Besides generating illuminance for fulfilling the visual task, generating luminance is also an important effect of the luminaire in the room. Today, it can be calculated and represented in good quality with commercially available design programs (see chapter "Photometric design software DIALux and Relux"). An important aspect in this is the surface luminance of the luminaire itself, which can be viewed from different angles. Particularly for direct-distribution luminaires, it determines melanopic efficiency, which must be promoted, as well as lighting glare, which must be prevented.
When rating direct glare, e.g. according to EN 12464-1 and reflected glare, e.g. according to DIN 5035-7, the average luminance of the light-emitting surface is the most important parameter. Even when the luminance on the light-emitting surface visible from various angles can exhibit significant localised variations in case of optical systems, e.g. specular louvre luminaires, the average luminance is defined as the rating parameter regarding the glare effect. Luminance L(γ) is determined using the luminaire's luminous intensity I(γ) in the relevant direction of distribution g and the projected luminous surface A(γ) perpendicular to it. A direct measurement of the luminaire's average luminance is possible using imaging instruments or, with corresponding effort, also using optical luminance measuring instruments.
The distribution of the luminaires’ average luminance is required to rate reflected glare on VDU screens according to EN 12464-1 in the planes C0 to C360 at 15° intervals, meaning all around the luminaires; namely for the distribution range of γ = 55°, 65°, 75° and 85° from the vertical according to EN 13032-1 (cf. chapter "Office", section "Luminance thresholds").
The unified glare rating method UGR standardised in EN 12464-1 does not require any luminance distribution data, since the UGR glare rating is determined by calculation, using luminous intensity and the extent of the solid angle occupied by the luminaire’s luminous surface, among other things.
The mechanisms of light’s melanopic efficiency are currently the object of numerous scientific investigations (see also chapter "The melanopic efficiency of light"). A quantifying rating method for these mechanisms has therefore not yet been conclusively defined. What is clear though is that size, location and luminance of all surfaces located in the field of vision – meaning the luminance distribution in the field of vision – as well as the light's spectral composition are crucial.