Peter Defoe’s article “Light: How Much Is Right?” (BSj 10/08) addresses an important issue, but in terms of the conclusion and methodology it is dangerously misleading.

The research sets out to identify the illuminance at which “text became distinguishable and could, with effort, be discerned and read”. This is a subjective assessment and was done by gradually raising or lowering the task illuminance. Defoe considers several parameters (age, eye colour, health) that may explain the variation in the results but fails to mention the lack of agreement expected anyway in such tasks, even when using well matched subjects. Raising the issue of gender but failing to use a single female subject is astounding. While 12 subjects is slightly better than the “six or seven” used in the earlier work, it is still a woefully inadequate sample.

Statistical analysis would have been useful, to demonstrate whether differences in preferred illuminance between subjects, dimming direction, eye colour etc were real or chance effects. This is not mentioned, and it is therefore not safe to place any value on the results.

There are many questions that should have been addressed in the article. For example, when adjusting illuminance to find the preferred level, the starting illuminance will affect the outcome. This can be seen in differences between results from the low and high (100 lux) starting illuminances used by Defoe, which produced preferred illuminances in the range 4 lux to 53 lux. If the high starting illuminance had instead been 50 lux the upper value of this range would have been smaller, and similarly if the high starting illuminance had been 200 lux the range would have been larger. In either case, the conclusion that the appropriate level is “likely to be … 25 to 30 lux” would change and it is therefore not a robust statement.

Why such a strong reaction to this article? Because before you can say Kruithof effect, findings like these can be misquoted, misused, and end up in lighting codes. Perhaps more so when accompanied by the claim that this is associated with work for a PhD thesis.

Dr Steve Fotios, senior lecturer, School of Architecture, University of Sheffield

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Building Sustainable Design