Basic position: 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
The tabular method is a procedure which is followed in order to determine the UGR value of a lighting installation in a standard room. However, the designer must bear in mind that the standard room usually has very little to do with real situations.
According to the tabular method the floor has a maximum degree of reflectance of 20 %, walls of 30 % to 50 % and the ceiling of 50 % to 70 %. White walls or ceilings with a degree of reflectance of 75 to 90 %, such as frequently occur in architecture, are not taken into consideration in the tabular method. In the tabular method the observer can be positioned either across or along the luminaire axis. The tabular method does not recognize an angle of vision diagonal to the luminaire axis and is based exclusively on rectangular room geometries. This method must be applied for each individual type of luminaire if different luminaires are present in one room, since each type of luminaire has its own UGR table.
Why is the UGR value shown as a figure in the luminaire data sheet?
Though the UGR value is not in itself a property of a product, nevertheless, in the data sheets of many manufacturers details such as UGR < 19 can often be found. It is, however, not correct to deduce that this is a luminaire property. Unless the manufacturer provides further details, this figure refers to the UGR value which the luminaire would have in a reference situation with room dimensions of 4H/8H and degrees of reflectance of 20 % for the floor, 50 % for the walls and 70 % for the ceiling. In real situations this value could be lower or even higher.
Despite the restrictions of the tabular method it is of benefit in evaluating products when comparisons are desired. A luminaire with a UGR value of 16 is, in practice, less likely to cause psychological glare than a luminaire which has the same parameters but a UGR value of 25.
Therefore it is important for the designer to be aware of the limitations of the tabular method. If he wishes to determine the UGR value for a specific position of the observer with greater accuracy, we recommend calculating this with the aid of a software which will then take into account the degrees of reflectance and room geometries as they are in reality. All the luminaires present in the room will also be included in the calculation.