I’m not sure Cycles is the rendering engine you want for this kind of thing. While Cycles can use physical units for light brightness, albedo, etc. it is by no means geared towards scientific measurement. To get a really accurate luminous flux values, you may want to consider a spectral rendering engine: those can path trace at arbitrary frequencies of light, while Cycles only traces at red, green and blue wavelengths.
If you just want a plausible linear monochrome output, you can do that in compositing fairly simply. Just make sure you set your color space to linear and export to OpenEXR (or some other floating point image format).