Glare Rating Systems

In order to achieve comfortable glare free conditions in the workplace it is necessary to control the luminance (brightness) emitted from a luminaire at certain angles. This is especially important in situations where computer monitor screens are being used.

One glare rating system used overseas is the British CIBSE (Chartered Institute of Building Services Engineers) standard LG3. This system uses three categories of performance rating for luminaires used in computer monitor areas of the workplace. It sets two maximum luminance limits over a range of angles specified in the standard.

The average luminance is an average over the entire area of the luminous element. This parameter is calculated.

The patch luminance is a measurement of any 25mm diameter area on the luminous element. This parameter is measured.

The luminance limits for each of these parameters vary according to the type of computer screen (negative or positive image) and on whether the screen has an anti reflective system.

In Australia a glare index system is used which takes into account the actual installation parameters as well as the luminaire. This system is a relative measurement of the visual discomfort of a seated observer who is looking straight ahead. It is calculated in a computer program (from Lighting Analysts) which divides the given room area into 100 sections and places the observer in each of those positions. The program then effectively rotates the observer through 360 degrees and calculates the glare at each angle for each of the 100 positions. The program prints the worst case figure only.

Room reflectances have a major part in this calculation. If the floor or walls are too dark the background luminance will be reduced due to there being less inter reflected light. As the glare calculation involves background luminance, a brighter background will give lower glare figures.