Uniformity of Pavement Luminance

The essence of a good outdoor lighting job lies in the uniformity of the pavement luminance. Uniformity of appearance free from annoying light and dark areas allows the eye to quickly survey and identify elements in the visual field. Equal footcandles do not produce uniformity of pavement luminance and economically are counter productive.

Traditionally the levels of illumination were stated in terms of average footcandles falling on the pavement. This has been shown to be inadequate and misleading.

For example, a spotlight directed vertically down on a stage might illuminate l0 square feet out of a total area of 200 square feet, to say, a level of 20 footcandles. By the average footcandle method of calculation, the stage would have an average illumination of l footcandle, when outside the lighted area, there is almost no measurable light. This exaggerated example demonstrates the dangerous flaw in designing with average illumination levels. Dramatic lighting belongs on stage, not in a parking lot.

All too often this flawed concept is included in lighting requirements as the ratio of average illumination to minimum illumination at any point on the pavement. The recommended average to minimum ratio for roadways is 3:1 and parking lots is 4:1. While it is possible to calculate average illumination levels, there is no practical means to field measure the results. The average to minimum calculation still allows hot spots, dark spots and streaks on the pavement. all of which reduce visibility.

Gardco Lighting, however, has always held the position that the only reliable measure of uniformity is the ratio between the maximum and minimum illumination on the pavement. Uniformity ratios of between 101 and 15:1 are considered optimum, and appear very uniform.

Illumination levels are still very important, especially the minimum level that can be tolerated in an installation. Illuminances are readily calculated and measured and should be an integral part of lighting specifications.