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.