Transforming RGB data to wavelength information

Here, I'm presenting a reverse transformation to wavelength to RGB. Right now I have no means of testing its accuracy on some real data. As a first check, I've run it on the mercury lamp image shown in my older post. There are small deviations caused probably by low intensity of light in some places. If used on some real stuff, I would definitely attempt to first fit a line (or some other meaningful function) through the estimated wavelengths. Then of course plot the spectra against fitted values. The domain of this function is limited to 380 - 645 nm. So, here's the algorithm in Python:

def RGB2lambda(R, G, B):
   """Returns 0 if indeciferable"""
   # selects range by maximum component
   # if max is blue - range is 380 - 489
   # if max is green - range is 490 - 579
   # if max is red - range is 580 - 645
  
   # which colour has highest intensity?
   high = float(R)
   highind = 1
   if G > high:
       high = float(G)
       highind = 2
   if B > high:
       high = float(B)
       highind = 3
      
   # normalize highest to 1.0
   RGBnorm = [R / high, G / high, B / high]
  
   # start decoding
   RGBlambda = 0
   if highind == 1: # red is highest
       if B >= G: # there is more blue than green
           return 0 # max red and more blue than green shouldn't happen
       # wavelength linearly changes from 645 to 580 as green increases
       RGBlambda = 645 - RGBnorm[1] * (645. - 580.)
      
   elif highind == 2: # green is max, range is 510 - 579
       if R > B: # range is 510 - 579
           RGBlambda = 510 + RGBnorm[0] * (580 - 510)
       else: # blue is higher than red, range is 490 - 510
           RGBlambda = 510 - RGBnorm[2] * (510 - 490)
          
   elif highind == 3: # blue is max, range is 380 - 490
       if G > R: # range is 440 - 490
           RGBlambda = RGBnorm[1] * (490 - 440) + 440
       else: # there is more red than green, range is 380 - 440
           RGBlambda = 440 - RGBnorm[0] * (440 - 380)
  
   return RGBlambda

And here is an accuracy check made on my older image of mercury lamp spectrum:

Accuracy check (synthetic test): x-axis is expected value, y-axis is algorithm output. Circles are the data, the ideal is presented by red line (y = x).

Please note that this was NOT produced by using a real photo of a spectrum. As someone who has some spectroscopic background I strongly discourage you from using this algorithm to get wavelength information. This was derived from a simplified version of simple wavelength-to-RGB algorithm. Nevertheless, I intend to make some photos of monochromatic light and test it ;-)