SELECTION OF LUMINAIRE

Luminaires are designed to provide lighting to fit many conditions. For street and area lighting, five basic patterns are available, as shown

Figure 1-9.\Light distribution patterns for roadway lighting.

in figure 1-9. While many luminaires can be adjusted to produce more than one pattern, no luminaire is suitable for all patterns. Care must be used, especially in repair and replacement, to install the proper luminaire for the desired pattern, as specified in the manufacturer's literature. Even when the proper luminaire is installed, care must be used to ensure that all adjustments have been properly made to produce the desired results.

1. Type I (fig. 1-9a) is intended for narrow roadways with a width about equal to lamp mounting height. The lamp should be near the center of the street. A variation of this (fig. 1-9b) is suitable for intersections of two such roadways with the lamp at the approximate center.

2. Type II (fig. 1-9c) produces more spread than does Type I. It is intended for roadways with a width of about 1.6 times the lamp mounting height, with the lamp located near one side. A variation (fig. 1-9d) is suitable for intersections of two such roadways, with the lamp not near the center of the intersection.

3. Type III (fig. 1-9e) is intended for luminaires located near the side of the roadway with a width of not over 2.7 times the mounting height.

4. Type IV (fig. 1-9f) is intended for side-ofroad mounting on a roadway with a width of up to 3.7 times the mounting height.

5. Type V (fig. 1-9g) has circular distribution and is suitable for area lighting and wide roadway intersections. Types III and IV can be staggered on opposite sides of the roadway for better uniformity in lighting level or for use on wider roadways.

MOUNTING HEIGHT AND SPACING

There are two criteria for determining a preferred luminaire mounting height: the desirability of minimizing direct glare from the luminaire and the need for a reasonably uniform distribution of illumination on the street surface. The higher the luminaire is mounted, the farther it is above the normal line of vision and the less glare it creates. Greater mounting heights may often be preferable, but heights less than 20 feet cannot be considered good practice.

You must be somewhat familiar with the terminology relating to how fixtures are located down a roadway. Figure 1-10 shows these

Figure 1-10.\Luminaire arrangement and spacing.

relationships graphically. The following information will be useful when determining the most appropriate mounting arrangements:

The transverse direction is defined as back and forth across the width of the road, and the longitudinal direction is defined as up and down the length of the road.

Modern roadway fixtures are designed to be mounted in the vicinity of one of the curbs of the road. The overhang is defined as the dimension between the curb behind the fixture and a point directly beneath the fixture.

A luminaire overhang should not exceed 25 percent of the mounting height.

No attempt should be made to light a roadway that is more than twice the width of the fixture mounting height. A roadway luminaire produces a beam in both longitudinal directions and is limited in its ability to light across the street.

There are three ways that a luminaire may be positioned longitudinally down the roadway. (See fig. 1-10.) Note that the spacing is always the dimension from one fixture to the next fixture down the street regardless of which side of the street that fixture is on.

A staggered arrangement generates better uniformity and possibly greater spacing than a one-side arrangement. This is particularly true when the width of the road becomes significantly greater than the mounting height. When the width of the road starts approaching two mounting heights, an opposite arrangement should definitely be considered. This would, in effect, extend the two-mounting-height width limitation out to four mounting heights.

The classification of a road and the corresponding illumination levels desired influences the spacing between luminaires. On a residential road it may be permissible to extend the spacing so that the light beams barely meet (fig. 1-11). For traffic on business roadways where uniformity of illumination is more important, it may be desirable to narrow the spacing to provide 50- to 100-percent overlap.

Figure 1-11.\Pavement brightness.