Having obtained the number of footcandles
, the next step is to determine the spacing of the lamps and their height.
Television coverage requires much higher light levels than those given in the standards, including footcandles
on vertical surfaces.
The amount of illumination is expressed in a footcandles
rating as read by the testing method described.
Houghton recommends comparing the light levels in each space to the code for your city, county or state: "Look at the minimum of light you have to have, then walk around the parking lot, sidewalk and sides of the building with a footcandle
meter to see if you're within the codes in regards to minimum light.
The National Institutes of Health's design guide lines specify that laboratories should have lighting levels of approximately 80 footcandles
. For surface mounted lighting, they recommend wrap-around type diffusers.
Once the metal-halide lights were installed, 85% of the yard was lit at 2.0 footcandles
, 15 times more light than the fluorescents provided.
Selkowitz also referred to the 2013 field study of the New York Times Company building which was designed in 2003, at which time the client did not know how much illuminance was needed for employees in each of the many diverse divisions of the company; some visual tasks required 50 footcandles
, some 40 fc and some 30 fc.
The lamp has a rated life of 4000 hours, and provides 3700 footcandles
at 36 inches from the work plane.
One-hundred-and-eighteen Prismalume fixtures are installed at 20 ft., with illumination levels at 80 to 90 footcandles
. As noted, fixtures are arranged in a checkerboard pattern, spaced at 35 ft.
All residential streets are lighted with 100-watt HPS lamps offering 0.38 footcandles
(maintained) and a uniformity ration of 6:1; collector streets use 200-watt HPS lamps that offer 0.84 footcandles
(maintained) and a uniformity ratio of 4:1.
The formerly washed-out parking area is now cheerfully lit, with an average light level of 3.1 footcandles
(a 109% increase) and uniformity at an average/minimum ratio of 4.4:1.
The best lighting advice: How perceived lighting is more than actual footcandles
. I carry a light meter around with me often, and I'm amazed at how effective vertical lighting can be to make the space look more illuminated than the meter measures.