Definitions
149
with a sun zenith angle of 60 degrees, the median wavelength in the 400
to 700 nm waveband is about 550 nm, for which the photon energy is
2.17 x 10' Jlmol. This is an approximate but useful number for converting between total energy in the PAR waveband and number of photons
for solar radiation. This conversion factor, which is often expressed as
4 . 6 ~
mol quanta per J, is within about ten percent of the estimate for
cloudy skys and various artificial lights used in growth chambers.
10.2 Blackbody Radiation
As was mentioned, photons are emitted or absorbed because of discrete
energy transitions in the emitting or absorbing medium. Each allowable
transition produces photons at a single wavelength. If there are many allowable transitions at closely spaced energy levels, the spectral lines tend
to merge into an emission or an absorption band. If there are an infinite
number of transitions spaced throughout the electromagnetic spectrum,
the medium is a perfect radiator or absorber. It will absorb all radiation
falling upon it and will radiate the maximum amount of energy that a
medium at its temperature is capable of radiating. Such a medium is
called a blackbody or full radiator. No such material exists in nature,
but some materials approach this behavior over parts of the electromagnetic spectrum. Thus we may speak of a blackbody radiator at visible
wavelengths or a blackbody radiator at thermal wavelengths, but would
not necessarily expect the same material to be a blackbody in both wave
bands. Snow is a very poor absorber of visible radiation, but almost a
perfect blackbody in the far infrared.
10.3 Definitions
Radiative transfer results in the transport of energy from a source through
a medium to a receiver. This exchange of energy is characterized by the
direction of the ray between the source and the receiver, wavelength of
radiation, time, coordinates of the point of interest, and the area of the
region under consideration. A slight variation on this idea occurs when a
surface is being viewed from one direction while an incident ray is from
another direction. Although the object being viewed could be considered
the source and the viewer could be considered the receiver, it is more
useful if both source and view directions are considered simultaneously
and so the term bi-directional arises because of the two directions: source
and view. Terminology in radiative transfer can be quite complex so the
terms used here are only a small subset of the complete terminology.
Nicodemus et al. (1977) give a much more extensive set of terms and
definitions.
Terms that we use are listed as follows:
RadiantJEux (W): The amount of radiant energy emitted, transmitted,
or received per unit time.
Radiantpux density (@, w/m2): Radiant flux per unit area.
149
with a sun zenith angle of 60 degrees, the median wavelength in the 400
to 700 nm waveband is about 550 nm, for which the photon energy is
2.17 x 10' Jlmol. This is an approximate but useful number for converting between total energy in the PAR waveband and number of photons
for solar radiation. This conversion factor, which is often expressed as
4 . 6 ~
mol quanta per J, is within about ten percent of the estimate for
cloudy skys and various artificial lights used in growth chambers.
10.2 Blackbody Radiation
As was mentioned, photons are emitted or absorbed because of discrete
energy transitions in the emitting or absorbing medium. Each allowable
transition produces photons at a single wavelength. If there are many allowable transitions at closely spaced energy levels, the spectral lines tend
to merge into an emission or an absorption band. If there are an infinite
number of transitions spaced throughout the electromagnetic spectrum,
the medium is a perfect radiator or absorber. It will absorb all radiation
falling upon it and will radiate the maximum amount of energy that a
medium at its temperature is capable of radiating. Such a medium is
called a blackbody or full radiator. No such material exists in nature,
but some materials approach this behavior over parts of the electromagnetic spectrum. Thus we may speak of a blackbody radiator at visible
wavelengths or a blackbody radiator at thermal wavelengths, but would
not necessarily expect the same material to be a blackbody in both wave
bands. Snow is a very poor absorber of visible radiation, but almost a
perfect blackbody in the far infrared.
10.3 Definitions
Radiative transfer results in the transport of energy from a source through
a medium to a receiver. This exchange of energy is characterized by the
direction of the ray between the source and the receiver, wavelength of
radiation, time, coordinates of the point of interest, and the area of the
region under consideration. A slight variation on this idea occurs when a
surface is being viewed from one direction while an incident ray is from
another direction. Although the object being viewed could be considered
the source and the viewer could be considered the receiver, it is more
useful if both source and view directions are considered simultaneously
and so the term bi-directional arises because of the two directions: source
and view. Terminology in radiative transfer can be quite complex so the
terms used here are only a small subset of the complete terminology.
Nicodemus et al. (1977) give a much more extensive set of terms and
definitions.
Terms that we use are listed as follows:
RadiantJEux (W): The amount of radiant energy emitted, transmitted,
or received per unit time.
Radiantpux density (@, w/m2): Radiant flux per unit area.