Using Artificial Light with cool white fluorescent lamps as the standard:
Low designates a minimum high level of 25 foot-candles, preferred level of 75 to 200 foot-candles. (Just enough daylight to read by)
Medium designates a minimum of 75 to 100 foot-candles and a preferred level of 200 to 500 foot-candles. (A well-lighted room with natural light)
High designates a minimum of 200 foot-candles and a preferred level of 500 to 1000 foot-candles. (Many windows, lots of natural light)
Very High designates a minimum of 1000 foot candles and a preferred level of over 1000 foot-candles.
A foot-candle is the illumination of a surface one foot from the light of a standard candle. Light meters measure light intensity in foot-candles, luxes, or micro-Einsteins.