Photopic Lumens and Scotopic Lumens
What are they and why does it matter to me? By understanding how the human eye registers light, an energy auditor can design an energy efficient project using less power and still achieve sufficient light levels. Thus meaning, more money back in your pocket at the end of a lighting retrofit.
Photopic lumens are the actual lumens read by any measuring device such as a foot-candle meter. Scotopic lumens (or Pupil lumens) is a measurement based on how the human eye registers light. The more “bright white” the light is, the brighter the light will appear to the human eye. A real world example would be this:
Late at night you are driving down the street and you notice two parking lots that have two different colors of light. One is more “bright white” and the other is “orange” in color, the “bright white” light source is called Metal Halide and the “orange” light source is called High Pressure Sodium (HPS). It would be safe to assume that both parking lots are using 400w lamps in each pole head, the 400w Metal Halide’s initial lumen output is 36,000 lumens and the 400w HPS’s initial lumen output is 50,000 lumens. Even though both parking lots have the same number of lights in them, the Metal Halide parking lot appears brighter than the HPS parking lot.
“Why is this true?” you may ask. Scotopic lumens or Pupil lumens is calculated with a correction factor applied to the initial lumen output. The more “bright white” the light source, the higher the correction factor is. The Metal Halide’s correction factor is 1.49, so the actual Pupil lumens for the 400w Metal Halide lamp is: 36,000 * 1.49 = 53,640 Pupil lumens. The HPS’s correction factor is .68, so the actual Pupil lumens for the 400w HPS lamp is: 50,000 * .68 = 34,000 Pupil lumens.
Pupil lumens is now a major deciding factor in which is the better light source for a given application. This is one of the main principles in designing the most energy efficient lighting package for our clients. After all, shouldn’t we be worried about the visible light produced rather than the light read by a foot-candle meter.