Is there any way to get intensity value of a certain wavelength for given pixel of a normal  image? I understand I can get a color from wavelength but it's not true for the opposite direction.
In case this picture would help.



To make this question simple, let's lighting and calibration aside to an optimized condition. 

I wonder if there is any way to get intensity value of a certain spectral wavelength for given pixel of a common image. That is, Wavelength and rgb of a pixel are both known. Based on that, I want to compute intensity of the wavelength for the pixel.
If I understand correctly - you want to know what wavelength a given RGB value will show?

Only some RGB values represent a pure wavelength, those on the edge of CIE diagram (there is no wavelength of brown for example) see http://www.fourmilab.ch/documents/specrend/ for details and code