Calculate optical image irradiance
irradiance = oiCalculateIrradiance(scene,optics)
The scene spectral radiance (photons/s/m2/sr/nm) is turned into optical
image irradiance (photons/s/m2/nm) based on information in the optics.
The formula for converting radiance to irradiance is
irradiance = pi /(1 + 4*fN^2*(1+abs(m))^2)*radiance;
where m is the magnification and fN is the f-number of the lens.
Frequently, in online references one sees the simpler formula:
irradiance = pi /(4*fN^2*(1+abs(m))^2)*radiance;
(e.g., Gerald C. Holst, CCD Arrayas, Cameras and Displays, 2nd Edition,
pp. 33-34 (1998))
This second formula is accurate for small angles, say when the sensor
sees only the paraxial rays. The formula used here is more general and
includes the non-paraxial rays.
On the web one even finds simpler formulae, such as
irradiance = pi/(4*FN^2) * irradiance
For example, this formula is used in these online notes
http://www.ece.arizona.edu/~dial/ece425/notes7.pdf
http://www.coe.montana.edu/ee/jshaw/teaching/RSS_S04/Radiometry_geometry_RSS.pdf
Reference:
The formula is derived in Peter Catrysse's dissertation (pp. 150-151).
See also http://eeclass.stanford.edu/ee392b/, course handouts
William L. Wolfe, Introduction to Radiometry, SPIE Press, 1998.
Copyright ImagEval Consultants, LLC, 2005.