Jul 27, 2021

Is individual control the only way to tackle visual discomfort?

By Willard Warren

With all the bare lamp lighting in vogue today, we need a glare metric to reduce visual discomfort. We’ve adopted, and discarded, glare predictive systems including the Scissor Curve, Glare Factor, ESI (equivalent sphere illuminance), VCF (visual comfort factor) and VCP (visual comfort probability), and are now considering the CIE’s UGR system.

For reference, here’s the 10th Edition of the IES Handbook’s take on glare: “Discomfort Glare Predictive Systems give reasonable predictions of the discomfort of a group of people, but give only poor predictions of an individual’s response.” That’s like having one hand in ice water and the other in hot water—the average temperature might be acceptable, but the person would still be in pain.

We can obtain a luminaire’s luminance at any angle from the manufacturer’s photometric data—and calculate a source’s projected area and its position index—yet know nothing about the visual comfort of any occupant, who is free to move their head and shift their gaze to avoid a glare source. Maybe the only glare metric is one that mandates a maximum source brightness for luminaires and requires a minimum ambient light level in the space to mitigate the glare’s effect—plus no more dark walls and ceilings.

When the source is daylight, we could look to the example of one Zero Net Energy school in New York City that reduces daylight glare by using a window with three different horizontal window panels. The top panel is clear glass with a short reflecting shelf projecting into the room to reflect daylight deeper into the room. The middle section is an opaque solar panel that generates the building’s electricity, while the bottom panel is clear glass positioned just above the window shelf to provide a view of the weather outside.

AS OPPOSED TO AN INDIVIDUAL’S response—and when dealing with our senses, in particular we’re stuck with “consensus mandates.” To make matters worse, the biggest unknown is the tolerance in our lighting calculation. For example, the equation for calculating a room’s average illumination has the initial lamp lumens in the numerator. However, labs have reported that some companies ship lamps with up to 20% less lumens than claimed. That’s multiplied by the luminaire’s coefficient of utilization (CU), which is based upon assumed room finish reflectances, and also multiplied by an assumed maintenance factor (MF) of 0.75 or 0.80 for LEDs, by custom. The equation’s denominator has the room’s floor area, which ignores the width of the HVAC units under the windows and other built-ins.

What’s worse is the tolerance of 7-8%, plus or minus, allowed in a 120/208-V electrical system. You can get 128 volts early in the day, which can drop to 110 volts in the afternoon after heavy electric loads have been added. That alone severely reduces lamp lumen output, easily resulting in a 25% lower light level than expected.

It would be more practical to provide a dimmable task/ambient lighting system that’s controlled by the room’s occupants. At the least, that’s better than following the Lighting Library’s latest criteria of having different light levels depending on age, specifically for those over the age of 65. How do you implement that criteria—by segregating a room’s occupants by age?

The European solution is to have a low adjustable general light level for safety and to provide occupants with adjustable task lighting. In the U.S., we’ve now gone from open-plan office systems to virtual and remote communication during the pandemic—maybe the next step is to give everybody a private office so we can spend all day adjusting the lighting level.


Willard Warren

Willard Warren

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