Blackbody radiation describes the physics process by which all heated objects emit radiation to their surroundings. Based on this principle, all the objects with temperature above 0 K emit heat (radiation) and the wavelength distribution of this radiation depends on the temperature of the object. For instance, as objects are heated beyond the red temperature, approximately 700 °C, the amount of radiation being emitted in the visible range increases. However, the most of the light being emitted by hot objects is infrared.

All blackbodies at the same temperature emittes radiation with the same spectral distribution, regardless of their composition. The spectral distribution of the radiation emitted by a blackbody is known as spectral radiancy, R_T(v), where v is the frequency of the radiation. The quantity R_T(v) is defined, and thus R_T (v)dv is the energy emitted per unit time in the frequency range v to v + dv from a unit area on a surface at temperature T.

Stefan's Law describes the relationship between the radiancy and temperature:
         R(T) = σT⁴ ------------------------------------------------- [4908a]
where,
         σ - Stefan-Boltzmann constant.

The relationship between the wavelength (λ) peak and temperature is given by Wein's displacement law,
          ---------------------------------- [4908b]
where,
         λ -- the wavelength in µm,
         T -- the temperature in degrees Kelvin.

Figure 4908 (a) shows that even for objects at 2000 K, there is only a small portion of the radiated energy in the visible portion of the spectrum. The majority of the information relating to the object's temperature is well into the infrared region. In the semiconductor industry, only few devices operate at such high temperatures. Figure 4908 (b) shows the temperatures around and slightly above room temperature with the particular interest between 300 and 350 K. It is clear that most of the energy is emitted at wavelengths greater than 5 µm, which is in the infrared region. Then, for such thermal analysis, the detector needs to be sensitive well into the infrared range.

(a)	(b)

[1] Paiboon Tangyunyong and Christian Schmidt, Thermal Defect Detection Techniques, Sandia National Laboratories is a multimission laboratory managed and operated by National Technology & Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.