Figure 6-12. - Luminaire arrangement and spacing.

A staggered arrangement generates better uniformity and possibly greater spacing than a one-side arrangement. That 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 definitely should be considered. That

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 luminaries. On a residential road, it may be permissible to extend the spacing so that the light beams barely meet (fig. 6-13). 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.

MANUFACTURER'S LITERATURE

The performance specifications of each model, type, and size of luminaire are provided with the fixture or obtained from the manufacturer's ordering information. A working knowledge of this information will assist you in selecting and installing the correct luminaire to accomplish the job. Manufacturers provide technical literature for every luminaire they make. This literature includes utilization and isofoot candle curves. These curves are important in calculating the lighting intensity of a particular lumunaire. Figure 6-14 is a sample of manufacturer's literature for a 250- or 400-watt light fixture.

Utilization Curve

The utilization curve (fig. 6-14A), a measure of luminaire efficiency, shows the amount of light that falls on the roadway and adjacent areas. The amount of light that is usable or actually falls on the area to be lighted is plotted as a percentage of the total light generated in the luminaire for various ratios of transverse distance (across the street from the luminaire on both the house side and street side) to the mounting height (fig. 6-15). The coefficient of