49
History and Theory of Infrared Technology
chosen. They include: the near infrared
(0.75–3 µm), the middle infrared (3–6
µm), the far infrared (6–15 µm), and the
extreme infrared (15–100 µm). Although
the wavelengths are given in µm
(micrometers), other units are often used
to measure wavelength in this spectral
region, for exmple, nanometer (nm)
and Ångström (Å).
The relationships between the dierent
wavelength measurements is:
10,000 Å = 1,000 nm = 1 µm
Blackbody Radiation
A blackbody is dened as an object
that absorbs all radiation that impinges
on it at any wavelength. The apparent
misnomer black relating to an object
emitting radiation is explained by
Kirchho’s Law (after Gustav Robert
Kirchho, shown in Figure 6), which
states that a body capable of absorbing
all radiation at any wavelength is equally
capable in the emission of radiation.
Figure 6. Gustav Robert Kirchho (1824–1887)
The construction of a blackbody source
is, in principle, very simple. The radiation
characteristics of an aperture in an
isothermal cavity made of an opaque
absorbing material represents almost
exactly the properties of a blackbody. A
practical application of the principle to
the construction of a perfect absorber
of radiation consists of a box that is light
tight except for an aperture in one of
the sides. Any radiation that then enters
the hole is scattered and absorbed
by repeated reections, so only an
innitesimal 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
calibrating thermographic instruments,
such as a FLIR Systems camera, for
example.
If the temperature of blackbody radiation
increases to more than 525°C (977°F), the
source begins to be visible so that it no
longer appears black to the eye. This is
the incipient red heat temperature of the
radiator, which then becomes orange
or yellow as the temperature increases
further. In fact, the denition of the so-
called color 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
blackbody.
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