Unified Glare Ratings explained
Lighting has become an extremely important necessity in our continuous 24 hour a day environment. Whilst the technological development of artificial lighting has moved ever forward since the inception of the incandescent lamp way back in the 1800’s, it has become clear that artificial light has created a number of issues in respect of light pollution and glare.
If we consider only the aspects of glare and leave light pollution for another time, then it is important that some clarification on the definition of glare is initially provided. Glare is the visual sensation caused by an excessive and uncontrolled brightness of light within the individual’s field of view.
Expanding on this statement, glare is produced when the eye is exposed to an intensely excessive and uncontrolled brightness, within the visual field, which is more than the eye can accept. Glare is subjective and the sensitivity to glare can differ greatly between individuals which as previously stated, is caused when the eye is exposed to a more intense light than it is normally adapted to. This can be natural light, such as by sunlight or artificial light, such as from the headlamp of a car or from a floodlight. It can be distracting or simply uncomfortable or can be more serious in being disabling or dangerous. Glare can prevent an individual distinguishing objects and detail. It is known that the eyes of older people are considered as being more sensitive to glare, a condition which is caused by the aging characteristics of the eye.
There are two forms of glare, Disability Glare and Discomfort Glare
Disability Glare – defined as directly impairing vision
Disability glare generally occurs when an object within the field of view has an excessive brightness, which is more than the eye can deal with. It is usually encountered externally, such as from oncoming vehicle coming over the brow of a hill, however internally this could be from the sun entering the space through a window or from the direct brightness produced by a light source within the space. It arises from stray light falling on the retina, usually from scatter by the media of the eye. Scattered light falls as a patch of veiling illuminance on the fovea and reduces the contrast of the retinal image, and as a result it impairs visibility and visual performance.
Discomfort Glare - defined as perceived discomfort
Discomfort glare is the discomfort, annoyance or in extreme cases, the pain caused by an excessively bright light source whose luminance is greater than the eye can adapt to. The degree of discomfort depends on several factors such as dimensions and luminance of the source of the glare. Discomfort glare may be different for each individual and will also depend upon the geographical position of the source to that of the line of sight of the individual as well as the contrast between the source and the area surrounding the source.