Glare meaning is the sensation produced by an exaggerated luminance within the visual field which alters sensitivity of the eye, causing discomfort, reducing visibility or both.
Glare may take place in two different ways. Sometimes they occur separately, but generally, they take place simultaneously. The first is
known as physiological glare (or disability glare). It impairs visual capacity and visibility, but it does not necessarily produce discomfort.
The second is known as psychological glare (or discomfort glare). This type is discomforting but it does not necessarily impairs object observation.
In indoor lighting, psychological glare (discomfort) is likely to be more of a problem than physiological glare (disability). Measurements taken to control discomfort glare will have to take discomfot glare into account, too. The sensation of discomfort experimented by discomfort glare tends to increase with the passing of time and contributes to nervous tension and fatigue.
Any given type of glare meaning may be direct or by reflection. Direct glare is the glare directly caused by luminances of the sources of light, such as lamps, luminaires and windows, which appear in the observer’s field of vision. Glare by reflection is the glare produced by reflected luminances from surfaces with high reflectance, especially specular surfaces such as polished metals,
except when these form part of the luminaire. Glare by reflection must be distinguished from other types of reflection which produce a reduction of the task contrast. They are more correctly described as veiling reflections (high luminance is reflected by the task towards the eyes, veiling it and reducing its contrasts).
Control of direct glare of lamps and luminaires consists in controlling their luminance in the direction of the observer’s eyes.
Nevertheless, the degree of experimented glare is not only a function of luminaires in the worker’s visual field, but it also depends on the type of activity performed. The more light demanded by the visual task, and the higher the need of concentration, the higher discomfort will be, too. However, in those situations where the worker must move to perform the task, the experimented discomfort will be less.
Therefore, the luminance degree of control will differ according to the type of task or activity. The C.I.E. has classified tasks and activities in five groups depending on the required luminance degree of control. In Chart 1, five groups referring to Quality Classes are enumerated.
In general terms, the highest luminances in an indoor area produced by the lighting installation are those coming from lamps.
Generally speaking, such luminances are too high to use lamps without controlling their brightness in the direction of the eyes. This is the reason why one of the luminaire functions is to limit luminance in the critical directions at an acceptable level.