The glare rating of a lighting installation according to the UGR method is based on the average luminance of the luminaires in the direction of the observer’s eye and not on the lamp or illuminant luminance, which is a lot higher most of the time. For luminaires where illuminants or parts thereof are directly visible from the critical viewing angle range for direct glare, glare and thus a decrease in visual performance can occur in spite of positive glare limitation ratings. In order to limit this glare caused by increased individual illuminant luminance as well, the illuminants must be shielded through suitable measures. The degree of shielding is characterised by the shielding angle, which must comply with various minimum values (table) according to the illuminant’s luminance.
According to EN 12464-1 the shielding angle is the angle between horizontal plane and viewing direction underneath which the luminous parts of the lamps/illuminants in the luminaire are just visible. According to this definition, only viewed, luminous lamp parts are rated, not bright luminaire surfaces (fig.). This angle is also called upper shielding angle according to EN 12665 (see also chapter 1.2.1).
It must be distinguished from the cut-off angle of a luminaire between the downward vertical and the direction from which the illuminants and the surfaces of high luminance are just not visible, which is defined in EN 12665. When determining the cut-off angle according to EN 12665, surfaces of high luminance need to be considered in addition to the luminance of the illuminants. The lower cut-off angle α depicted in fig. according to EN 12665 only applies, e.g. to narrow-distribution specular reflectors which mostly feature a rather low luminance in the viewing direction. A white reflector would have high luminance which means the shielding angle would be roughly 90°.
For the illuminant luminance values specified in table, the corresponding minimum shielding angle according to the definition in EN 12464-1 must be observed for the luminous parts of the illuminants only. The values do not apply for luminaires with exclusive light emission into the upper half-space or for luminaires mounted below eye-level.
Luminance values for commercially available lamps are listed in table. The luminance of a single white LED depends on the angle (see also chapter 18.104.22.168, "LED lamps"), however, it would usually exceed the limit value of 50 kcd/m2 under flat angles of ≤ 15° as well as the limit value of 500 kcd/m2 for angles of ≤ 30°. This leads to non-permissible glare. When looking directly (under 90°) into the LED, values > 10.000 kcd/m2 are reached.
For LED luminaires, the height of the directly visible individual luminance values depends on the structure of the inserted LED modules or the luminaire. The luminaire manufacturer must ensure that the visible luminous surfaces comply with the requirements of EN 12464-1 (see table). To achieve this, it is possible to insert e.g. a diffuser or a lens to flare LED luminance (see also figure). Open individual LEDs are only used in luminaires for high mounting heights.