There are two sorts of glare  discomfort glare and disability glare. The mechanisms of both are unknown, but the conditions under which discomfort glare occurs and the ways in which disability glare can affect performance are well known. Discomfort glare: When a portion of the visual field has a much higher luminance than its surround, a feeling of discomfort around the eyes and brow may occur. This increases with an increase in the luminance of the glare source, and with an increase in the angular size of the glare source at the eye, and decreases with an increase in the luminance of the background and with an increase in the angularposition of the source relative to the line of sight. Disability glare: An extraneous light source can affect visual performance. The problem with disability glare is that it reduces contrast, causing a washing out  the whole scene looks grey. Like discomfort, the disability glare is often reduced by increasing the light level. Think about a car's headlights on full during the day; there's lots more light, and as a result, the car's headlights are less of a problem