The recommended illumination levels for interior lighting are measured in lux or lumen. Illumenance is measured by "foot candles." According to Engineering Tool Box: "A foot candle is actually one lumen of light density per square foot, one lux is one lumen per square meter." Engineering Toolbox also states that the recommended indoor light level is 500 to 1,000 lux. However, the light level may increase to 2,000 in areas where detailed work is being done, such as in a workshop or office.
Low Light Levels (20 to 250 Lux)
Interior areas with lux levels below 250 are considered low light. For example, public areas that have dark surroundings, like parking garages, may have lux levels of 20 to 50. Areas where individuals spend a short amount of time and do not undertake visual tasks may have light levels of 50 to 100 lux. A working area where visual tasks are sometimes performed; like a storage closet, may have a lighting level of 100 to150 lux. Most homes, warehouses and theatres have lux levels around 150. Classrooms and some offices may have lux levels of 250.
Normal Light Levels (500-1000 Lux)
Most offices, areas with computers, libraries, grocery stores, show rooms and labs have lux levels of 500. Large supermarkets, technical offices (like architecture) or mechanic workshops have lux levels of 750. Areas where drawing work or detailed mechanical work is taking place will typically have lux levels of 1,000.
Highlight Levels (1,500 to 20,000 Lux)
Very detailed drawing or mechanical work areas will have 1,500 to 2,000 lux levels. Areas where work is being performed on object that require increased contrast or that are very small in size (microchips) may require 2,000 to 5,000 lux light levels. Areas where specialised work is being performed for long hours on very small objects could require anywhere from 5,000 to 20,000 lux.
Lux Levels and the Human Eye
The human eye and each person have preferences for certain light levels. Some people prefer low light, others prefer lots of light. The type of environment is important to the light level as well. A small intimate romantic restaurant may want lower lux levels to create a softer feeling for the eyes, while a kid's playroom may need high lux levels to keep the children awake and alert.