Photopic Lumens vs. Scotopic Lumens & Usable Light

In lighting design there are two distinct kinds of lumen output produced by lamps. The first is called photopic lumens, which represent the relative sensitivity of the eye under intense lighting such as full outdoor sun. Photopic lumen output is the amount of light registered by the cones in the human eye and is measured by standard lumen and footcandle meters.

The second type of lumens are called scotopic, which represent the sensitivity of the eye under typical interior lighting conditions and cannot be measured directly with a standard light meter. Scotopic lumen output is the amount of light registered by the rods of the human eye and also controls pupil size directly effecting visual acuity for tasks.

For lighting interior spaces a correction factor may be applied to photopic lumen readings to find the usable light produced by a given light source, also called scotopic lumens. As shown below, two different light sources having similar photopic lumen readings taken by a standard light meter can have drastically different usable light outputs and apparent brightness.

The factor S/P is the ratio of scotopic luminous quantity to photopic luminous quantity for the lamp spectral power distribution. From a given photopic quantity, the analogous scotopic quantity can be found by multiplying the photopic quantity by the value of the S/P ratio. Natural daylight has an S/P ratio of close to 2.5 which means that it is highly scotopically enriched.

Click here for a definition of scotopic, photopic and mesopic vision.

Light Source Photopic Lumens/Watt S/P Ratio Scotopic Lumens/Watt
5000k Ultra II™ LED White Lite 90 2.00 180
4100k T5 Fluorescent 90 1.62 145
Clear Metal Halide 85 1.49 126
5000k Triphosphor Fluorescent 70 1.58 111
4000k Multi-Vapor Metal Halide 85 1.26 107
6500k Daylight Fluorescent 55 1.72 95
3500k Triphosphor Fluorescent 69 1.24 85
Vitalite Fluorescent 46 1.71 79
5000k 90 CRI Fluorescent 46 1.70 78
2900k Warm White Fluorescent 65 0.98 64
Low-Pressure Sodium 165 0.38 63
50watt High-Pressure Sodium 65 0.76 49
Deluxe Mercury Vapor 40 0.86 34
35watt High-Pressure Sodium 55 0.57 31
Tungsten Halogen 22 1.32 29
Standard Incandescent 15 1.26 19

What is Photopic and Scotopic Vision?

The retina, a light sensitive membrane at the back of the eye, contains millions of very tiny light receptors that convert light into electrified signals sent to the vision centers of the brain. The two major categories of light receptors (photoreceptors) are called cones and rods because of their shapes. The very central part of the retina, the fovea, contains only cones. The rest of the retina contains both rods and cones, with the number of rods dominating the cones by about ten to one.

Up until now, it's been widely accepted that cones handle day vision and rods are designed for night vision. Consequently, lighting manufacturers have utilized light meters to measure a lamp's lumen output that are calibrated by examining the eye's sensitivity to only cone activated vision in the very central part of the retina (photopic), completely ignoring the effect of rod activated vision (scotopic).

But, according to a study by Dr. Sam Berman and Dr. Don Jewett, the roles of rods and cones are not that exclusive - they actually share responsibility depending on lighting conditions. Dr.'s Berman and Jewett's experiments, sponsored by the U.S. Department of Energy, have shown that rods (scotopic) do indeed play a role in typical workplace lighting conditions. Thus, human perception of lighting conditions is not consistent with the devices we generally use to measure light output.

This and other studies lead us to the conclusion that both photopic and scotopic responses to lighting need to be evaluated when measuring light effectiveness. Ideally, this would require light meters with a calibration for conventional (photopic) illuminance as well as an addition calibration for scotopic illuminance.