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Figure 1.1
Flux is a measure of radiant power.
formed from simultaneously changing electric and magnetic fields. At a very
small scale, light also behaves as if it exists in discrete bundles or packets (pho-
tons). However, at the macroscopic scale where rendering is normally done, par-
ticle effects really do not come into play, and wave effects have limited impact.
In real environments, light is more naturally regarded as a flow of energy, the
measurement of which is known as radiometry . The physical quantities of light
measurement are called radiometric quantities. 1
1.1.1 Radiant Energy and Flux
A basic physical (radiometric) quantity of light is flux
Φ
, which is defined as the
radiant energy Q per time:
dQ
dt .
Φ =
(1.1)
Flux, the temporal flow of radiant energy, is thus a measurement of radiant power
and is typically expressed in watts. Flux normally refers to the radiant power
reaching or flowing through some real or hypothetical surface. In the case of a
light source, the flux through an imaginary sphere centered at the source repre-
sents the total flux output of the source ( Figure 1.1 ) . This flux divided by 4
π
,the
area of a unit sphere, is the intensity of the source.
1.1.2 Irradiance
Flux measures the total radiant power reaching a surface, but without regard to
surface position: the power might be concentrated on only part of the surface, or
spread evenly across it. A position-dependent measure of radiant power is flux
1 Terminology of radiometry is not entirely standardized. The notation here follows Chapter 2 of
Radiosity and Realistic Image Synthesis by Cohen and Wallace [Cohen and Wallace 93]; the chapter
was written by Pat Hanrahan.
 
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