Imagine driving along a dark road and a car approaches in the opposite lane. The headlights’ brightness contrasts with the surrounding darkness, making them easily visible, but uncomfortable to look at directly. The driver switches to high beams and the sudden increase in brightness is nearly blinding.

Glare is a visual sensation produced when a light source is too bright or the brightness contrast is too high. Direct glare is caused by directly viewing a light source, such as, in this example, reflected glare caused by light reflecting off a surface.

The effects can include visual discomfort or, in extreme cases, disability. One type of discomfort glare is overhead, which is distinctive because the glare source is not in the field of view. It is produced by direct, downward light scattering at the eyebrow and the eye’s cornea and light reflected by facial features.

Efficiency versus comfort

Controlling brightness is at the heart of good lighting practice. Many manufacturers design sophisticated luminaire optics to control it, contending with the trade-off between luminous efficiency and visual comfort. To properly control brightness in an application, it must be predictable, and for that we need standardized metrics.

In the 1960s, visual comfort probability (VCP) was introduced. It expressed the percentage of people likely to find a certain lighting scene visually comfortable in terms of discomfort glare. A related metric is discomfort glare rating, a number expressing the capacity for light sources in a given visual environment to produce discomfort glare.

Over time, VCP fell out of favor as the International Commission on Illumination’s unified glare rating (UGR) gained prominence. Developed for interior fluorescent lighting, it has since evolved to accommodate LED. Today, the DesignLights Consortium (DLC), LEED, WELL, ISO and EN 12464-1 rely on or require UGR as a criterion.

While it’s good to see prominent organizations address glare as a lighting quality measure, there’s a problem, said Jeremy Yon, industry relations leader at Current Lighting. He represents the manufacturing community on this issue as chair of the National Electrical Manufacturers Association’s Luminaire Section UGR Task Force, which recently published LS 20001-2021, "White Paper on Unified Glare Rating (UGR)" (available to download at www.nema.org).

"The broad aim of UGR is to provide a better way to evaluate glare in indoor settings," Yon said. "However, it is not a simple matter, because UGR depends strongly on the specific application, not only in regards to the luminaires, but also in their layout, the shape of the room, and the reflectances of the surfaces in the room. Additionally, the location of people in the room and the tasks conducted can affect the experience of glare."

Using UGR

The problem therefore lies in how UGR is used. It can be evaluated as three distinct metrics: UGR-Lum (for a particular luminaire under specific conditions), UGR-Appl (averaged for the application) and UGR-Point (UGR at a specific point, which should be properly averaged for a room).

"Luminaire UGR is the most common use of the term today and is the most problematic use," Yon said. "This metric evaluates a single luminaire type in a specific evaluation space without taking into consideration the space itself. It therefore cannot appropriately predict the likelihood of glare in any space other than that single specific condition."

Risks of relying solely on UGR-Lum include choosing a product that has the "right number" but produces higher glare once it’s installed, or a product without the right number might be discounted while actually being suitable for the project. Also, a designer might choose a product that makes the space feel dim, similar to the deep-parabolic troffers of the 1980s.

The lesson for designers is that UGR is a guide, not an answer, and a tool for design rather than design itself. In short, a designer may check a box for UGR-Lum to satisfy DLC, LEED or WELL, but they have not necessarily addressed the potential for glare by doing so. UGR-Appl is the best overall guide, Yon said, but it is complex to calculate. In the end, designer experience and interest may matter most.

If an interior application has potential for glare, consider the source (photometric report, visible lamps, brightness contrast against surroundings), task (location, orientation, brightness contrast) and their relationship with each other and the user within the particular room characteristics. Adjusting one or a combination of these can address common glare problems. When possible, mock-ups can help.

"UGR is an imperfect predictor of glare, but Application UGR is the best method we have today," Yon said. "If UGR’s limitations are not understood, such as using Luminaire UGR as a singular number inaccurately across a variety of applications, it undermines the purpose of the metric and can result in poor lighting.