Assessing Glare - The CIBSE Glare Index

The CIBSE glare index is a system for the evaluation of discomfort glare defining two methods of calculation:

1. The calculation of the glare index based on a formula.
2. The finding of the index from tables based on the photometric data as supplied by the luminaire manufacturer.

The first method requires the use of computer software, since the formula is rather complex. It is, however, more accurate than the use of a table.

The formula for the CIBSE glare index is as follows:

With:

Lj	. . .	Luminance of the jth glare source
Lb	. . .	Average luminance of the field of view
Pj	. . .	Position index of the jth glare source
	. . .	Subtended area of the jth glare source

The second method is sufficiently accurate for most purposes. The tables being used provide an uncorrected glare index. They are based on a few assumptions:

• the luminaires are at a spacing-to-height ratio of 1
• they are at a height of 2.0m above eye level
• the total light output of the lamps in the luminaire is 1000lm
• the observer is located at the mid-point of a wall, with a horizontal line of sight towards the centre of the opposite wall
• the eye level is 1.2m above floor level

Correction factors need to be applied if any of the above assumptions doesn't hold true. At the present time there is no such correction factor for other spacing-to-height ratios.

The limiting glare indices according to the CIBSE code for interior lighting are roughly identical to the ones derived with the UGR formula. For instance, CIBSE recommends a limiting glare index of 19 for offices.

The CIBSE glare index is used mostly in the United Kingdom. It is anticipated that it will be replaced with the unified glare rating.