Spectral radiance

In radiometry, spectral radiance or specific intensity is the radiance of a surface per unit frequency or wavelength, depending on whether the spectrum is taken as a function of frequency or of wavelength. The SI unit of spectral radiance in frequency is the watt per steradian per square metre per hertz (W·sr−1·m−2·Hz−1) and that of spectral radiance in wavelength is the watt per steradian per square metre per metre (W·sr−1·m−3)—commonly the watt per steradian per square metre per nanometre (W·sr−1·m−2·nm−1). The microflick is also used to measure spectral radiance in some fields.[1][2]

Spectral radiance gives a full radiometric description of the field of classical electromagnetic radiation of any kind, including thermal radiation and light. It is conceptually distinct from the descriptions in explicit terms of Maxwellian electromagnetic fields or of photon distribution. It refers to material physics as distinct from psychophysics.

For the concept of specific intensity, the line of propagation of radiation lies in a semi-transparent medium which varies continuously in its optical properties. The concept refers to an area, projected from the element of source area into a plane at right angles to the line of propagation, and to an element of solid angle subtended by the detector at the element of source area.[3][4][5][6][7][8][9]

The term brightness is also sometimes used for this concept.[3][10] The SI system states that the word brightness should not be so used, but should instead refer only to psychophysics.

The geometry for the definition of specific (radiative) intensity. Note the potential in the geometry for laws of reciprocity.
  1. ^ Palmer, James M. "The SI system and SI units for Radiometry and photometry" (PDF). Archived from the original (PDF) on August 2, 2012.
  2. ^ Rowlett, Russ. "How Many? A Dictionary of Units of Measurement". Retrieved 10 August 2012.
  3. ^ a b Planck, M. (1914) The Theory of Heat Radiation, second edition translated by M. Masius, P. Blakiston's Son and Co., Philadelphia, pages 13-15.
  4. ^ Chandrasekhar, S. (1950). Radiative Transfer, Oxford University Press, Oxford, pages 1-2.
  5. ^ Mihalas, D., Weibel-Mihalas, B. (1984). Foundations of Radiation Hydrodynamics, Oxford University Press, New York ISBN 0-19-503437-6., pages 311-312.
  6. ^ Goody, R.M., Yung, Y.L. (1989). Atmospheric Radiation: Theoretical Basis, 2nd edition, Oxford University Press, Oxford, New York, 1989, ISBN 0-19-505134-3, page 16.
  7. ^ Liou, K.N. (2002). An Introduction of Atmospheric Radiation, second edition, Academic Press, Amsterdam, ISBN 978-0-12-451451-5, page 4.
  8. ^ Hapke, B. (1993). Theory of Reflectance and Emittance Spectroscopy, Cambridge University Press, Cambridge UK, ISBN 0-521-30789-9, page 64.
  9. ^ Rybicki, G.B., Lightman, A.P. (1979/2004). Radiative Processes in Astrophysics, reprint, John Wiley & Sons, New York, ISBN 0-471-04815-1, page 3.
  10. ^ Born, M., Wolf, E. (1999). Principles of Optics: Electromagnetic theory of propagation, interference and diffraction of light, 7th edition, Cambridge University Press, ISBN 0-521-64222-1, page 194.