What is the reference spectrum? I need to know how to calculate the reference spectrum of a wavelength 500nm.
As far as I can say, a "reference spectrum" isn't really a purely physics term. These are two words, not just one term, much like "interesting spectrum". It's just a spectrum – with some intensities listed for many frequencies – that may be found in a book or in a reference library and used as a reference for comparisons. Many spectra may be tabulated and listed in this way.
For each reference spectrum, one may look at the intensity for 500 nm. But there's no unique answer because there are many different spectra, even "reference spectra".