text: The field of spectroradiometry concerns itself with the measurement of absolute radiometric quantities in narrow wavelength intervals. It is useful to sample the spectrum with narrow bandwidth and wavelength increments because many sources have line structures  Most often in spectroradiometry, spectral irradiance is the desired measurement. In practice, the average spectral irradiance is measured, shown mathematically as the approximation:
  Where  is the spectral irradiance,  is the radiant flux of the source (SI unit: watt, W) within a wavelength interval  (SI unit: meter, m), incident on the surface area,  (SI unit: square meter, m). The SI unit for spectral irradiance is W/m. However it is often more useful to measure area in terms of centimeters and wavelength in nanometers, thus submultiples of the SI units of spectral irradiance will be used, for example μW/cm*nm
  Spectral irradiance will vary from point to point on the surface in general. In practice, it is important note how radiant flux varies with direction, the size of the solid angle subtended by the source at each point on the surface, and the orientation of the surface. Given these considerations, it is often more prudent to use a more rigorous form of the equation to account for these dependencies
  Note that the prefix "spectral" is to be understood as an abbreviation of the phrase "spectral concentration of" which is understood and defined by the CIE as the "quotient of the radiometric quantity taken over an infinitesimal range on either side of a given wavelength, by the range".