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Calculation of the glare index

The degree of discomfort glare for indoor installations can be estimated by calculating the glare index according to the UGR method.

Following the introduction of the latest EN 12464-1, UGR will replace the previous European calculation method (NB-method). The demands on the glare index documented in EN 12464-1 refer to the highest average value for the lighting. The UGR-method (Unified Glare Rating) is described in CIE’s publication no 117-1995.

The UGR glare index for discomfort glare is ranked on a scale, which in practice runs from 13 to 28 where the higher the glare index the higher the level of glare. The smallest change in the glare index denoting a significant difference is 3.

The glare number should be calculated according to the ‘table method’ to check whether the recommended glare index in the standard’s tables is met by a specific lighting installation. (The method is described in CIE’s publication no 117-1995). However, this method assumes that the luminaires in the room are positioned symmetrically and feature symmetrical light distribution across and along the luminaire.

Luminaire manufacturers provide information/table data as a part of the luminaire’s photometric data to calculate the lighting installation’s glare index.