Calculate R:FR photon ratio from spectral irradiance.Source:
This function returns the red:far-red photon ratio of a light source spectrum.
an object of class "source.spct"
select which definition of red and far-red should be used, defaults to "Smith"
logical indicating whether multiplier values should be cached between calls
logical indicating whether to use hinges to reduce interpolation errors
a single numeric nondimensional value giving the R:FR photon ratio, with name attribute set to the name of the wavebands, with "(q:q)" appended.