A filter and method for filtering an optical beam are disclosed. Oneembodiment of the filter is an optical filter for filtering an incident lightbeam, comprising an optically effective material characterized by: a lighttransmittance of less than 1% for wavelengths below 420 nm； and a lighttransmittance for wavelengths complimentary to wavelengths below 420 nm that,combined with the transmittance for wavelengths below 420 nm, will yield afiltered light beam having a luminosity of about 90% and an excitation purityof 5% or less. The complimentary wavelengths can be wavelengths above about640 nm, wavelengths above about 660 nm, and/or wavelengths from about 540 nmto about 560 nm. Further, in one embodiment the difference between the lighttransmittance just below 420 nm and the light transmittance just above 420 nmcan be greater than 90%. Above 420 nm, in some embodiments, the lighttransmittance can be arbitrarily determined. The optically effective materialcan be optical-grade glass, an optical-grade plastic or polymer, a thin-filmdielectric coating, or an optical-grade glass or plastic coated with adielectric coating. The optical filter can be mounted downstream of anillumination source exit aperture, wherein the illumination source producesthe incident light beam, and upstream of a site to be illuminated by thefiltered light beam. Alternatively, the optical filter can be operably mountedon a set of viewing optics of a surgical microscope to filter a reflectedportion of a light beam produced by an illumination source and used toilluminate a site, such as a surgical site.
