DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

Eq B.1
Radiant exitance sums emitted and
reflected light. The former term
emittance excludes reflected light.
Some thermal engineers use the
term radiosity for radiant exitance.
In computer graphics, radiosity
refers to a specialized technique to
compute illumination; it does not
refer to any particular quantity.
In photography, the symbol I
is often used for irradiance or
illuminance, instead of intensity.
Radiometry and photometry involve light, in space. No
surface is necessary! Light properties may be described
at a real or imaginary surface; however, they are not
properties of a surface. Absorptance (α), reflectance (ρ),
and transmittance (τ) are intrinsic properties of surfaces,
not properties of light.
In what follows, I use the usual physics convention of
writing letter symbols in italics and units in Roman type.
Radiometry
Radiometry starts with energy (symbol Q) at wavelengths
between 100 nm and 1 mm. Energy is
expressed in units of joules, J. A photon’s energy (Qp ) is
related to its wavelength (λ):
Qp h
The rate of flow – or formally, the time-derivative – of
radiant energy is power (P), also known as radiant flux
(F, or preferably, Φ), expressed in units of watts, W.
c −34
h ≈ 6. 6260755⋅10 J⋅s (Planck's constant)
= ;
λ
-1
c ≡ 299972458 m ⋅s
(speed of light)
Radiant flux per unit area – that is, flux density – arriving
at a real or imaginary surface, throughout all directions
in a hemisphere, is irradiance (E). Irradiance is
expressed in units of watts per meter squared, W·m -2
(or W/m 2). Solar irradiance (insolation) at noon is about
1 kW·m -2.
Radiant flux per unit area leaving a surface, in all directions,
is radiant exitance (M). Formally, this is the derivative
of radiant flux with respect to area. Radiant
exitance is expressed in units of watts per meter
squared, W·m -2 . Radiant exitance from a nonemissive
surface is simply its irradiance times its reflectance.
Radiant flux in a specified direction – or formally, radiant
flux per unit solid angle – is radiant intensity
(symbol I ). The unit for intensity is watts per steradian,
W·sr -1 (or W/sr). Intensity must be specified in
a particular direction; it is independent of the
observer’s location.
602 DIGITAL VIDEO AND HDTV ALGORITHMS AND INTERFACES