All matter--animate or inanimate, liquid, solid, or gas--constantly exchanges thermal energy in the form of electromagnetic radiation with its surroundings. If there is a temperature difference between the object in question and its surroundings, there will be a net energy transfer in the form of heat; a colder object will be warmed at the expense of its surroundings, a warmer object cooled. And if the object in question is at the same temperature as its surrounding, the net radiation energy exchange will be zero.
  In either case, the characteristic spectrum of the radiation depends on the object and its surroundings' absolute temperatures. The topic of this volume, radiation thermometry, or more generally, non-contact temperature measurement, involves taking advantage of this radiation dependence on temperature to measure the temperature of objects and masses without the need for direct contact.

Radiation Basics
The development of the mathematical relationships to describe radiation were a major step in the development of modern radiation thermometry theory. The ability to quantify radiant energy comes, appropriately enough, from Planck's quantum theory. Planck assumed that radiation was formed in discrete energy packages called photons, or quanta, the magnitude of which are dependent on the wavelength of the radiation. The total energy of a quantum, E, is found by multiplying Planck's constant, h = 6.6256 x 10^-34, and, the radiation frequency, , in cycles per second.   In 1905, Albert Einstein postulated that these quanta are particles moving at the speed of light, c = 2.9979 x 10⁸ m/s. If these photons traveled at the speed of light, then they must obey the theory of relativity, stating E² = c²p² , and each photon must have the momentum The frequency can be found by dividing the speed of light by its particle wavelength Substituting for momentum:

  From this equation, it is apparent that the amount of energy emitted depends on the wavelength (or frequency). The shorter the wavelength, the higher the energy.
  Emitted radiation consists of a continuous, non-uniform distribution of monochromatic (single-wavelength) components, varying widely with wavelength and direction. The amount of radiation per unit wavelength interval, referred to as the spectral concentration, also varies with wavelength. And the magnitude of radiation at any wavelength as well as the spectral distribution vary with the properties and temperature of the emitting surface. Radiation is also directional. A surface may prefer a particular direction to radiate energy. Both spectral and directional distribution must be considered in studying radiation.
  Wavelength can be thought of as a type of address to find where a ray's energy is located. The map containing all the wavelengths of electromagnetic radiation is called the electromagnetic spectrum (see the inside front cover of this volume). The short wavelengths are the gamma rays, x-rays, and ultraviolet (UV) radiation, containing the highest amount of energy emitted. The intermediate portion of the spectrum, the heat region, extends from approximately 0.1 to 1000 (micrometers or microns: 1,000,000 microns = 1 meter), and includes a portion of the ultraviolet and all of the visible (VIS) and infrared (IR) waves. This portion is termed thermal radiation, and is important in the study of heat transfer and radiation thermometry.   Non-contact temperature sensors work in the infrared portion of the spectrum. The infrared range falls between 0.78 microns and 1000 microns in wavelength, and is invisible to the naked eye. The infrared is region can be divided into three regions: near-infrared (0.78-3.0 microns); middle infrared (3-30 microns); and far infrared (30-300 microns). The range between 0.7 microns and 14 microns is normally used in infrared temperature measurement. The divisions have been related to the transmission of the atmosphere for different types of applications.

Figure2-1:Radiation Energy Balance