Lighting Calculations

First step is determine the occupancy and use of the room. Organizations, such as IES (Illuminating Engineering Society) prescribe recommended illumination for specific room types. For meeting rooms and classrooms, 50 foot-candles are generally recommended; Kitchens are typically 50-60 foot-candeles; Lounges, living rooms, etc. are usually 25-35 foot-candles.

Once proper illumination level is determined, select a specific light fixture. Volumetric, fluorescent (T5 or T8) fixtures are currently king for commercial office areas. The manufacturer's specification sheet for the selected light fixture will provide a table of "coeficients of utilization" for specific room reflectance and "room cavity ratios".

Once these determinations are made, the room cavity ratio, which is based on fixture mounting height, width and length of the room, is calculated and this calculation yields a coeficient of utilization, which, when plugged into the manufacturer's table, yields illumination in foot-candles or lumens.

It seems like a complicated process but it is really fairly simple, and an excel spreadsheet can be easily setup to do the calculations. There are numerous websites that provide guidance if you search under "room cavity ratio", "room lighting calculation", etc. Best is to have an engineer or lighting designer, who is an old hand at this, guide you through the process first time.