FLIR Ranger HRC MS - Wiens Displacement Law

Ranger HRC™ operator´s manual – Theory of thermal imaging
240 Publ. No. TM G007971 Rev. A1 – ENGLISH (EN) – Sept 09. 2008
17.3.2 Wien’s displacement law
By differentiating Planck’s formula with respect to l, and nding the
maximum, we have:
[ ]
m
T
µλ
2898
max
=
This is Wien’s displacements law after Wilhelm Wien, 1864–1928),
which expresses mathematically the common observation that colors
vary from red to orange or yellow as the temperature of a thermal radia-
tor increases. The wavelength of the color is the same as the wavelength
calculated for l
max
. A good approximation of the value of l
max
for a given
blackbody temperature is obtained by applying the rule-of-thumb 3000/
T μm. Thus, a very hot star such as Sirius 11.000 K), emitting bluish-
white light, radiates with the peak of spectral radiant exitance occurring
within the invisible ultraviolet spectrum, at wavelength 0.27 μm.
Figure 17.5 Wilhelm Wien (1864–1928).
The sun approx. 6.000 K) emits yellow light, peaking at about 0.5 μm in
the middle of the visible light spectrum.
At room temperature 300 K) the peak of radiant exitance lies at 9.7 μm,
in the far infrared, while at the temperature of liquid nitrogen (77 K) the
maximum of the almost insignicant amount of radiant exitance occurs
at 38 μm, in the extreme infrared wavelengths.