The results of these studies are combined in the following glare formula which yields a UGR value. The procedure is described in detail in the Technical Report CIE 117 (1995) "Discomfort Glare in Interior Lighting". The UGR method is applicable for most glaring light sources (particularly luminaires). A UGR rating is not possible forindirect-distribution luminaires, single-workstation luminaires and accent lighting spotlights.With asymmetric-distribution luminaires, it is important to rate the illuminance values in the observer’s direction. A UGR reference value labelling is thus not sensible for such luminaires. Psychological glare from windows has not yet been conclusively researched. Currently, there is no suitable glare rating method available for it. Psychological glare cannot be described using precise figures (meaning with some positions after the decimal dot), instead it is described using a figure range (levels) which reflects the statistically recorded glare perception of a multitude of observers. For this reason, UGR values are divided up into levels with the standardised UGR limits 13, 16, 19, 22, 25 and 28.
