Different software programs use different methodologies for calculating the UGR. Keep this in mind when comparing different lighting plans. This value ultimately provides nothing more than an expectation of the expected light comfort of a lighting design. Therefore, always assume the properties of the light source and use it as a tool to get a good idea of lighting design and eye comfort.
Everyone knows it, a lamp with such sharp light that you turn your head away or squeeze your eyes. Sometimes even hurting the eyes. After looking into the light source, you sometimes see spots in front of the eyes. This is due to the high light intensity of the light source. Such high intensity causes irritation, glare, headaches and eye fatigue.
UGR value in this acronym stands for Unified Glare Rating. Freely translated into Dutch: the Uniform Glare Value. It works on the basis of a point system from 5 to 40
[1]
. Where most test results typically have an outcome in the 10 to 31 point range
[2]
. A score of 10 means comfortable light and a score of 31 means absolutely intolerable light. The UGR value always comes about within a lighting plan. Stand-alone lamps with a UGR value basically say nothing without a clear description of the chosen parameters. Thus, UGR value is not a product property.
The photo-receptors (rods and cones) that provide the stimulus to the brain -so people can see- are overexcited. You temporarily lose sight of that particular spot because of the destruction of the photo-pigment by the bombardment of photons. The photo-pigment is rapidly regenerated and during this regeneration process, you will see a dark spot in this particular area of your vision.
It usually involves a relatively small part of the retina where this phenomenon occurs. But consider that a light source of sufficient intensity can burn a hole in the retina. Therefore, never look into strong light sources.
The method for establishing the UGR value is described in: CIE 117-1995, CIE 190-2010 and CIE 232-2019. Basically, three different values are calculated:
Lb= the background luminance, which means the luminance excluding light measured directly from the light source. The total contains factors for each light source within the field of view.
Ls: the average luminosity of the light source in the direction of the observer. (brightness/illuminated area)
ω: the fixed angle from the observer’s eye (illuminated area / viewing distance^2)
p: Guth Position Index (more visual discomfort when closer to the light source)
To make the UGR-appl formula easier to apply, a standardization was chosen. The chosen position, viewing direction, lamp distribution and reflection values are standardized in the tabular UGR value calculation. This makes it easier to compare different lamp installations.
LED lighting has many optical capabilities where different non-uniform beam angles can be achieved. This gives lighting designers many additional opportunities to truly customize lighting plans. The CIE 117 UGR methodology is not suitable for this purpose. Also, in light sources that use different light paths, so that the lamp itself is not uniformly illuminated, give an unrepresentative value of the CIE 117 UGR methodology. In many cases, the UGR is displayed more favorably than it actually is. Because of this, comparison with other uniform light sources is always an apples-to-apples comparison.
The center of the luminous surface of the lamp serves as the measurement point. With this, higher representativeness is obtained.
Overall, the UGR value says a lot in a lighting design, even with LED lighting. Where the UGR value partly lacks representation is when using COB (chips on board). These are enormously powerful LEDs that are usually hidden deep within a fixture. The COB is often used in conjunction with a reflector. This combination provides a well-controlled light pattern and reduces angles from which direct viewing is possible.
However, the direct viewing angle is so blinding that, if the lamp is positioned incorrectly, there is always uncomfortable glare for the user of the room. Be it from direct radiation from the light source, or be it from a reflective surface.
Light sources that have different light-emitting surfaces are also subject to representativeness deviation. Blind application of UGR-Appl and thus making a mutual comparison between different light sources and lighting designs is impossible. The lighting designer should consider all factors in his design and the lighting designer should be aware of the various shortcomings regarding the chosen light sources, lighting design and the chosen UGR methodology.
