What is glare? I use the word glare frequently and therefore I should define it. What is the meaning of glare for a light sensitive person? I have given some clues about what glare feels like to a person with photophobia (see Why Glarminy  and About Glarminy) but again, I haven’t taken the time for an explanation.

What is glare?

We all know what glare is, but formulating the answer to the question: What is glare? is not so easy. Glare might be defined in terms of a contrast in brightness between different objects in one’s field of vision. The objects that tend to cause the sensation of glare are usually either sources of light (light bulbs, electronic display devices, the Sun), or those whose surfaces reflect most or all of the light (a mirror, water, snow, objects with very smooth surfaces).

If you prefer, you can also view a 2 minute video titled What is glare which has been posted by PhillipsLighting.

What is the meaning of glare? Why is glare a problem?

Some (eye doctors) claim that our eyes are “designed” so well, that when in good shape, they can look straight into the Sun. (Before you try it I should warn you, that there are many more who say that the intensity of light of the Sun would cause permanent damage to your eyes). I only state this to say that generally the intensity of light is not the main problem. Rather, the trouble is in the brightness contrast, which is precisely what glare is.

Glare is a problem, because our eyes can only adjust to one level of brightness at a time. When the eyes adjust to darkness, the pupils contract to allow less light into the eyes. In this case a brighter object anywhere in the field of vision will emit too much light for the eyes to take in such that one experiences the uncomfortable sensation of glare. Something similar happens when one object is poorly lit in otherwise bright environment. Both situations of contrast cause difficulty in seeing detail. The eyes try to adjust continuously to the two levels of brightness and this leads to weariness, tension around the eyes, headaches, etc…

The meaning of glare might be defined on a continuum between no contrast (no glare) and a very strong contrast. Obviously in the absence of contrast there is zero glare. The meaning of no glare is that one’s eyes feel comfortable. At the other extreme, the contrast is too strong. In this situation the meaning of glare is that the eyes instinctively close or one looks away. This is called disability glare or loss of visibility.

Between the two extremes (no glare and disability glare) there is a continuum of increasing contrast. When the contrast is mild the meaning of glare is a slight interference with seeing detail in the vicinity of the contrast. This type of glare is often referred to as discomfort glare (it is frequent when working with a computer and leads to eye strain and headaches – computer vision symptom, CVS). As the contrast increases, the meaning of glare is a stronger discomfort and interference with seeing detail (an example might be driving into the sunset/sunrise or at night).

The meaning of glare for a person with photophobia

The above definition refers to the meaning of glare in general, i.e. for any person. However, different people perceive glare differently – some people tolerate the same contrast in brightness better than others. It is therefore meaningful to speak of glare in subjective terms, i.e. as it is perceived by a specific person. Moreover, our eyes’ light tolerance tends to decrease as we age. Subsequently, the negative effects of glare (eye strain, weariness and other problems) increase. Therefore the meaning of glare for the same person changes over time as her/his tolerance to light decreases.

The meaning of glare for a person with photophobia is therefore that they enter the zone of discomfort glare (orange in the figure above) at lower brightness contrast than a person who is not light sensitive. Similarly, a light sensitive person has a lower disability glare threshold than a person without photophobia. Hence photophobia or light sensitivity is also referred to as intolerance to light.

Is there anything else that a definition of glare and the meaning of glare for a person with photophobia should take into account? Given that glare bothers everyone, do you think that everyone could benefit from light ergonomics (glarminomics), not just people with low light tolerance?