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. The optically effective material can be optical-grade glass, an optical-grade plastic or polymer, a thin-film dielectric coating, or an optical-grade glass or plastic coated with a dielectric coating. The optical filter can be mounted downstream of an illumination source exit aperture, wherein the illumination source produces the incident light beam, and upstream of a site to be illuminated by the filtered light beam. Alternatively, the optical filter can be operably mounted on a set of viewing optics of a surgical microscope to filter a reflected portion of a light beam produced by an illumination source and used to illuminate a site, such as a surgical site.