Definitions

Luminous intensity, I, is the solid angular flux density in a given direction measured in candlepower in American National Standards Institute (ANSI) units and candela (cd) in SI units. The candela and candlepower have the same magnitude. See Figure 6.1.

Figure 6.1: Relationship of light source, illumination, transmittance, and reflectance. ( Source: GE lighting Business Group )

Lumen (lm) is the unit of luminous flux equal to the flux in a unit solid angle of 1 steradian (sr) from a uniform point source of 1 cd. On a unit sphere, an area of 1 ft ² (or 1 m ²) will subtend an angle of 1 sr. Because the area of a unit sphere is 4 pi, a source of 1 candlepower (1 cd) produces 12.57 lm.

Illuminance (E) is the density of luminous flux incident on a surface in lumens per unit area. One lumen uniformly incident on 1 ft ² of area produces an illuminance of one footcandle. The unit of measurement, therefore, is the footcandle (fc) in ANSI units. In SI units, the measurement is lux (lx), or lumens per square meter.

As a rule of thumb, 10 lx is taken as being approximately equal to 1 fc.

Luminance, L, is the luminous flux per unit of projected area (apparent) area and a unit solid angle leaving a surface, either reflected or transmitted. The unit is the footlambert (fL), in which...