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Luminance ( Photometric Brightness )
The light that we actually see, brightness can be measured as the light leaving a lamp, or the light reflecting from an object's surface. It is measured in footlamberts (English) or candelas/square meter (metric).
Brightness can be used for a variety of purposes, from producing a sense of drama to creating sparkle and glitter elements in a space. The brighter a task is, the easier it is to see and the lower the amount of light that is required. Too little brightness decreases contrast and calls for a higher light level. But if not properly controlled, high brightness can produce levels of glare that either impair or prevent a desired task being performed. Glare can be described as direct or reflected glare, which can then result in discomfort or disability.
Direct glare comes straight from the light source. Reflected glare shows up on the task itself, such as a computer screen. Discomfort glare does not prevent seeing makes it uncomfortable. Disability glare prevents vision--a popular example is holding a glossy magazine at a certain angle; a veiling reflection results, impairing our reading of the page.
Strategies for Reducing Unwanted Glare
Strategies commonly employed to reduce unwanted levels of glare include:
- Indirect lighting that throws more light upward than downward, diffusing the light and reducing glare on computer screens
- Parabolic louvers, special lenses or other diffusing media on fixtures that diffuse the fixture's light output
- In an office, it may be possible to de-emphasize the ambient lighting system with reduced light output and diffusing media, while providing adjustable task fixtures at workstations
- Relocating the light source
- Relocating the task or changing its orientation until the glare is removed
- Changing the surface reflectance of the task
- Use blinds or shades on windows to control the amount or transmittance angle of sunlight entering the space
Visual Comfort Probability
Visual comfort probability (VCP) is a rating on a scale of 0-100 given to indoor fixtures (in a uniform system with identical fixtures) to indicate how well accepted they are likely to be by the area's occupants. A VCP rating of 75, for example, indicates that 75% of the occupants in the poorest location would not be bothered by direct glare. Generally, office environments require that fixtures have a VCP rating of 70 or more, although this figure has been revised by some in recent years to 80 or more for environments where visual-task computers are used. The VCP rating for a given fixture can be found in its photometric test report. Generally, again, the higher the VCP rating, the lower the fixture's efficiency at transmitting light to the task.
Shielding Media Characteristics for Fluorescent 2x4 Recessed Troffer Fixtures:
|Shielding Medium||Fixture Efficiency||VCP Rating|
|Clear Prismatic Lens||60-75%||50-70|
|Low-Glare Clear Lens||60-75%||75-85|
|Deep-Cell Parabolic Louver||50-70%||75-95|
|Small-Cell Parabolic Louver||35-45%||99|
Brightness ratios in a space can affect how it is perceived. While high ratios of bright to dark in the space can produce contrast or a sense of drama, it can also be visually fatiguing during transient adaptation, which describes the eye adapting to changes in brightness. This can reduce productivity and can even be hazardous. The right approach is determined by the application; the IESNA has recommended brightness ratios for a wide range of environments.
Uniform light and brightness levels across a space can be desirable but may also be boring; in such cases, sparkle elements, color and/or other methods can be employed to create visual interest without causing fatigue.
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