Photometric room design does not only include artificial lighting, but also combinations of daylight and artificial lighting. In principle, work rooms must feature a visual connection to the outside (see also EC directives 89/654/EEC and 92/58/EEC, implemented in Germany via the work place ordinance of 12.08.2004 currently in revision). Significantly increased illuminance as a consequence of sufficient daylight supply compared to the values specified in EN 12464-1 for artificial lighting can have a positive influence on subjective well-being (see also chapter "Light and non-visual effects"). However, in screen work, this can lead to impaired visibility of screen information. This is why, according to Council directive 90/270/EEC of 29 May 1990 on the minimum safety and health requirements for work with display screen equipment (implemented through national ordinances in many countries, e.g. in Germany through the VDU workstation ordinance), windows in rooms with VDU screen workstations have to be equipped with a suitable system of adjustable covering.
The suitable systems of adjustable covering required by 90/270/EEC must not lead to windows being obscured and rooms being illuminated exclusively with artificial lighting during the day. This would lead to a severing of the visual connection to the outside required for work places, particularly offices. Hence, the suitable systems of adjustable covering must provide screening from highly glaring luminance, e.g. the sun, as well as ensure the visual connection to the outside.
Generally, users accept higher levels of luminance at covering systems on windows and skylights than they do at other room surfaces. Luminance levels between 2.000 cd/m² and 4.000 cd/m² at covering systems in the lateral field of vision are generally not described as disturbing. This means that e.g. slightly translucent and adjustable blinds (maximum transmittance 30%) which shield but also allow looking through them are a possibility to realise covering without shutting the room off of daylight completely.