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 Infrared Theory
Planck's Blackbody Distribution Law
The intensity of
radiation emitted from an object is a function of its temperature,
wavelength, and emissivity. A perfect emitter, also known as a
blackbody, is a material that radiates 100% of the electromagnetic
energy that is theoretically possible for a material at a specified
temperature.
Planck’s blackbody equation defines the intensity of radiation for a
blackbody as a function of temperature and wavelength. Each curve shown in
the figure below represents the energy distribution at a different
temperature. As temperature increases, the intensity of radiation
increases. Additionally, as the temperature of an object increases, the
peak intensity moves to shorter wavelengths. The surface of the sun, at
6000°C, has its peak in the yellow region of the visible portion of the
spectrum, and therefore, appears yellow. We can also observe this
phenomenon by slowly heating a piece of metal. At room temperature, the
metal does not emit any light visible to the human eye. However, if we heat
it to approximately 500°C, it begins to glow red. At 500°C, the metal emits
visible light primarily in the red portion of the visible spectrum. As the
temperature of the metal increases to approximately 1500°C, it begins to
glow white. At 1500°C, the metal emits energy at all visible wavelengths.
White light is the combination of all visible colors.
Objects that emit energy well also absorb energy well. Therefore, a perfect
emitter is also a perfect absorber. The term blackbody arose because a
perfect absorber absorbs all visible electromagnetic energy (light) and
appears black. Blackbodies are useful as a reference to characterize the
behavior of materials. The relationship between the intensity of energy
emitted by a blackbody at various wavelengths as a function of temperature
was discovered by Max Planck in 1900.
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