Tutorial: Theoretical Considerations When Planning Research on Human Factors in Lighting

ABSTRACT

Research on human functioning is notoriously difficult. This particularly holds for the study of light effects, at least if one wants to go beyond establishing that changes in light “have an effect” and understand why this effect occurs—in other words, if one wants to make causal inferences about the mechanism behind it. The latter is, of course, crucial for generalizing insights and being able to use them effectively in other contexts. The culmination of many decades of research has taught us that light affects psychological functioning in numerous ways and through various pathways. This implies that, regardless of the investigator’s particular interests in either of those mechanisms, generally all will be at play, simultaneously, for participants in any lighting study. The present tutorial aims to address this complexity and how to deal with it by concisely describing the most important pathways that we currently are aware of. Such awareness is important both in contemplating the design and methodology of a study and in interpreting results from other studies and generalizing them to a particular application or light design.

KEYWORDS:

• Epistemology
• human factors in lighting
• image-forming pathway
• methodology
• non-image-forming pathway
• theory

Acknowledgments

The author thanks the editor, the guest editor, and two anonymous reviewers for their thoughtful comments and suggestions on earlier versions of this article.

Disclosure statement

No potential conflict of interest was reported by the author.

Notes

1. The debate regarding the causal mechanism behind these findings is still ongoing—some attribute them to observer effects, novelty effects, or demand characteristics (Olson et al. 2004); a fairly recent analysis of the original data even claims that the data patterns were not really as remarkable as often reported and showed “Monday morning effects” rather than what is generally implied with Hawthorne effects (Levitt and List 2011).

2. A third important class of mechanisms through which light exposure affects functioning pertains to light falling on the skin. Extreme exposure to ultraviolet, visible, and infrared radiation can damage the skin through both thermal and photochemical mechanisms (e.g., see Boyce 2010). But, at the positive end, light on the skin is also responsible for the production of vitamin D and serotonin. Via this route it potentially affects mood, cognitive performance, and health (Wacker and Holick 2013), but in the present text we limit ourselves to effects of light information received and processed by the eyes.

3. Admittedly, photic information is also processed through additional neural pathways not belonging to either group, but we will limit ourselves to these two broad classes (Ward 2015).