Further issues

In side-by-side matching trials the participants have been instructed to use a variety of equality objectives, such as equal brightness, equal clarity, equal pleasantness and equal satisfaction with visual appearance. To what extent does he choice of terminology matter?

A matching experiment was conducted where participants matched two lighting scenes for a range of visual equality objectives. The results did not suggest a significant difference. One explanation is that the meaning of some terms is not well understood, and in which case observers assume a task they do understood – equal brightness.

Fotios S, Gado T. A comparison of visual objectives used in side-by-side matching tests. Lighting Research and Technology 2005; 37(2); 117-131.

Two items commonly evaluated in rating scales are (spatial) brightness and visual clarity. While the definition of brightness is, probably, well understood by naïve participants, it was proposed that the meaning of visual clarity is not. One reason to suspect this is that not even lighting experts agree on what it means.

A review of past rating studies was conducted. Within individual studies, and across the collated studies, there is a strong association between mean ratings of brightness and visual clarity for a given stimulus.

There are two interpretations of this outcome. First, that by coincidence, brightness and clarity do evoke the same magnitude on a rating scale. Second, that when participants do not understand the meaning of an item they are asked to evaluate they assume instead the meaning of something they do understand, and here evaluations of brightness were substituted for the requested evaluation of visual clarity.

Fotios S, Atli D. Comparing judgements of visual clarity and spatial brightness through an analysis of studies using the category rating procedure. Leukos 2012; 8(4); 261-281.

This paper questioned the influence of adaptation on the results of spatial brightness evaluations. Typically there are two situations: (1) studies using category rating allow some degree of chromatic adaptation and do not tend to find an effect of SPD; and (2) studies using simultaneous matching do not allow adaptation to the individual spectra and tend to find a significant effect of SPD.

This leads to a question: when do room occupants tend to evaluate their lighting? If it is on immediate entry, then the results of matching studies are likely to be pertinent. If it is after spending some time in the room, then it is likely the results of category rating studies are pertinent.

Fotios SA. Chromatic adaptation and the relationship between lamp spectrum and brightness. Lighting Research and Technology 2006; 38(1); 3-17.

While many brightness studies have used simultaneous (i.e. side by side) evaluations, others have used sequential evaluations – the two light settings are observed one after another in the same spatial location.

An experiment was conducted using sequential evaluations and compared to the results of previous work using simultaneous evaluations for same pair of stimuli. The results did not suggest a difference between simultaneous and sequential evaluations.

Fotios SA, Cheal C. A comparison of simultaneous and sequential brightness judgements. Lighting Research and Technology 2010; 42(2); 183-197.

Interpretation of experimental results should use statistical tests to determine whether differences and relationships are real and not just a chance effect. In this paper Jim Uttley describes some of the limitations common in lighting research including sample size analysis, testing the assumption of normality and reporting effects size.

Uttley J. 2019. Power analysis, sample size, and assessment of statistical assumptions – Improving the evidential value of lighting research. Leukos. 15(2-3): 143-162.