"Glare" Is to Light as "Noise" Is to Sound - CIE Research Roadmap on Glare

Excessive light intensity, regardless of the source type, can have several unwanted effects on people. The presence of bright sources in the periphery of the field of view may cause a reduction in visibility or cause feelings of discomfort. The discomfort might be associated with the bright peripheral light sources detracting the eye from the intended gaze direction, although the exact mechanism causing this discomfort is not fully understood. Another effect occurs when the field of view itself is experienced as too bright or dazzling, typically in daylight conditions that require sunglasses.

Glare has been under study for over a century, but it really took off in the 1940s when increasing electric light levels gave rise to complaints about discomfort caused by excessive light. As a result, a multitude of glare formulae were proposed, each formula being based on a limited set of experimental results on a specific light source (gas discharge lamps, fluorescent tubes, daylight through windows, …) in a specific application (office lighting, street lighting, sports lighting, …).

Disability caused by glare has been successfully attributed to light scattering in the eye. The scattered light of bright sources creates a veiling luminance on the retina, which reduces the contrast of the retinal image. As a result, this glare effect could be well defined and tested experimentally. The prediction formula for disability glare, as described in CIE 146:2002 "CIE equations for disability glare", is therefore widely accepted and applied without reference to a specific application.
The situation is completely different for discomfort, where the mechanism, or possibly multiple mechanisms, is not clearly identified nor easily determined experimentally. Although discomfort metrics based on direct measurement of pupillary constriction, eye movements, or contractions of facial muscles have been examined, most scientific studies on glare that causes discomfort are based on subjective evaluations of the discomfort experienced in response to various stimuli.
The various formulae to predict discomfort experienced in interior lighting were successfully replaced by the Unified Glare Rating (UGR) in 1995 (CIE 146:1995 "Discomfort glare in interior lighting"). The UGR, like its predecessors, is an empirically-derived formula, based on discomfort ratings of lighting characteristics as produced by lighting technologies and applications predating the 1990s. It is generally considered as a practical method based on consensus, rather than science.

Meanwhile, the introduction of LED light sources has enabled luminaires and lighting systems with characteristics (such as peak luminance, luminance contrast or intensity cutoff) that were not previously possible. A recent CIE publication (CIE 205:2013: Review of Lighting Quality Measures for Interior Lighting with LED Lighting Systems) questioned the validity of conventional lighting quality measures for interior lighting with LED lighting systems. The report identified an important problem: A luminaire with visible LED sources in some cases seems to provoke more discomfort than a more conventional looking luminaire (which might also be LED based) with the same intensity and source area, but with a uniform source luminance. This issue has been addressed by CIE Joint Technical Committee JTC 7. The observed issues related to glare from sources with a strongly non-uniform luminance appear to be linked to ambiguity in the definition of the glare source area. The committee proposes a luminance measurement protocol to eliminate this ambiguity, such that UGR may also be correctly applied to the non-uniform glare sources that may be encountered in LED lighting. The report of JTC 7 is expected to be published mid-2019.

For application areas other than interior lighting no such consensus has been achieved. Consequently, the lighting community has been forced to work with many different glare formulae for applications like daylight through windows, road and street lighting, or sports lighting (Daylight Glare Probability (DGP), Daylight Glare Index (DGI), Cumulative Brightness Effect (CBE), Glare Control Mark (GCM), Glare Rating (GR), et cetera). Another complicating factor is the large variation in individual glare sensitivity, which may depend on, for instance, age, chronotype ("being a morning or an evening person"), or the type of activity. The visual interest of the scene can also influence whether or not viewers experience discomfort from glare sources.

Because of this scattered approach and large variation in individual responses, the evaluation of glare, though seen as relevant to lighting design, is often considered too difficult to quantify. To counteract this tendency, a unified glare prediction method that can be applied to any lighting application and is independent of the glare source technology needed. Being able to predict discomfort arising from glare will be beneficial to end users because it will enable its prevention. Manufacturers will be better able to develop suitable products and to categorize the suitability of their products for various configurations and applications. This will facilitate better choices by lighting designers and specifiers to balance cost to lighting comfort.

Although there exist differences between the various discomfort prediction formulae for glare, the formulae for different application areas such as interior lighting and roadway lighting are very similar in form (see J.J. Vos, "Reflections on glare" Lighting Res. Technol. 35,2 (2003) pp. 163–176, or P.R. Boyce, "Human Factors in Lighting" 3rd edition, New York: Taylor and Francis, 2013). They all predict an increase in discomfort with increasing glare source intensity, a decrease when this intensity is spread over a larger source area, and a decrease when the background or adaptation luminance increases. This points in the direction of a commonly shared mechanism that causes the discomfort, independent of the source technology or application area.

The identification of the psychological or physiological mechanism or mechanisms that cause discomfort from glare has been identified as one of the key research questions in the CIE research roadmap. When excessive light in the field of view causes discomfort, is the problem caused by distraction of the eye toward bright peripheral sources, a mismatch between local and global adaptation, uncomfortable variations in pupillary constriction, a lighting pattern or light direction that deviates from what is "natural", a combination of these mechanisms, or by another, unknown mechanism?

The identification of such glare mechanisms is a prerequisite for the construction of a discomfort prediction model in which multiple application conditions and light source types may be incorporated by measurable physical, physiological and psychological parameters such as lighting characteristics (position, size and orientation of glare sources, luminance patterns, lighting dynamics, spectral distribution, etc.) and observer parameters (positions or trajectories, gaze directions, field of view, age, chronotype, visual task, type of activity, etc.). The advancements of neurological science and perception research in unravelling the pathways of the visual system may be instrumental in reaching this goal. This fundamental knowledge will enable a robust and science-based discomfort prediction method that, as for disability caused by glare, can withstand changes caused by future innovation in light sources and can be applied to all lighting applications.