Disability Glare

Disability glare is a degradation of visual performance caused by a reduction of contrast. It can occur directly, by reducing the contrast between an object and its background (i.e. directly affecting the visual task), or indirectly by affecting the eye.
Two examples of direct disability glare are the reflection of the sun from a car dashboard, and the view through a misted up windscreen. When the sun is being reflected on the car dashboard, the luminance of both the object (e.g. the figures on the speedometer) and the background (the surrounding dial) are raised by the addition of extra light, and this increase is by the same amount.  However, proportionally the difference between the figures and the background is reduced, and thus it is harder to read the instrument.  When looking through a misted-up windscreen the world appears as though viewed through a veil. The whole scene looks grey and washed out, as both luminance contrast and colour contrast are diminished.  The physical situation where direct disability glare occurs can be characterised by the fact that the contrast reduction in the visual stimulus could be measured by an instrument.

The importance of contrast can be seen by considering the fact that stars cannot be seen in the sky on a cloudless day (even though they are still there) because the contrast between them and the rest of the sky is below the person’s contrast threshold for those ambient conditions.  However, when the sun goes down the sky darkens, raising the contrast between the stars and the sky, and the stars ‘come out’.

Indirect disability glare affects the eye and not the object being viewed. It is seen, for example, when a car approaches at night with its headlights on full beam, causing the eyes to be dazzled. In this scenario, the disability is caused by a reduction of retinal image contrast caused by light scattering within the eye and also by the raised adaptation level of the eye as the car approaches. Disability is prolonged because the eye takes time to re-adapt to the ambient light level when the car has passed.

The primary factor in determining individual differences in sensitivity to indirect disability glare (affecting the eye) is the age of the person. Although there are many changes that occur within the eye as it ages, the major change in the current context is that of scatter.  The increase in scattering with age and disease can be compared with the increase in dust in the atmosphere after a volcano eruption, and the consequence of the additional scatter is an increase in the disabling effect of a light shone into the eye.

In general, disability glare can be reduced by lowering the luminance of the glare source.  Other remedies are generally specific to the particular situation, and the context of the disability.