The most common measure of light output (or luminous flux) is the lumen. Light sources are labeled with an output rating in lumens. For example, a T12 40-watt fluorescent lamp may have a rating of 3050 lumens. Similarly, a light fixture’s output can be expressed in lumens. As lamps and fixtures age and become dirty, their lumen output decreases (i.e., lumen depreciation occurs). Most lamp ratings are based on initial lumens (i.e., when the lamp is new).
Light intensity measured on a plane at a specific location is called illuminance. Illuminance is measured in footcandles, which are workplane lumens per square foot. You can measure illuminance using a light meter located on the work surface where tasks are performed. Using simple arithmetic and manufacturers’ photometric data, you can predict illuminance for a defined space. (Lux is the metric unit for illuminance, measured in lumens per square meter. To convert footcandles to lux, multiply footcandles by 10.76.)
Another measurement of light is luminance, sometimes called brightness. This measures light “leaving” a surface in a particular direction, and considers the illuminance on the surface and the reflectance of the surface. The human eye does not see illuminance; it sees luminance. Therefore, the amount of light delivered into the space and the reflectance of the surfaces in the space affects your ability to see.
Luminous flux is commonly called light output and is measured in lumens (lm). Illuminance is called light level and is measured in footcandles (fc). Luminance is referred to as brightness and is measured in footlamberts (fL) or candelas/m2 (cd/m2).
Determining Target Light Levels
The Illuminating Engineering Society of North America has developed a procedure for determining the appropriate average light level for a particular space. This procedure used extensively by designers and engineers recommends a target light level by considering the following.