FLIR Ranger HRC MS - Page 254

Ranger HRC™ operator´s manual – Theory of thermal imaging
244 Publ. No. TM G007971 Rev. A1 – ENGLISH (EN) – Sept 09. 2008
For highly polished materials ε
l
approaches zero, so that for a perfectly
reecting material i.e. a perfect mirror) we have:
1=
λ
ρ
For a graybody radiator, the Stefan-Boltzmann formula becomes:
[ ]
24
/ mWM Τ= εσ
This states that the total emissive power of a graybody is the same as a
blackbody at the same temperature reduced in proportion to the value of
ε from the graybody.
1
2
5
4
3
Figure 17.8 Spectral radiant exitance of three types of radiators. 1: Spectral radiant exi-
tance; 2: Wavelength; 3: Blackbody; 4: Selective radiator; 5: Graybody.
1
2
5
4
3
1,0
0,5
0
Figure 17.9 Spectral emissivity of three types of radiators. 1: Spectral emissivity; 2: Wavel-
ength; 3: Blackbody; 4: Graybody; 5: Selective radiator.