Q: I have a room defined on my drawing that shows a value for Foot-candles Actual. I’ve also calculated the average foot-candles for the room at workspace height using point-by-point photometrics, which shows a different value. Why are these values different?

A: The foot-candle calculation for rooms is less precise than the calculation for point-by-point photometrics.

The room calculation uses the zonal cavity method, which is an older formula used before computers were powerful and available enough to perform more sophisticated calculations. Only the size of the room and the total lumens produced by a single type of light fixture are taken into account; multiple light fixture types are not accounted for. The calculation does not account for light fixture placement within the room; it assumes you have a competent lighting design. The calculation also does not make full use of IES files. For more information about how this calculation is used in the software, see the Edit Room Label List page of the user manual.

Point-by-point calculations account for the size of the room, the placement and properties of each light fixture, all information included in IES files, and more; the result is a much more accurate and precise calculation.

Typically, the foot-candle calculation for rooms should only be relied on for general estimations. If you are using multiple light fixture types, are not sure about the quality of your lighting design, have IES files for your fixtures, or just want to be sure you’re getting accurate figures, use point-by-point photometrics.