This coefficient of utilization table is published for a given indirect general lighting product considered for installation in a room with a 20 percent floor reflectance, 30 percent ceiling reflectance and 50 percent wall reflectance.

The room cavity, defined as the volume of space between the luminaires and the workplane, is 5 ft. high x 15 ft. long by 15 ft. wide, so the room cavity ratio, using the formula RCR = [5 x H x (L + W)] ÷ (L x W), would be 3. The dimensions are fixed and cannot be changed, so looking up an RCR of 3 in the below table, we get a CU of 0.19.

If Required Lumens = [Maintained Illumination (fc) x Area (sq.ft.)] ÷ (Ballast Factor x CU x Light Loss Factors), and our maintained light level is 35 footcandles, ballast factor is 0.88 and the overall light loss factor is 0.75, then 63,000 lumens are required for this 225-sq.ft. space.

Now suppose lighter finishes are used that raise ceiling reflectance to 80 percent—this is, after all, a 100 percent uplight luminaire, so 80-90 percent is recommended—with wall reflectance increased to 70 percent for non-window walls and 80 percent for window walls (to reduce contrast between bright windows and their surround). Looking at the CU table for the luminaire, CU is now 0.64. And our lumen requirement drops to about 19,000 lumens—an instant 70 percent savings on capital and operating costs.

As shown, controlling surface reflectances can impact overall application efficiency, particularly when indirect luminaires are being considered.

Craig DiLouie

See Full Bio