A traditional efficacy measure for lamps (light bulbs) has been lumens per watt. Recent studies suggest consideration of an added measure for determining efficacy as it relates to human visual acuity for night vision. The Canadian government and ASSIST scientists have determined dual lumen parameters should be calculated to determine visual acuity, Photopic Lumens and Scotopic Lumens.

The eye contains two light receptors, cones and rods. Each has unique spectral sensitivity. At night both are active. For straight-ahead viewing to see objects in your direct line of motion, only cones are active. Detection of objects not in your direct line of motion is done by both rods and cones. Objects are best discerned in central vision. We know not everyone has the same peripheral vision, readers will recall the great Magic Johnson and Larry Bird.

To assure a lighting system meets standards for object detection/recognition, it would seem that the recommended light levels should be given in both Photopic and Scotopic photometric quantities, which vary by light source.


The graph below shows the luminous efficiency functions of the eye at various wavelengths of light, normalized at 683 lumens at 555 nanometers. The cone achromatic channel for photopic vision has a maximum sensitivity at the green color of 555 nm. Rods for scotopic vision have maximum sensitivity illustrated by the peak of the Scotopic sensitivity function at the blue-green color of 507 nm. The differing peaks of the two sensitivity functions are due to the normalization of 683 lumens at 555 nm and not innate differences in rod or cone light sensitivity.


The Scotopic and Photopic lumens seen by the eye associated with a lamp with a given spectral density should be determined by weighting that spectral density with the Scotopic and Photopic spectral sensitivity. The integrated result over the range of visible wavelengths of the weighted spectral density yields the Scotopic and Photopic lumens. The ratio of Scotopic to Photopic light for a particular lamp (S/P) is independent of absolute light level to the extent that the spectral density remains constant.

Comparison of a tested high pressure sodium lamp with a tested LED lamp array shows LED to have a significantly higher S/P values, about 2.65 times greater.

090420-100w-hps-scotopic-v-photopic-lumens090420-88w-led-scotopic-v-photopicThe Photopic lumen output of both sources, 100W HPS and 88 input W LED, is equal. The Scotopic lumen output advantage of the LED is 11,179 to 4,226 Scotopic lumens for the 100 W HPS. At a lamp output 0f 6650 Photopic lumens, the LED has an efficacy of 70 to 94 lumens per input watt (plug load) and the 100W HPS has an efficacy of 48.5 lumens per input watt (plug load), including a standard magnetic ballast with a power supply efficiency of 73%. Though efficiency of the HPS could be improved with a more efficient but more costly and less robust electronic ballast, the  advantage related to the S/P ratio cannot be matched by the 100W HPS source.  In applications such as walkway or roadway lighting where both straight-ahead and peripheral vision are required, the LED array has distinct electrical energy and visual acuity superiority.

Photo of red LED sculpted rear lights courtesy Porsche USA.


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