Infrared, meaning "below the red", describes a specific span of the electromagnetic spectrum, which is a scale used to classify various forms of radiating energy. Electromagnetic radiation is everywhere and comes in many forms. Visible light that comes from a lamp and radio waves used in cell phones, are both examples of electromagnetic radiation; other examples are microwaves, X-Rays and of course Infrared energy.

Notice in the EM diagram, the different types of radiation. Also, note where Infrared lies, between visible light and microwaves. As you may have noticed, the electromagnetic spectrum describes a fairly broad categorical array of EM radiation. Each form is produced and detected in very different ways, but are all the same in that they can be commonly described in terms of a stream of photons (massless particles) moving in a wavelike pattern at the speed of light. Each photon contains a certain amount (or bundle) of energy, and all electromagnetic radiation consists of these photons. The only difference between the various types of electromagnetic radiation is the amount of energy found in the photons.

Radio waves have photons with low energies, microwaves have a little more energy than radio waves, Infrared has still more, then visible light, ultraviolet, X-Rays, and the most energetic of all are gamma-rays. Hotter, more energetic objects and events create higher energy radiation than cool objects. As a natural effect of metabolism, humans are constantly releasing varying levels of energy in the Infrared wavelength; this information can be expressed or measured as heat. This concept is a key to understanding how Infrared technology can see heat or body temperature in humans.

Unlike X-Ray, sonography, magnetic resonance imaging (MRI) and virtually all other modern medical imaging modalities designed to capture anatomical information, medical Infrared technology is designed to capture physiological information.

Physiological imaging such as Infrared used in Thermology, images the natural superficial Infrared (heat) emission of the skin and accurately represents its temperature. The temperature of the skin is a complex phenomenon that is very much influenced by the perfusion of small blood vessels regulated by the vaso-constrictive and vaso-dilatory influences of the autonomic nervous systems sympathetic and parasympathetic components respectively.

However, skin temperature also is affected by larger blood vessels and the metabolic character of tissues deeper in the body. These energy features "float" to the skin surface and can greatly influence its temperature. The energy features are characteristically diffused by depth and non-homogeneity of the underlying tissue. While characteristics of skin perfusion are of interest in Thermology studies of the peripheral nervous system, more typically the vascular and metabolic features of deeper tissue is of principle interest to a diagnostic study.

Modern imaging devices provide a way for physicians to instantly capture large arrays (images) of quantitative thermographic information. In this image, each pixel represents a different temperature. for example: the new Eclipse imaging system by IMS provides a 320/240 pixel resolution, this means there are 76,800 separate points of recorded temperature data in each image.