Source: Living Textbook

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Radiant Exitance

Understanding Radiant Exitance

Radiant exitance, also known as emittance, is a term used in radiometry to describe the radiant flux emitted by a surface per unit area. It is essentially the optical power emitted by a light source over a certain surface area.

Units and Measurement

In the International System of Units (SI), radiant exitance is measured in watts per square meter (W/m²). This measurement is crucial in understanding the amount of optical power emitted by a surface.

Spectral Exitance

Another important quantity is spectral exitance, which measures the exitance per unit frequency or wavelength interval. It is expressed in units of W/(m² Hz) or W/(m² nm).

Example: Blackbody Radiation

A common example of radiant exitance is thermal radiation from a black body at a certain temperature. According to Planck’s law, the spectral radiance of a black body can be used to calculate the spectral exitance and ultimately determine the radiant exitance using the Stefan–Boltzmann law.

The Stefan–Boltzmann law states that the radiant exitance of thermal radiation from a black body is directly proportional to the fourth power of the temperature, with the Stefan–Boltzmann constant being approximately 5.6704 × 10^-8 W m^-2 K^-4.

Conclusion

Understanding radiant exitance is essential in various fields, including radiometry and thermal physics. It provides valuable insights into the emission of optical power from surfaces and is crucial for calculations related to blackbody radiation and other forms of thermal emission.

Source: ResearchGate
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