Blackbody Radiation; Planck's Law - FLIR ThermaCAM PM575 Operator's Manual

[9.3 — Blackbody radiation]
The relationships between the different wavelength measurements is
10 000 Å = 1 000 nm = 1µ = 1 µm
9.3

Blackbody radiation

A blackbody is defined as an object which absorbs all radiation that impinges on it
at any wavelength. The apparent misnomer »black« relating to an object emitting
radiation is explained by Kirchhoff's Law, which states that a body capable of
absorbing all radiation at any wavelength is equally capable in the emission of
radiation.
The construction of a blackbody source is, in principle, very simple. The radiation
characteristics of an aperture in an isotherm cavity made of an opaque absorbing
material represents almost exactly the properties of a blackbody. A practical appli-
cation of the principle to the construction of a perfect absorber of radiation con-
sists of a box that is light tight except for an aperture in one of the sides. Any radi-
ation which then enters the hole is scattered and absorbed by repeated reflections
so only an infinitesimal fraction can possibly escape. The blackness which is
obtained at the aperture is nearly equal to a blackbody and almost perfect for all
wavelengths.
By providing such an isothermal cavity with a suitable heater it becomes what is
termed a »cavity radiator«. An isothermal cavity heated to a uniform temperature
generates blackbody radiation, the characteristics of which are determined solely
by the temperature of the cavity. Such cavity radiators are commonly used as
sources of radiation in temperature reference standards in the laboratory for cali-
brating thermographic instruments, such as a ThermaCAM™ 500 series camera
for example.
If the temperature of blackbody radiation increases to more than 525 °C, the
source begins to be visible so that it appears to the eye no longer black. This is the
incipient red heat temperature of the radiator, which then becomes orange or yel-
low as the temperature increases further. In fact, the definition of the so-called
»colour temperature« of an object is the temperature to which a blackbody would
have to be heated to have the same appearance.
Now consider three expressions that describe the radiation emitted from a black-
body.
9.3.1

Planck's law

Max Planck was able to describe the spectral distribution of the radiation from a
blackbody by means of the following formula:
52 ThermaCAM™ PM575/595 Operator's Manual