Lighting Research Center Lighting Research Center
    Volume 6 Issue 1
July 2002    
How do luminaire manufacturers prevent glare?

The smaller bulb diameters and higher light outputs of T5 lamps increase the light emitted per unit area—or the luminance—of the lamp bulb walls. A T5 or T5 high output (T5 HO) lamp has 60% of the surface area of a T8 lamp. Because, however, a standard T5 lamp has nearly identical light output to a T8 lamp (2,900 lumens versus 2,950 lumens), the surface luminance of a standard T5 lamp is 1.64 times as much as a T8 lamp. Further, the light output of a T5 HO lamp is 5,000 lumens, so the surface luminance is 2.83 times as high as a T8 lamp (see the CASE STUDY Case Study). The higher surface luminance, without any shields or diffusers, results in more glare and discomfort for room occupants. Unless the lamps are properly shielded, these high-luminance light sources may also annoy workers by reflecting off of computer screens or other visual display units.

To avoid glare from the high-luminance bulb walls of T5 lamps, luminaire manufacturers design indirect and direct/indirect luminaires that keep the lamps out of sight. Acrylic guide panels [see Figure 18-2.], parabolic louvers, and metal mesh filters are often used with these luminaires. Direct luminaires with parabolic louvers, metal mesh filters, or diffusers may be able to reduce glare. When using louvers, shielding angles must be at least 30° in open ceiling luminaires in order to be effective. A recent study suggests that even a luminaire overhead can cause discomfort glare when the luminance of the luminaires is high enough, such as bare T5 or T5 HO lamps (Ngai & Boyce 2000). If a typical diffuser is used, a distance of at least 20 millimeters (0.8 inches) is needed between the lamps and the diffuser in order to obtain effective diffusion (Philips 2001/2002). Designers should realize, though, that indirect lighting and most glare control filters or diffusers reduce the amount of light delivered to the workplane compared to unshielded direct fixtures.

Another measure to avoid glare is to decrease the apparent size of high-luminance light sources. For example, in high-bay applications, such as factories and warehouses, even high-luminance luminaires using T5 HO lamps may be acceptable because the apparent size of the luminaires near the high ceiling is small to the observer.

© 2002 Rensselaer Polytechnic Institute. All rights reserved.

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