Glare can be caused by the light source itself (direct glare) or by reflection of the light source (reflected glare).
The extreme case of physiological glare is absolute glare. This happens when luminances of more than 104 cd/m2 are present in the field of vision, e.g. when looking at the sun or looking directly into artificial light sources. Absolute glare is independent of the luminance contrast to the surroundings: it cannot be avoided by a brighter environment. To prevent the eye from being exposed to danger, a protective reflex is triggered which causes the eyes to close or even the head to be turned away.