The Chartered Institute of Building Services Engineers' "Lighting Guide 3" describes three performance categories for aluminium louvres. These three categories define levels of ambient light control of the Health and Safety (Display Screen Equipment) Regulations 1992. The categories are defined in terms of light cutoff angle to the vertical.

Category 1
Describes the lowest cutoff angle - 55 degrees - and therefore the tightest control of ambient light. Category One louvres should be used in environment where highly intensive workstation activity - perhaps with large, high definition screens - is undertaken.

Category 2
Describes a medium cutoff angle - 65 degrees. Category Two louvres are intended for use in environment where fairy widespread and intensive workstation activity is undertaken.

Category 3
Describes the greatest relaxation of cutoff angle - 75 degrees. Category Three louvres are intended to be used in non-critical areas where use of computer workstation is predominantly casual - and never intensive. The increased coverage provided by Category Three louvres may allow wider lamp spacing, however, any computer workstation that are present may have to be carefully placed in order to avoid reflections.

In addition to the Lighting Guide 3 Louvre categories, a further critical parameter that should be considered when lighting computer workstations is the "patch test". The patch test is a procedure used to ensure that there are no high luminance spots along the length of the louvres (the "ladder" effect). Such an effect can cause particularly distracting reflection on workstation screens. To past a patch test, patch luminance should not exceed 500cd/m2 at any angle above the luminance limit angle. The procedure for carrying out patch is described in BS 5225.