In some past studies researchers have concluded either that current light levels (typical for offices) are too high, while in others they conclude light levels are too low. Why this confusion?
The conflicting studies tended to use an adjustment method where test participants were asked to adjust the light level to their preferred level. A review of these studies suggested a centering tendency – the mean preferred illuminance was always located near the mid-point of the adjustment range. Thus, experiments using a low range (a low upper limit) tended to conclude a low preferred illuminance, while experiments using a high range (a high upper limit) tended to conclude a high preferred illuminance.
We confirmed this proposal by experiment.
An experiment was carried out to explore luminance adjustments and confirmed the bias associated with stimulus range and anchors (initial illuminance settings). This work was conducted by Asta Logadóttir as part of her PhD thesis.
The two previous experiments explored bias in the adjustment procedure related to changes in brightness, which has an obvious change in magnitude. In this third experiment we examined instead settings of preferred CCT using an adjustment paper. Once again, the results confirmed a centering tendency. This work was conducted by Asta Logadóttir as part of her PhD thesis.
Logadóttir Á, Fotios SA, Christoffersen J, Hansen SS, Corell DD, Dam Hansen C, Investigating the use of an adjustment task to set preferred colour of ambient illumination, Colour Research & Application, 2013; 38(1); 46-57.
This fourth experiment examined luminance adjustment but included also evaluations of satisfaction. Settings of preferred luminance again revealed significant bias associated with stimulus range and anchors.
After making a setting of preferred brightness, participants were asked to evaluate their satisfaction with that setting. Given that the participant had control over the light setting and did not tend to set the upper limit available, a response of low satisfaction would have been surprising.
Consider the luminance set by a participant within a low luminance range. Participants tended to report that to be a satisfactory luminance. If, however, that same light level were set by another (the experimenter in this case) then there was a lower degree of satisfaction. These results show the benefit of personal control.