CHAPTER 7 INFRARED PRINCIPLES

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