I am using a QuickTiter Retrovirus Quantitation Kit and the protocol says to "read the microplate using a 480/520 nm filter set", but I am unsure as to what this means. Is this suggesting that there should be two excitation wavelengths, or that one wavelength is for background? I am currently using a SpecrtaMax L luminescence microplate reader with Softmax Pro software and I cannot determine the correct settings to use.