This paper presents part of the findings of a research project conducted at the Martin Centre University of Cambridge, which looked into how daylighting quality affects the users of a library interior. It discusses the importance of luminance in the field of view for the perception of quality and introduces a new index named Luminance Differences (LD) which represents luminance variability (‘noise’). It will be illustrated that this index was successful in relating to the users’ subjective assessments of daylight quality, suggesting that it can be used to predict user preferences in a daylit space.
My own work using electric lighting, 1,2 suggests that both the average luminance for the band and the variation are important and that it is a combination of the two that indicates lighting preference with regard to the application. There is little doubt that a low level of uniform lighting is likely to be unacceptably gloomy but equally a high level of luminance variation is likely to be unacceptably glaring, somewhere in between would be acceptable. I would appreciate the authors' views on this and ask whether the average luminance for the band was considered either on its own or in combination with luminance variation?There is no doubt, in my view, that the lit appearance of a space is an important element of lighting design whether it is daylighting or electric lighting and it was good to see the authors making the same point. However, to say that the effect could be created by variations in re ectance rather than lighting is extremely unlikely unless all the cues about how the space is lit are removed. Of course room surface re ectance and colour will have an in uence on the room appearance but not at the expense of light intensity and variation -the eye sees them differently unless the lighting cues are removed then you have an optical illusion.This brings me to a fundamental question about de ning the lit appearance of a space using the luminance distribution. Luminance is just one element in de ning brightness. The authors have used a relatively coarse resolution of measurement which will conceal luminance extremes which I am sure are important. For example small high luminance elements could provide sparkle but they could be sources of glare. Did the authors consider this and did the work provide any information on this important area?
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