Avoiding glare caused by reflections is decisive for good visual comfort at workplaces. Downlights should be positioned to the side of the workstation. This avoids unpleasant reflections, for example on the keyboard. Luminaires with a 30° cut-off provide a good basis for visual comfort. Pleasant brightness on background walls also contributes to high levels of acceptance at the workplace.
Optimising the luminaire arrangement
Luminaires installed vertically above the workstation increase the risk of reflected glare. The same applies to luminaires with insufficient cut-off and with wide light distributions that cause disruptive mirror images on the screen. Such reflections reduce levels of visual acuity and can also cause fatigue and headaches. The UGR (Unified Glare Rating) value is used for evaluating the glare caused by luminaires in office lighting.
EN 12464-1 reference values:
– UGRL <19 at the workstation
– UGRL <22 at the reception
Using glare-controlled luminaires
Two factors are especially important for minimising direct glare from luminaires: firstly the cut-off angle of the luminaire and secondly the arrangement of luminaires. The UGR value as the specific value for direct glare is however not a characteristic of the individual luminaire but always depends on the room and the position of the viewer.
Validity range of UGR tables
Glare depends particularly on the luminaire arrangement. The tabular specifications for UGR values listed in product data sheets are only valid with a uniform matrix of one luminaire type in a rectangular room. If a calculation is made for a specific viewer position in this room though, the UGR value may deviate from the specification on the data sheet. If for example the data sheet specifies UGR <22, the lighting calculation for a specific workstation may result in a UGR value of 19.4.
Luminaire arrangement and observer position
Calculating the UGR value with a computer is necessary in situations with a non-uniform matrix of luminaires, or if various luminaires with various luminous flux levels and different light distributions are used, or if non-rectangular rooms are used. In such cases it must be remembered that the calculation result is only valid for the specified viewer position. The example demonstrates how a luminaire with a UGR value of 22 on the data sheet achieves a value of UGR <10 in the computer calculation. This corresponds to the lowest glare on the UGR scale.
Individual light calculation for UGR
If the desk arrangement is changed, the UGR value must be re-evaluated for the modified viewer position. The individual calculations are more precise and also allow designers to use a wider spectrum of luminaires. The reason is that luminaires with a tabular value of UGR < 22 for the general room can also achieve an acceptable value of UGR <19 with appropriate arrangement of the luminaires and individual computer calculation.