Mar 4, 2021

The luminance of our surroundings is key to mitigating glare

By Willard Warren

The IES Handbook defines “discomfort glare” as a sensation of annoyance and “disabling glare” as painful when high luminance (brightness) light sources are in our field of view. While at work, for example—where we move our bodies and eyes quite a bit, but our light sources are generally fixed—we measure the “glare” by four factors: (1) the luminance of the source, (2) its size, (3) its location in the field of view and (4) the viewer’s light adaptation level.

The fourth factor is most often ignored, but it has the unique ability to mitigate glare. For instance, when you’re driving at night and the cars coming at you in the opposite lane have their “brights” on, you’re going to suffer disabling glare, but when driving in daylight, your light adaptation levels are much higher than at night, and those oncoming headlights don’t bother you at all. When you’re indoors, bright light sources can be discomforting, but a room with light finishes and abundant general lighting mitigates that glare.

Incorporating daylight into designs is one way to provide abundant general lighting. Jim Benya of Benya Burnett Consultancy in Davis, CA, reports that his most recent designs provide sufficient daylight for 80% of the time, while room lights are only on 20% of the time. In addition to mitigating glare, this also reduces the lighting load density of those rooms to around 0.3 watts per sq ft, which is very energy efficient.

If you are now working from home, consider locating your desk near a window to allow daylight in to keep your light adaptation level high, maintain your circadian rhythm and improve your color discrimination. In commercial offices, on the other hand, most luminaires use lenses to refract high-angle rays of light downward, or use parabolic or dark louvers to cut off those rays, but we should be directing as much light as possible upward to brighten the walls and ceilings in the rooms we work in.

At night when we’re outdoors, we should illuminate sidewalks, buildings and store fronts with wide-angle streetlights to increase our light adaptation levels and improve visual acuity. For additional safety, we should follow London’s example and require cameras on all buildings. Indoors, we should not sleep in totally dark rooms, which have been proven to be dangerous for seniors.

AN IES COMMITTEE is now studying the adoption of an indoor glare evaluation system for our lighting code, modeled after Europe’s CIE “United Glare Rating” (UGR) system. The article “UGR on the Rise” by James K. Eads, published in the October issue of LD+A, included a table of the UGR values compared with the Hopkins Rating Scale, where in seven steps, the UGR goes from 10 (imperceptible) to 28 (uncomfortable).

The luminance of our surroundings must be kept high to mitigate glare, because by increasing the luminosity of the background we can lower the UGR and move up on the Hopkins Rating Scale. That could be considered “UGR-LITE.”

My college engineering/economics professor lectured that we must always consider alternatives in our designs. His favorite recommendation was: “When your city’s bridges become too low to allow the new, taller ships to clear under them—instead of replacing the bridges, consider lowering the riverbeds.”

The Brits are doing that now on the Thames River to eliminate flooding in basements adjacent to the riverbanks. We now have the opportunity to mitigate glare, both indoors and out, by flooding the spaces we spend time in with surround light, to raise our light adaptation and comfort levels.


Willard Warren

Willard Warren

Willard L. Warren, PE, LC, Fellow IES, DSA, is principal of Willard L. Warren... More info »