Luminaire specifications generally include a luminance limiting diagram (Söllner) pursuant to EN 12464 or CIE 29/2. This shows the directly visible mean luminance as calculated from the luminous intensity of the polar curve and the light emission area of the luminaire. The diagrams are normally plotted for 45° to 85° for the planes C0/C180 and C90/C270.

Luminance limiting diagram

The standards prescribe specific classes of glare control based on service Illuminance for certain types of interior or activity (EN 12464, Part 2 and CIE 29/2). The luminance limiting curves of the diagrams represent the dividing line between the various classes. Class A applies in general where particularly high requirements have to be met with regard to direct glare control, especially where the main line of vision is not downward (VDU workstations, classrooms). Class 1 is required for general office work, whereas Class 3 is adequate in those cases where there is no need for enhanced glare control.

Glare assessment

Glare is only measured along and transverse to the axis of the luminaire (the planes C0/C180 and C90/C270). Two types of diagrams (A and B) - showing a slight relative displacement of the limiting curves - are employed, depending on the type of luminaireand luminaire orientation. In addition to the above glare classification system, a minimum cut-off angle is also required for open luminaires and luminaires with clear, non-textured covers. The angle varies between 0° and 30° depending on the type of light source, luminance and glare class. Where the cut-off angle requirement is not met, the luminance of the light source itself is used as the basis for glare control calculations.

Söllner limiting curve method

The luminance limiting method is based on research carried out on opal and prismatic diffusers by Söllner in the Sixties. For more advanced luminaires, e.g. with narrow-beam light distribution patterns or a high proportion of indirect light, it is often not possible to provide more than general guidance. Moreover, the method only takes account of individual luminaires while completely ignoring the interior and thus the overall impression created by the lighting system.