It is important to understand that there is not a clear-cut line between comfort and glare. For instance, many people will happily work away in a daylight space with a window, turning their backs to the sun and avoiding the glare while generally enjoying a cheerful space. In other cases, people will tolerate some glare as long as it contributes to an overall impact of sparkle (see Sparkle) or a festive mood. Yet a modest difference in brightness caused by an unattractive unchanging source like a lensed troffer is often deemed too much glare.
In an attempt to quantify the visual comfort of direct luminaires in a space, Visual Comfort Probability (VCP) was developed. VCP is a calculation taking into account the relative brightness of a lighting system from a given viewing angle, resulting in the likelihood (as a percent) that a lighting system will be visually comfortable from a glare control standpoint. VCP data were confirmed experimentally using uniform layouts of lensed fluorescent luminaires. While it is typical practice to extrapolate the VCP concept to apply it to various size louvers and luminous ceilings, it should be noted that VCP data have not been experimentally confirmed using these systems. As such, one should be cautious in using VCP to evaluate the potential visual comfort of lighting systems using other than lensed luminaires, making it of limited practical use.
In other words, there is acceptable glare and unacceptable glare from a comfort standpoint and the definition is not quantitative but qualitative. Discomfort glare can be the result of either direct glare or reflected glare. Overhead glare is discomfort glare caused by a glare source above the normal field of view.
For a long time, it was believed that discomfort glare could only occur with a glare source in the field of view and that the further away from the centre of view, the less the glare sensation. By this reasoning, working under an open parabolic luminaire should not be uncomfortable because the bare lamps are above the field of view. However, many people do express discomfort due to the brightness of the lamps in this situation. And there is a wide range of individual sensitivity to this overhead glare condition. This is because the field of view is not an abrupt cut-off, nor consistent between individuals. Likewise, the sensation of brightness does not end abruptly, but diminishes. The overhead glare sensation is believed to be a result of the interaction of the high-angle light with the viewer’s facial physiology – light scattering at the cornea and eyebrow and reflecting into the eye from nose, cheekbones and other facial objects such as bangs or eyeglasses.
There are several variables associated with the level of discomfort from a glare source:
Spectral Composition: Current research is evaluating the impact of spectral composition on glare sensation. Theory is that the higher the color temperature and the stronger the short wave length composition (blue portion of the spectrum), the greater the discomfort. For long term occupancy and commercial spaces, be more concerned about glare caused by fluorescent lamps, lenses, and other overly bright sources of light. (Luminaires & Distribution/Luminaire Components provides more information about glare issues related to specific types of luminaires.) Be less concerned about the glare in interior spaces of short-term duration sunlight and small point sources like incandescent filaments. Be most concerned about sources of glare in relation to stationary tasks when the building occupants cannot easily relocate themselves or their task.
Direct glare is caused by a view of the light source, often with high contrast to the surroundings. While the bare light bulb is technically the most efficient “luminaire” because it has no internal losses, it is a poor choice in almost every application because the glare from the bulb walls is visually unpleasant. Thus, the control and distribution of light from a source is an essential characteristic of its visual efficiency. Glare is associated not just with lamps, but also with daylight, especially when one is exposed to low angle, direct sunlight. Any excessively bright source, not just electric lamps, but also low-angle sunlight or overly bright luminance, can cause glare from windows or skylight diffusers. Thus IES design guides promote modest ratios of brightness.
Luminance is a very important concept in lighting, since luminance is what we actually see. Rigorously, luminance is defined to be the ratio of the intensity of light produced by a surface in a given direction to the projected area of the emitting surface. In International System of Units (SI), luminance is generally expressed as candelas/square meter (cd/sqm). In Inch-Pound (IP) units, luminance should generally be expressed in candelas/square foot (cd/sqft). The foot-lambert unit for luminance that has been used in the past has been deprecated and should be avoided. Brightness is used to describe the strength of the perception caused by viewing surfaces (or volumes). Brightness is related to luminance, but takes account of the fact that there is not a linear relationship between relative luminance values and perceived brightness. For example, a surface with a luminance of 100 cd/sqm will not appear twice as bright as a surface of 50 cd/sqm.
Occupant control of light sources is an important strategy in reducing the impact of glare. Adjustable task lights and individually dimming ceiling luminaires provide control over glare conditions from electric lighting. Individually controllable window blinds and curtains provide occupants with the opportunity to adjust lighting conditions to their particular needs and wants. When feasible, such personal control of lights will greatly reduce the negative impacts of potentially glaring sources. For more about controls, refer to Lighting Controls.