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TM 9-258
CHAPTER 7
INFRARED PRINCIPLES
discontinuous spectrum source, a line source, or a band
7-1.
General.  Infrared light is considered to be
source.
radiant energy in the band of wavelengths between about
e.  An intense source of infrared radiation can
0.76 and 100 microns. The portion of the band between
be devised using the maser (Microwave Amplification by
0.76 and 3 microns is sometimes referred to as near
Stimulated Emission of Radiation) principle. This type of
infrared light, and the portion between 3 and 100 microns
source is very directional, is concentrated within a narrow
has been called far infrared light. Most of the infrared
spectral interval, and possesses a high degree of
light  band  overlaps  the  heat  radiation  band  of
coherence.
electromagnetic radiation. By definition, infrared pertains
7-3.
Thermal  Radiation.
The  electromagnetic
to or designates those radiations, such as are emitted by
a hot body, with wavelengths just beyond the red end of
energy that is emitted from the surface of a heated body
the visible spectrum.
is called thermal radiation. This radiation consists of a
continuous spectrum of frequencies extending over a
7-2.
Infrared Radiation.
wide range. The spectral distribution and the amount of
a.  The term infrared radiation is used to
energy radiated depend chiefly on the temperature of the
describe electromagnetic radiation whose wave-length
emitting surface. Careful measurements show that at a
lies just beyond the red end of the visible spectrum, and
given temperature there is a definite frequency or
the beginning of the region that can be detected by
wavelength at which the radiated power is maximum,
microwave radio techniques.
The distinction, for
although this maximum is very broad. In addition, the
example,  between  infrared  astronomy  and  radio
frequency of the maximum is found to vary in direct
astronomy is, therefore, an arbitrary one based entirely
proportion to the absolute temperature.  This rule is
on differences in detection techniques.
known as Wien's law.
At room temperature, for
b.  A  study  of  source  characteristics  is
example, the maximum occurs in the far-infrared region
important because of the help such knowledge provides
of the spectrum, and there is no perceptible visible
in the choice of detectors and the design of optical
radiation emitted.  But at higher temperatures, the
elements. In many applications the radiation source is
maximum shifts to correspondingly higher frequencies.
not subject to control. This is most often true for passive
Thus at about 5000C and above, a body glows visibly.
systems where the objects are detected by their natural
7-4.
Infrared Detectors.
radiation. It also may be true for those active systems
a.  The  central  element  in  any  infrared
that use a source for the illumination of a scene so that
detection system is the detector.  This is the device
objects may be detected by the reflected radiation.
c.  The natural division or grouping of infrared
which transduces the energy of the electromagnetic
radiation falling upon it into some other form (in most
sources depends upon the nature of the wavelength
cases, electrical). The mechanisms used to perform this
distribution of the emitted energy. One type of source
function are broadly classified as either photon detectors
emits radiation over a very broad and continuous band of
or thermal detectors.
wavelengths. A plot of its emission versus wavelength is
b.  When radiation of wavelength less than a critical
a smooth curve which usually passes through only one
value falls upon the surface of certain materials,
maximum. This type is called a continuous spectrum
electrons are emitted from the surface. Photo tubes
source, or simple a continuous source.
d.  Another type of source is one which
using this photoemissive effect are very widely used.
However, the nature of the effect is such that
radiates strongly is some relatively narrow spectral
photoemissive detectors operate in the ultraviolet, visible,
intervals, but, in other wavelength intervals, the source
or very near infrared.  The photoemissive effect, also
does not radiate at all.  A plot of emission versus
termed the external photoelectric effect or simply the
wavelength reveals a series of emission bands or lines.
photoelectric effect, was discovered by Hertz in 1887 and
The curve is discontinuous and the source is called a
explained by Einstein in 1905. The image converter is
the only type of detector which has found much
application in infrared technology, but its use is
7-1

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