Figure 6-13. - Pavement brightness.

utilization for any specific situation is obtained from this curve. The utilization curve will determine the amount of light that actually strikes the roadway surface. This percentage of light has an impact on the spacing distance of the luminaries.

Isofootcandle Curve

The isofootcandle diagram (fig. 6-14B) shows the distribution of illumination on the road surface in the vicinity of the luminaire.

The lines on this diagram connect all points having equal illumination, much as the contour lines on a topographical map indicate all points having the same elevation. Thus, at any point on the diagram (or roadway), we know the magnitude and direction of the illumination with respect to nearby points. To make this data more universal, you are given both the top horizontal and left vertical axes in terms of mounting-height ratios.

It is sometimes convenient for you to replot the isofootcandle data to the same scale as that used on a drawing containing a lighting layout. By superimposing this diagram, you can study the distribution of light. Under the unity correction factor in the mounting-height table (fig. 6-14B), one can find the mounting height for which the data are calculated. The numbers beside each line represent the initial