Alliance for Lighting Information

Luminances in Outdoor Lighting

by David M. Keith

The following diagram shows the mesopic luminance range, over a logarithmic scale, from 0.001 past 3.4 (to 10) cd/m2. Shown in the diagrams are luminances associated with 7%, 18% and 50% reflective surfaces for three different illumiannce leveles. The lowest line corresponds to moonlight, for 0.1 lux illuminance. The middle line corresponds to luminances for the three different reflectances at the very darkest point (the minimum at maintained conditions) of a roadway with the lowest value criteria that meet the American National Standard Practice for Roadway Lighting, IESNA RP-8-00, criteria for roadway lighting (Table3, classification of local low). The highest line corresponds to luminances at the brightest point (the maximum at maintained conditions) of a roadway with the highest value criteria that meet RP-8-00 (major high).

The next diagram shows the same mesopic luminance range, over a logarithmic scale, and again the lowest line corresponds to moonlight, for 0.1 lux illuminance. The middle line corresponds to luminances for the three different reflectances at the very darkest point of a parking area with the lowest value criteria that meet the IESNA RP-20-98 criteria for parking lot lighting (basic). The highest line corresponds to luminances at the brightest point of a parking area with the highest value criteria that meet RP-20-98 (enhanced).

The last diagram shows the same mesopic luminance range, over a logarithmic scale, with the same line corresponding to moonlight. The middle line corresponds to luminances for the three different reflectances at the very darkest point of a walkway with the lowest value criteria that meet the IESNA DG-5-94 criteria for walkway lighting (residential). The highest line corresponds to luminances at the brightest point of a roadway with the highest value criteria that meet DG-5-94 (commercial).

Clearly the recommended design luminances for roadways and parking lots extend from the middle of the mesopic range up into the photopic range. Certainly with the contribution from headlights, roadway and parking lot design luminances are in the "meso-photopic region" where foveal and color perception is present, if not up into the photopic range. There is additional discussion of the mesopic visual range and outdoor lighting criteria here.

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