Research Note: On the magnitude of error in the calculation of mean room surface exitance

Abstract

Two formulae currently exist for calculating mean room surface exitance. This research note explains and demonstrates that one of these formulae is erroneous under certain conditions and stresses that the alternative expression should generally be used for computing mean room surface exitance.

A fresh approach to interior lighting design: The design objective – direct flux procedure

“…Duff also recorded levels of horizontal working plane illuminance, which showed only weak correlation with SB. 10,11 There is strong indication here of a linear SB/MRSE relationship expressed by the indicated trendline, but some caution needs to be applied. The limited range of MRSE levels, 25-100 lm/m 2 , has effectively restricted SB responses to the range of SB2 (dim) to SB4 (neither dim nor bright).…”

“…The scope of Duff's studies of exitancebased metrics is extensive. He developed procedures for measurement and calculation; 9 he compared the accuracy of alternative prediction formulae 10 and he conducted experiments which involved subjects making subjective assessments of their surrounding environments. 11,12 The outcomes of these experiments are examined in the following section and are shown to be particularly relevant to this paper.…”

“…As part of Duff's assessment of the accuracy of alternative predictive formulae, 10 he compared MRSE values calculated for a reference room by his own definitive procedure (equation 1) 9 with values calculated by Cuttle's The relative values shown in the chart indicate that if all room surfaces are of the same reflectance, then for providing MRSE, it makes no difference how the flux is distributed. For conventionally decorated spaces, where ceilings are lighter and floors are darker than walls, then as reflectance diversity increases, so the efficiency of indirect lighting to provide MRSE increases, and the efficiency of direct lighting reduces.…”

Calculation and measurement of mean room surface exitance: The accuracy evaluation

“…The idea of MRSE attracts much attention and discussion. 5–11 However, the successful adoption of this new concept in lighting applications may largely depend on research advances in the following aspects: (i) experimental evidence of MRSE being clearly linked to people’s assessment of brightness appearance, (ii) an easy way to calculate MRSE, and (iii) a practical method to measure MRSE in the field. For the link between MRSE and brightness appearance, Cuttle 4 has shown that, through subject assessments, MRSE and scene brightness are linearly related.…”

“…Equation (2) provides an insightful and easy approach to calculate MRSE for practical lighting design. Duff 9 examined the magnitude of error of using equation (2) by comparing its prediction with that based on simulation data and equation (1). It was concluded that ‘ one of these formulae is erroneous under certain conditions ’.…”