The use of filters in optical microscopy has become so commonplace that it is hard to find a biological microscope that does not have some type of filter accessory kit. For particular modalities, such as two-photon imaging, the use of optical filters is essential to make the method viable. Filter technology has progressed steadily over the past few decades, moving away from simple colored glass filters to complicated thin-film–based optical components. With optical densities commonly reaching values of 4 or greater, spectrally selective imaging has never been more accurate.
