Page 214 - Algae Anatomy, Biochemistry, and Biotechnology

Working with Light 197

2
projected area of 1 m at a distance of 1 m, a 1 cd light source will similarly produce 1 lm m 22 ,
(Figure 5.12).

Typical Values for Luminous Intensity (in cd)
Flashtube 1,000,000
Automobile headlamp 100,000
100 W incandescent bulb 100
Light emitting diode (LED) 0.0001

LUMINOUS ENERGY
Luminous energy is a photometrically weighted radiant energy. It is measured in lumen per seconds
(lm sec 21 ).

LUMINOUS FLUX DENSITY (ILLUMINANCE AND LUMINOUS EXITANCE)

Luminous flux density is a photometrically weighted radiant flux density. Illuminance is the
photometric equivalent of irradiance, whereas luminous exitance is the photometric equivalent
of radiant exitance.
Luminous flux density is measured in lux (lx) or lumens per square meter (lm m 22 ).

FIGURE 5.12 Luminous intensity: a 1 cd light source emits 1 lm sr 21 in all directions (isotropically).
According to the inverse square law, the intensity varies in inverse proportion to the square of the distance,
that is, at a distance of 2r from the source the intensity will be equal to 1/4lm sr 21 .