What is the UGR value? When is it required and used?
The abbreviation UGR stands for »unified glare rating«. The UGR value is a dimensionless parameter which provides information about the degree of psychological glare of a lighting installation in an indoor space. UGR values are defined in steps within a scale of 10 to 30. In DIN EN 12464-1:2011-08 the steps within this scale are 13, 16, 19, 22, 25 and 28. In the final instance these steps express the statistical perception of glare experienced by a large number of observers. So UGR 19, for example, means that 65% of observers »did not really feel disturbed« by the glare. Conversely, of course, this also means that the remaining 35% felt disturbed by the glare. The lower the UGR value, the less direct glare is experienced by the observers.
The UGR value can only be calculated; it cannot however be directly determined photometrically. Where there are lighting installations with luminaires from which 65% of the light is emitted indirectly and where narrow beam spots or asymmetrically radiating luminaires are installed, then, by definition, it is not possible to indicate a UGR value.
Contrary to widespread opinion the UGR value is not really a property of a luminaire. Here we are dealing with much more than the interaction of the »brightness level« of the luminous surfaces of a luminaire in relation to the »brightness level« of the surroundings and the position and viewing angle of the observer. The average »brightness« of the light emitting surface of a luminaire is defined in this context as the average luminance of the luminaire and the »brightness« of the background or the surroundings as background luminance.
The following example taken from a real-life situation demonstrates clearly the influence which the ratio of these brightness levels to each other can have on the glare effect: Imagine that you are driving along a road at night with no street lighting. A car now comes towards you with headlights on full beam. You are blinded by the strong light and are hardly able to keep your eyes on the road. Imagine the same situation on a sunny summer’s day. The same vehicle approaches again with the headlights on full beam. Now you are far less likely to be blinded by the headlights. Yet the properties of the headlights have not changed at all. The degree of direct glare results here mainly from the contrast to the surroundings (i.e. the background luminance).
The position and the viewing angle of the observer also have to be borne in mind. For, if the luminaire is not in the field of vision of a person, then this same person cannot be affected by glare. In certain norms, depending on the field of activity, adherence to UGR thresholds is required. These can be found in the current DIN EN 12464-1:2011-08 under »5 Index of Lighting Requirements«. Since the issue here is maximum UGR thresholds, the term UGRL (Unified Glare Rating Limit) is used. In accordance with DIN EN 12464-1:2011-08 the lighting designer must provide evidence of the direct glare categorization with the aid of the tables of the CIE Unified Glare Rating method (acc. to CIE 117-1995). The purpose of the tabular method is to make it easier for the lighting designer to apply the very complex formula behind the UGR value.
Limitations of the tabular method when determining the UGR value