Principle of Thermal Imaging

All materials above 0 degrees Kelvin (-273 degrees C) emit infrared energy. The infrared energy emitted from the measured object is converted into an electrical signal by the imaging sensor (microbolometer) in the camera and displayed on a monitor as a color or monochrome thermal image. The basic principle is explained in the following sections.

Infrared Radiation

The infrared ray is a form of electromagnetic radiation the same as radio waves, microwaves, ultraviolet rays, visible light, X-rays, and gamma rays. All these forms, which collectively make up the electromagnetic spectrum, are similar in that they emit energy in the form of electromagnetic waves traveling at the speed of light. The major difference between each ‘band’ in the spectrum is in their wavelength, which correlates to the amount of energy the waves carry. For example, while gamma rays have wavelengths millions of times smaller than those of visible light, radio waves have wavelengths that are billions of times longer than those of visible light.

The wavelength of the infrared radiation ‘band’ is 0.78 to 1000μm (micrometers). This is longer than the wavelength of visible light yet shorter that radio waves. The wavelengths of infrared radiation are classifi ed from the near infrared to the far infrared.

Emissivity
Infrared radiation is energy radiated by the motion of atoms and molecules on the surface of object, where the temperature of the object is more than absolute zero. The intensity of the emittance is a function of the temperature of the material. In other words, the higher the temperature, the greater the intensity of infrared energy that is emitted. As well as emitting infrared energy, materials also refl ect infrared, absorb infrared and, in some cases, transmit infrared. When the temperature of the material equals that of its surroundings, the amount of thermal radiation absorbed by the object equals the amount emitted by the object.


The fi gure above shows the three modes by which the radiant energy striking an object
may be dissipated. These modes of dissipation are:
a = absorption
t = transmission
r = refl ection

The fractions of the total radiant energy, which are associated with each of the above modes of dissipation, are referred to as the absorptivity (a) transmissivity (t) and the refl ectivity (r) of the body. According to the theory of conservation of energy, the extent to which materials refl ect, absorb and transmit IR energy is known as the emissivity of the material.