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