Evaluating discomfort glare from non-uniform electric light sources

Abstract

This study tested the appropriateness of unified glare rating (UGR), CIE glare index (CGI), British glare index (BGI), and visual comfort probability (VCP) for evaluating discomfort glare from common non-uniform electric light sources. An experiment was conducted in a windowless room. High dynamic range photogrammetry was used for measuring three non-uniform fluorescent glare sources. The luminance and geometry data were acquired to calculate the glare ratings. The sub-divisibility and additivity of UGR, CGI, BGI, and VCP were tested via numerical method. Their predictability was evaluated using 67 subjects in two sessions. It was found that UGR, CGI, BGI, and VCP all have valid additivity, but only UGR and CGI have valid sub-divisibility. Current formulae of UGR, CGI, BGI and VCP all overestimate the glare sensation from non-uniform sources that young people may feel.

A review of indices for assessing visual comfort with a view to their use in optimization processes to support building integrated design

“…This index ranges with a three-unit step between 10, meaning that glare is 'imperceptible', to 34, meaning that glare is 'intolerable'; Cai and Chung [112] suggest 19 as a typical border value between comfort and discomfort glare (table 5); instead, according to Jakubiec and Reinhart [113], CGI lower than 13 means that glare is imperceptible and CGI higher than 28, means that glare is intolerable.…”

“…x presents the comparison of five discomfort glare indices as arranged in [27] and [112] on a nine-point glare sensation…”

“…UGR ranges between 10 (imperceptible) to 34 (intolerable) with a three-unit step; like CGI, a value of 19 is typically considered the frontier between comfort and discomfort glare [112]. EN 12464-1 reports UGR limit values for a number of visual tasks.…”

“…Cai and Chung [112] modified some numerical values to meet the semantic descriptions with the discomfort glare scale resolution; e.g., regarding BGI, the intolerable glare degree is shifted from 30 to 31 to match the three-unit steps of BGI. Jakubiec and Reinhart [113] instead propose a different comparison matrix based on a four-point glare sensation scale (Table 6).…”

“…Moreover, UGR cannot be used for light sources smaller than 3·10 -4 sr, since UGR tends to overestimate glare caused by those very small electric light sources [112], therefore CIE proposed a modified version of UGR called UGR small to be used with small light sources with a projected area (A 0 ) lower than 0.005 m 2 [68] UGR small = 8 log 10 0.25…”