LED Glare – the various causes potential ways to minimise it
‘Glare’ in the lighting industry can be quantified using the UGR (Unified Glare Rating) index, and this is becoming an increasingly contentious topic within the lighting industry. End-users everywhere are after ‘low-glare’ LED lighting
, especially in the office environment where, according to EN 12464-1 (the European standard for lighting of workplaces), the recommendation is for UGR<19. But what is ‘Glare’, how is the UGR-value calculation, and to what extent can LED lighting manufacturers actually control the UGR-value of a fixture?
Let’s face it, LED lighting is often blamed for excessive glare, and because they are point-sources of light, this is mostly true. But a recent Living Lighting Workshop I attended in London has highlighted for me that there are far more issues related to this topic than I had originally anticipated. I (shamelessly) refer to the presentation from Oliver Buchan for some of the facts in this article.
A complete description of Glare is: “the experience of reduction in visual ability due to a high dynamic range of luminance within the visual field”. That’s a bit of a mouthful. Simplified it means: “the huge difference between light and dark in your line of sight is causing you discomfort”. What’s interesting is the phrase: “between light and dark”. If the LED fixture itself is the “light” part, then what is the “dark” part referring to? It’s the background and completely unrelated to the LED fixture at all. So, hang on one second…are we saying that Glare is a result of both the LED lighting and the environment it’s installed in!? Yes, we are. And that’s what makes the subject of Glare and UGR values so controversial in the lighting industry. You can supply any fixture you like whether it’s marketed as an “ultra-low-glare, make-you-feel-good, super-dooper LED fixture”, but if it’s installed in the wrong environment then it may still end up having a UGR>19.
Don’t take my word for it – check the math:
Ok, so to keep this simple, you only need to know that the “L” on the top refers to the amount of light from the LED fixture. Because it’s on the top of the equation it means that the more light emitted by the fixture then the potential for a higher UGR-value right? But, what about the “Lb” below the line. This refers to the amount of light from the background (i.e. everything except the LED fixture). And because it’s below the line, if we increase this value then there is the potential for the UGR value to be lower. So, the ‘Glare’ associated with an LED fixture is also affected by the environment in which it’s installed.
There are lighting calculations that can be done that automatically calculate UGR values for certain scenarios and settings. One such piece of software we regularly use is Dialux. In this software you can control the reflectance of the walls, ceiling and floor; as well as the room dimensions, number of LED fixtures installed and the lumen output of the fixtures themselves. What’s interesting is that by adjusting one of these variables at a time, one can investigate the effect of this variable on the UGR value of the installation. Some variables affect the UGR values more than others; for e.g. adjusting the ceiling height has little affect, whereas adjusting the floor reflectance has a significant affect. Increasing the floor reflectance from 20% to 90% drops the UGR value from 19 down to 12! This is because more light will be reflected from the floor back onto the ceiling resulting in less of a difference in illuminance between the LED fixture and background ceiling. Adding glare control optics to the LED fixture is also a practical and obvious solution and can improve values from UGR 19 down to UGR 16. As one can’t just adjust the reflectance values of surfaces to suit the requirements of Glare, nor control the environment in which these LED fixtures are installed, the most practical way for an LED Manufacturer to control Glare is still by the design of the optics on the fixture itself.
So to summarise: (1) UGR is dependent on the fixture AND surroundings in which it’s installed, (2) You CANNOT define a specific UGR value on a LED fixture datasheet without outlining all the other variables associated with its installation, (3) Glare control optics are still the most practical means of controlling the Glare under any number of different environmental variables, (4) Glare, at its core, is still a subjective experience, but it’s still the best we have…