Abstract
When you look at a bright light, the light scattered within the eye reduces the contrast of the retinal image, interfering with vision. The interference is referred to as disability glare. The bright light can also be uncomfortable to look at, and result in so-called discomfort glare. The two types of glare are “likely influenced by different channels in the visual system.” Partly because of adaptation, vision may take a while to recover from exposure to the bright light source. The factors that affect disability glare, discomfort glare, and glare recovery include the intensity of the source, its luminance and duration, the location of the source within the visual field, and the background against which it is presented. These factors are reviewed in the recent book published by the International Society for Optics and Photonics (SPIE) from which the above quotation is taken (p.22).
The book is by John Bullough from the Light and Health Research Center, now at the Icahn School of Medicine in New York. The primary focus is on lighting applications that take place outdoors, such as vehicle headlighting and street lighting, the stated objective being to “help us tame the glare from headlights, streetlights, displays and windows” (Preface: p. v). While the focus is narrow, the largely atheoretical treatment is likely to be very useful for lighting engineers, although they should bear in mind that pattern glare from the repetitive distribution of ceiling luminaires and strings of LEDs is not considered.
There is a tradition within lighting engineering of referring to a “standard observer” and ignoring differences between people. Yet susceptibility to discomfort glare varies considerably from one observer to the next. Bullough considers age as a source of individual variation, but acknowledges that it is only partly responsible for the large differences between people. Susceptibility to discomfort glare is a component of the photophobia with which migraine and many other common neurological conditions are associated, and these provide clues as to the physiological mechanisms responsible (Wilkins et al. 2021). Bullough lists some of the psychological factors involved in discomfort glare but not the neurological.
Neurologists at the Harvard Medical School (Noseda et al. 2016) have measured discomfort glare from white light and narrow band blue, green, amber, and red light. The lights were equated for (photopic) luminance, which increased progressively from 1 to 100 cd.m−2. They concluded that green light was the most comfortable during a headache—a conclusion that has been widely reported and cited, but was unsound, given that the study failed to take account of the manner in which the spectral sensitivity of the eye changes from scotopic to photopic over this luminance range, with consequences for the relative brightness of light of different wavelengths. This is not an error that a lighting engineer would have made. Bullough considers glare from lights with various spectral power but not the large individual differences. The differences are of concern to neurologists because preference for a particular chromaticity is related to migraine diagnosis (Wilkins et al. 2021). It might be useful for lighting engineers and neurologists to liaise in their studies of glare.
It may well be the case that “there is not yet a firm explanatory mechanism that has been validated for discomfort glare” (p.18), but Bullough might consider contributions from neuroscience. The late Horace Barlow showed how the visual system evolved to process images from nature efficiently with a sparse code such that few neurons are active at any one time. Images that give rise to discomfort differ from those in nature and have an excess energy at those spatial frequencies to which the visual system is generally most sensitive. Such uncomfortable images evoke a relatively large hemodynamic response, consistent with the idea that the visual system is then functioning inefficiently, with more neurons active than usual, reducing the sparseness. The discomfort can therefore be seen as a homeostatic mechanism that prompts the observer to look away from the glare source, thereby reducing metabolic demand (Wilkins et al. 2021).
There is thus a theoretical interest in discomfort glare that extends beyond the literature considered by Bullock. It involves the concept of neural efficiency, and this helps to explain the photophobia in a range of neurological disorders that render the visual cortex hyperexcitable. Hopefully, it will eventually be possible to bring the phenomena of glare, photophobia, and cortical excitability within a general theoretical framework, one that encompasses individual differences, and considers discomfort from flicker and spatial patterns as well as bright light. Then and perhaps only then will we be able to understand glare in lighting and displays.
