The basic light paths of a fluorescence microscope. An excitation filter cube containing a dichroic mirror (DM 1) directs excitation light (from a bulb or laser; filtered with an excitation filter) to the specimen and passes emitted fluorescence to the emission cube for further separation (multiple indicators). The barrier filter B prevents excitation light from reaching the detectors. Emitted fluorescence is separated by DM 2 into two beams. The emission filters (Em 1 and Em 2) block unwanted light. Fluorescence is detected by cameras (wide-field) or PMTs (laser-scanning). For transmitted illumination of a bright-field image, long wavelength light is selected.