A filter and method for filtering an optical beam are disclosed. One embodiment of the filter is an optical filter for filtering an incident light beam, comprising an optically effective material characterized by: a light transmittance of less than 1% for wavelengths below 420 nm; and a light transmittance for wavelengths complementary and near complementary to wavelengths below 420 nm that, combined with the transmittance for wavelengths below 420 nm, will yield a filtered light beam having a luminosity of about 90% and an excitation purity of 5% or less. The complementary wavelengths can be wavelengths above about 640 nm, wavelengths above about 660 nm, and/or wavelengths from about 540 nm to about 560 nm. Further, in one embodiment the difference between the light transmittance just below 420 nm and the light transmittance just above 420 nm can be greater than 90%. Above 420 nm, in some embodiments, the light transmittance can be arbitrarily determined.