11
Fundamental Radiation
11.1 Thermal Radiation Intensity and Emissive Power
Any surface can emit energy as long as its surface temperature is greater
than absolute zero. Thermal radiation can refer to (1) surface radiation and
(2) gas or volume radiation. Surface radiation is that radiation which comes
from an opaque material surface (such as a solid or liquid surface,1 μm thick
from the surface). Gas radiation is that radiation which comes from a volume
of gas (such as, CO 2 , H 2 O, CO, or NH 3 ). But gas radiation does not occur
from a volume of air (air cannot emit or absorb radiation energy). Modern
theory describes the nature of radiation in terms of electromagnetic waves
that travel at the speed of light. The various forms of radiation differ only
in terms of wavelength. In this chapter, the discussion will be confined to
thermal radiation [1–4], as shown in Figure 11.1. Thermal radiation primarily
depends on wavelength (spectral distribution, 0.1–100 μm, from visible light
to infrared (IR)), direction (directional distribution, θ, φ), and material temperature (absolute temperature, ◦ K or ◦ R) and material radiation properties.
Figure 11.2 shows the nature of spectral and directional distributions.
What follows try to establish the relation between surface radiation flux
(i.e., radiation rate per unit surface area, or called emissive power) and radiation intensity. Here we assume that radiation intensity from a surface is
given. The later section will discuss how to obtain the radiation intensity value
from Planck. Figure 11.3 shows the conceptual view of a hemispheric radiation from a surface (consider radiation from the upper surface only) [4].
Monochromatic directional radiation intensity (function of wavelength and
temperature in all directions) is defined as differential radiation rate per unit
surface area and unit solid angle,
dq
I λ (θ, ϕ, λ, T) =
(11.1)
dA n dω
where dω is the unit solid angle,
dA n
rdθ · r sin θ · dϕ
dω =
=
= sin θ dθ dϕ
r 2
r 2
221
Fundamental Radiation
11.1 Thermal Radiation Intensity and Emissive Power
Any surface can emit energy as long as its surface temperature is greater
than absolute zero. Thermal radiation can refer to (1) surface radiation and
(2) gas or volume radiation. Surface radiation is that radiation which comes
from an opaque material surface (such as a solid or liquid surface,1 μm thick
from the surface). Gas radiation is that radiation which comes from a volume
of gas (such as, CO 2 , H 2 O, CO, or NH 3 ). But gas radiation does not occur
from a volume of air (air cannot emit or absorb radiation energy). Modern
theory describes the nature of radiation in terms of electromagnetic waves
that travel at the speed of light. The various forms of radiation differ only
in terms of wavelength. In this chapter, the discussion will be confined to
thermal radiation [1–4], as shown in Figure 11.1. Thermal radiation primarily
depends on wavelength (spectral distribution, 0.1–100 μm, from visible light
to infrared (IR)), direction (directional distribution, θ, φ), and material temperature (absolute temperature, ◦ K or ◦ R) and material radiation properties.
Figure 11.2 shows the nature of spectral and directional distributions.
What follows try to establish the relation between surface radiation flux
(i.e., radiation rate per unit surface area, or called emissive power) and radiation intensity. Here we assume that radiation intensity from a surface is
given. The later section will discuss how to obtain the radiation intensity value
from Planck. Figure 11.3 shows the conceptual view of a hemispheric radiation from a surface (consider radiation from the upper surface only) [4].
Monochromatic directional radiation intensity (function of wavelength and
temperature in all directions) is defined as differential radiation rate per unit
surface area and unit solid angle,
dq
I λ (θ, ϕ, λ, T) =
(11.1)
dA n dω
where dω is the unit solid angle,
dA n
rdθ · r sin θ · dϕ
dω =
=
= sin θ dθ dϕ
r 2
r 2
221