What is a Blackbody and Infrared Radiation?

In 1800, the English astronomer Sir William Herschel observed that while using a prism to spread sunlight  into a color swath, he detected changes in temperature when he moved a blackened thermometer across the spectrum of colors. Herschel found that the heating effect increased toward the red and continued to increase as he moved the thermometer into the dark portion beyond the red end of the visible light spectrum, He found the maximum heating occurred considerably beyond the red, in the region we now call "infrared". All objects radiate infrared energy (unless the object has a temperature of absolute zero).

The maximum energy which can be radiated by an object is called the blackbody radiation. A blackbody is a theoretical object, (i.e. emissivity = 1.0), which is both a perfect absorber and emitter of radiation. Common usage refers to a source of infrared energy as a "blackbody" when it's emissivity approaches 1.0 (usually e = 0.99 or better) and as a "graybody" if it has lower emissivity.

Atmospheric absorption of infrared energy or the use of optical windows and lenses can also significantly affect measured results.

The radiation given off by a blackbody occurs in a wide range or spectrum of wavelengths and, based on careful measurements and quantum theory, Max Planck produced an equation to model the observed blackbody radiation curve. His discovery is considered to be one of the most important in the field of quantum physics.

A hot object emits  radiation in a range of wavelengths of varying intensities. The radiation spectrum has a shape like that of the graph below with the peak wavelength directly related to the source temperature. As an object becomes hotter, the wave length becomes shorter and the total energy emitted increases. At around 500°C there is enough emitted energy in the visible spectrum to be seen as a red glow, changing to yellow as the temperature increases. In the spectrum of energy that is visible to the human eye, the peak wave length which relates to a specific color is at times referred to as the "color temperature".

The Planck energy distribution formula, describing the energy density per unit time per unit wavelength, is below:

which is known as Planck's Radiation Law. (A watt is a joule per second, i.e., W = J/s).

While this equation is complicated, both conceptually and analytically, the Planck curve predicts the power per unit area per wavelength produced by blackbody radiation given nothing more than the radiative temperature. The radiative spectrum for any object can be predicted to good accuracy by this model. The rate at which energy is radiated per unit area by an object is called the power or radiant exitance and this can be found by integrating the area under the graph.