I've come across plenty of figures demonstrating the spectral luminous efficiency as a function of wavelength (meaning the humans eye's sensitivity to different wavelengths) http://www.yorku.ca/eye/photopik.htm but I've been unable to find the equation which gives this function so I can plot it myself.
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1$\begingroup$ The reason you don't often see a function for it is that it's an empirical measurement, not a prediction from theory. Any function you find will simply be some sort of fit to the data. It may be useful for calculations, but it's really nothing more than an interpolation of the data. $\endgroup$– Colin KCommented Jan 30, 2013 at 16:13
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The equation is approximately:
V(\lambda)=exp-0.5[(((\lambda-559)/41.9)]^2)
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$\begingroup$ Do you have a reliable source for that? $\endgroup$– ptomatoCommented Jan 27, 2013 at 17:22
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$\begingroup$ It is taken from the lecture notes of Prof. Brian Cowan which I unfortunately cannot post as they are restricted. If you plot this function [ wolframalpha.com/input/… ] you will find that it appears identical to the plot I posted initially and the all of the other ones available online. As I also said this function only provides an approximation although I've found it agrees well with the available data. I should also point out that due to a quirk with WA you will have to recalculate that function after clicking the link by clicking '=' $\endgroup$– LlamaDCommented Jan 30, 2013 at 15:42