(t = Total flux transmitted
Da= Total flux absorbed
Equation 3-5 is often expressed as a fraction of the total incident flux and can be re-
written as follows:
l=p+r+a (3-6)
p = % of flux reflected by the surface (reflectivity)
T = % of total flux transmitted (transmissivity)
a = % of total flux absorbed absorptivityy)
1)1
Figure 3-1. Incident radiation ((i) is reflected, transmitted or absorbed (Maldague
2001)
For simplicity, the discussion will now be limited to opaque objects that do not
transmit incident radiation. If this is the case, Equation 3-6 can be simplified as:
1=p+a (3-7)
p = % of flux reflected by the surface (reflectivity)
a = % of total flux absorbed absorptivityy)
Finally, the relationship between absorptivity and emissivity can be expressed by
Kirchoff s law which states that the two quantities are equal.
Figure 3-2 A shows the electromagnetic (EM) radiation emission curves for several
common objects at different temperatures. The curves illustrate that the intensity
(brightness) of the EM emissions increase with the object's temperature and that the
wavelength containing the peak intensity increases as temperature decreases.