Brightness matching

Research methods for spatial brightness

In brightness matching, the brightness of one scene is varied by the experimenter until it matches the brightness of a second reference scene.

This paper demonstrated a tendency for test participants to set the illuminance of the varied scene to a slightly low illuminance. While the difference tends to be small, it can be sufficient to change a conclusion regarding the relative brightness of different scenes. This bias was demonstrated directly in the results of null condition trials, with an illuminance ratio (test/reference) of less than 1.0. The bias was also shown in the results of between-lamp trials. To overcome this, dimming should be applied to both the test and reference fields in successive trials.

Fotios SA, An error in brightness matching associated with the application of dimming. Lighting Research and Technology 2001; 33(4); 223-231.

This paper describes a bias revealed when a matching experiment was carried out at more than one reference illuminance (three in this case). Brightness settings were biased towards the middle illuminance, tending to be set low in trials with the high reference illuminance, and tending to be set high in trials with the low reference illuminance. This suggests a problem associated with the choice of reference illuminance (or other reference condition): it may be better instead to avoid choosing a reference and use all-possible-pairs.

Fotios SA, Cheal C. Evidence for response contraction bias in side-by-side matching tasks. Lighting Research and Technology 2007; 39(2); 159-169.

This paper reviews the results of past studies using simultaneous (side by side) matching tests and suggests a need to:

  • Counterbalance the stimuli between the left and right hand sides to counter position bias

  • Apply dimming to both stimuli in turn

  • Set the variable stimulus to both high and low initial settings at the onset of trials.

Fotios SA, Houser KW, Cheal C. Counterbalancing needed to avoid bias in side-by-side brightness matching tasks. Leukos 2008; 4(4); 207-223.