient heights. Table 13.6 shows the minimum mounting height recommended for different types of HID systems.

13.2.3.4 Exit Signs

Recent advances in exit sign systems have created

Table 13.6 Minimum mounting heights for HIDs

Lamp Type

feet above ground

400 W Metal Halide

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