The distribution of luminance involves such factors as the reflectance, shine and color of the surfaces at the workstation and within the interior. Surface finishes should be selected to avoid excessive differences in luminance (cf. Contrast) and irritating reflections in light-colored surfaces.
With regard to the risk of direct glare from luminaires, it should be borne in mind that the line of vision when working at a VDU is normally almost horizontal, whereas it is often angled downwards during other forms of office work. For that reason, Class A glare cut-off characteristics are required for such workstations pursuant to EN 12464, Part 7.
Reflected glare is caused by bright objects or areas reflecting in smooth or glossy surfaces, including computer screens. As such reflections depend on the angle of vision or incident light, the relative location of the workstations, VDU's and luminaires is a decisive factor in avoiding the problem, which can often be solved by simply changing the angle of tilt or swivel of the screen.
In accordance with EN 12464, Part 7, the Mean Luminance of surfaces that reflect in a VDU - including furnishings and windows - must be no higher than 200 cd/m², while maximum luminance must be limited to 400 cd/m². In the case of luminaires that reflect in a VDU, the provisions of EN 12464 are as follows. Beyond the luminance limit angle, i.e. the angle at which a luminaire starts to become visible to the operator in a VDU, mean luminance of the luminous surfaces in the planes C0-C180 and C90-C270 must not exceed 200 cd/m².
The use of special VDU luminaires , however, with a luminance limit angle of 50°, only makes real sense for large offices. In smaller rooms luminance at the emission angle provided with such sophisticated glare protection would not in any case be visible in the computer screen. In the case of computer-supported workstations, therefore, the use of luminaires with a 60° luminance limit angle (specular darklight reflector optics) is to be recommended.
In the modern office computer screens are now often viewed at an angle of more than the 20° on which the standard is based. Notebook and other displays may even be used lying almost flat, which makes them susceptible to reflected glare from almost every direction. Picture tube technology has also made further progress in recent years, including antireflection coatings and the use of black lettering on a light background (positive contrast), so that there are now far fewer problems with irritating reflections. Where such modern equipment is in use, the standard explicitly permits higher luminance levelsthan the figures quoted, without specifying any maximum limits, however.