Heat Sources

All bodies emit electromagnetic radiation in the wavelength spectral range and with intensities that correspond to their temperature. This is called heat radiation or blackbody radiation. Spontaneous and stimulated emission by atoms are the basis for the Planck curve (function), which gives the intensity radiated by a blackbody as a function of wavelength for a definite body temperature. The normalized spectral irradiancy of heat sources versus wavelength shows that the emitting spectrum is very broad, and is shifted to IR range by decreasing temperature (see Fig. 3.1). A blackbody is an object that absorbs all the electro­magnetic energy that falls on the object, no matter what the wavelength of the radiation. Many objects made from condensed materials (for instance, metals, tissues) can be consid­ered as blackbodies. The area under the Planck curve increases as the temperature is increased (the Stefan-Boltzmann law); the peak in the emitted energy moves to the shorter wavelengths as the temperature is increased (Wien’s law). Follow Wien’s low maximum of heat radiation spectrum, is the function of body temperature as power four. Thus, strongly heated metals are used in light sources with a broadband visible and infrared radiation. For example, filament lamp with a temperature of 2000K emits light with peak wavelength at approximately 1400 nm.