To quantify the illuminance and energy consumption levels achieved by increasing the ceiling’s light reflectance (LR), ceiling system manufacturer Armstrong World Industries commissioned Brinjac Engineering to conduct two controlled studies. (Illuminance is a measure of the intensity of light on a unit area of a surface.)
In the first study, four different room configurations were outfitted with a direct recessed 2X2-foot parabolic troffer and an indirect pendant. One set of rooms was given a 75% reflective ceiling; the others, a 90% reflective ceiling. After measuring work plane illuminance in both sets of rooms, Brinjac found the 0.90 LR ceiling to increase direct lighting levels by just 2–5%; however, indirect lighting levels jumped by almost 22% and offered greater lighting uniformity.
Although work plane illuminance from direct lighting only increased modestly when changing the light reflectance from 0.75 to 0.90, the effect of indirect lighting on work plane illuminance was significant. In addition, the higher LR value enabled a reduction in indirect luminaire fixtures and a decrease in lighting power density.
Based on these results, specifiers can use fewer fixtures or lower wattage levels to achieve required lighting levels, which can reduce energy costs, as analyzed in the second study where the 90% reflective ceiling tile was installed in the same spaces with an optimized lighting design and compared to the 75% and 90% ceiling with the original lighting scheme.
After comparing the three ceiling designs, the Brinjac researchers found that:
• Spacing between indirect luminaire sections with the 0.90 ceiling could be increased, thereby reducing the total number of luminaires required to achieve light levels afforded by the 0.75 ceiling.
• The 0.90 reflective ceiling with indirect fixtures yielded a 23% lower lighting power density than the 0.75 ceiling, and 21% lower than the parabolic troffer layout.
• This lighting power density reduction was calculated to achieve 9% HVAC energy costs savings, as compared to the 0.75 ceiling, and 7% in relation to the troffer layout.
For more information on this study, see: http://www.armstrong.com/common/c2002/content/files/38652.pdf.