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1: Definitions of and Relations between Quantities used in Radiation Theory

1: Definitions of and Relations between Quantities used in Radiation Theory

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Mar 5, 2022
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- 1.1: Introduction to Radiation Theory

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Topic hierarchy
Tag directory

1.1: Introduction to Radiation Theory
1.2: Radiant Flux or Radiant Power, $\phi$ or P
1.3: Variation with Frequency or Wavelength
1.4: Radiant Intensity, I
1.5: "Per unit"
1.6: Relation between Flux and Intensity
1.7: Absolute Magnitude
1.8: Normal Flux Density F
1.9: Apparent Magnitude
1.10: Irradiance E
1.11: Exitance M
The exitance of an extended surface is the rate at which it is radiating energy (in all directions) per unit area. The usual symbol is M and the units are W m⁻² . It is an intrinsic property of the radiating surface and is not dependent on the position of an observer.
1.12: Radiance L
1.13: Lambertian Surface
A lambertian radiating surface is one whose intensity varies with angle according to Lambert's Law. The radiance of a lambertian radiating surface is independent of the angle from which it is viewed. Lambertian surfaces radiate isotropically. The radiance of a black body is lambertian. Since the Sun exhibits limb-darkening; the Sun is not a black body, nor is it lambertian.
1.14: Relations between Flux, Intensity, Exitance, Irradiance
1.15: A= $\pi$ B
1.16: Radiation Density (u)
This is merely the radiation energy density per unit volume, expressed in J m⁻³ , and usually given the symbol u .
1.17: Radiation Density and Irradiance
1.18: Radiation Pressure (P)
Photons carry momentum h/λ and hence exert pressure. Pressure is rate of change of momentum (i.e. force) per unit area.

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