May 22, 2017

Sometimes ‘glare’ is actually ‘useful lumens spelled differently

By Paul Mitchell and Kellie Koedel

While BUG ratings are a significant step forward from the old cutoff classifications, one area that warrants discussion for possible improvement is the method for defining and calculating glare.

The Glare rating is currently derived by totaling all lumens emitted in the range of 60 to 90 deg above nadir. By identifying them as “glare,” we are inherently saying that they are unwanted, which is simply not the case. Glare was originally relegated to lumens in the range of 80 to 90 deg. By tripling that range from a 10-deg to a 30-deg window, important problems arise.

For one, pole spacing is reduced where low G ratings are mandated. Adding poles to a new project often brings unnecessary cost. Remember that every additional pole location requires trenching, pipe, wire, etc., along with the material and installation cost associated with the actual poles and fixtures. With that in mind, adding more poles to a project is often not a realistic financial option, and consequently, uniformity suffers. This issue is exacerbated when poles are shorter. For an existing installation that has high-angle lumens, uniformity is an even more difficult challenge.

Another shortcoming is the reduction in vertical illuminance, which is important for identification purposes and where safety is a concern. Whether the site in question is a park pathway, retail center, campus walkway or downtown area, people want the ability to identify other people coming toward them. Vertical illuminance is essential to facial recognition and seeing facial expressions.

Figure 1
Figure 1

One of the improvements that BUG offers over the cutoff classifications is that BUG considers a luminaire’s forward direction and target separately from the back area. Likewise, it is worth considering that the light contributing to the Glare rating could be divided into four areas: the forward direction, the back side and lateral projection to each side, which is perhaps the most relevant with regard to spacing (Figure 1).

The determination of a G rating for a project should consider the mounting height, as glare is a more important issue where lower mounting heights bring the light source closer to eye level for both pedestrians and drivers. That mounting height also changes the angle of observation, potentially causing glare issues. Consider also that we do not perceive the fixture from 360 deg around all at once, or from more than one position at a time. Therefore, the current description of glare, combining lumens from all angles and the full 360-deg range, does not reflect the observer’s true experience.

Some might say that a higher G rating can be allowed at higher mounting heights, but what we’re suggesting is that those useful and often necessary lumens should not be classified as glare at all. Rather, BUG ratings might benefit from redefining a portion of these lumens via an additional metric, such as “intensity.” We need to find a way to measure and try to limit the direct luminous intensity of a light source, without penalizing necessary vertical illuminance by unfairly categorizing it as glare.

Perhaps we should remove the “G” altogether and come up with a different name without such a negative connotation attached to it. Glare is defined as “harsh, uncomfortably bright light.” That suggests an issue more related to intensity, which is defined as “the magnitude of a quantity per unit.” To suggest that all lumens emitted between 60 and 90 deg qualify as harsh and uncomfortably bright light is simply wrong.

LEDs have no doubt changed the perception of a light source. The ability to pack so much power into such a concentrated space can definitely cause issues with glare. Perhaps the suggested Intensity rating could consider the highest magnitude candela vector in that 60-to 90-deg range but also assign it a value at that angle, keeping in mind the observer’s position. For simplicity, keep 10-deg window ranges. So, Intensity would still be considered from 60 to 90 deg. Separate it into Intensity low, medium and high, representing 60 to 70, 70 to 80, and 80 to 90 deg, respectively. As an example, if the highest candela value is in the 78-deg direction, IM3 might represent a Level-3 Intensity in the 70- to 80-deg range. The Intensity rating should also have a component for the horizontal plane (e.g., region or quadrant). Rather than sum up all lumens 360 deg around the fixture at 78 deg vertical, only allow the Intensity rating to be calculated in 90-deg horizontal increments. No observer will ever be affected by more than one quadrant at a time.

To facilitate this change, photometric software would require an update. Whereas the G rating is currently derived from lumens in the four areas of BVH, BH, FVH and FH, those angles would now be divided into 12 areas. For each 10-deg zone (60 to 70, 70 to 80, and 80 to 90) you would have B (Back), F (Forward) and L (Lateral) zones, left and right. All 12 could still be combined into one letter (“G” or “I”), but the specifier could then view the breakdown of what’s actually being emitted toward a single vantage point, providing a more realistic expectation of real world visual comfort or discomfort.

There is a threshold at which illumination becomes detrimental, and ultimately becomes glare. Redefining the glare part of the BUG rating to specifically address that will allow better practices to be established. Specifiers will become familiar with which angles are truly detrimental for specific mounting heights. End users won’t be scared by something that might actually be beneficial to them. We do not have the perfect solution to offer, but we invite our industry and IES Committees to tackle this issue.

Paul Mitchell is the education director and VP of sales for Western North America for Sternberg Lighting.
Kellie Koedel is responsible for specification sales for the MH Companies.