FLIR Ranger HRC MS - Non-Blackbody Emitters

Ranger HRC™ operator´s manual – Theory of thermal imaging
242 Publ. No. TM G007971 Rev. A1 – ENGLISH (EN) – Sept 09. 2008
Figure 17.7 Josef Stefan (1835–1893) and Ludwig Boltzmann (1844–1906).
Using the Stefan-Boltzmann formula to calculate the power radiated by
the human body, at a temperature of 300 K and an external surface area of
approx. 2 m
2
, we obtain 1 kW. This power loss could not be sustained if it
were not for the compensating absorption of radiation from surrounding
surfaces, at room temperatures which do not vary too drastically from the
temperature of the body – or, of course, the addition of clothing.
17.3.4 Non-blackbody emitters
So far, only blackbody radiators and blackbody radiation have been dis-
cussed. However, real objects almost never comply with these laws over
an extended wavelength region – although they may approach the black-
body behavior in certain spectral intervals. For example, a certain type
of white paint may appear perfectly white in the visible light spectrum,
but becomes distinctly gray at about 2 μm, and beyond 3 μm it is almost
black.
There are three processes which can occur that prevent a real object from
acting like a blackbody: a fraction of the incident radiation α may be ab-
sorbed, a fraction ρ may be reected, and a fraction t may be transmit-
ted. Since all of these factors are more or less wavelength dependent, the
subscript l is used to imply the spectral dependence of their denitions.
Thus:
• The spectral absorptance α
l
= the ratio of the spectral radiant power
absorbed by an object to that incident upon it.
• The spectral reectance
ρ
l
= the ratio of the spectral radiant power
reected by an object to that incident upon it.
• The spectral transmittance
t
l
= the ratio of the spectral radiant power
transmitted through an object to that incident upon it.