A fluorescent equipped universal illuminator has three main filtering devices in it. The exciter filter takes the light from the light source and filters it to produce only the wavelengths needed to excite the specimen. It reflects all light below a certain wavelength and transmits above that wavelength. This center point wave length must be between the excitation and emission point for the fluorochrome. It must be greater than the excitation wavelength so it will reflect the excitation light down and into the objective.
Dichroic mirrors act as filters by transmitting or reflecting unwanted light away.
When the excitation light hits it any light that has to high a wavelength will be transmitted through the mirror and never reach the specimen. At the specimen plane the excitation light triggers the fluorochrome to emit light. It will always be at a longer wave length than the excitation due to conservation of energy. The objective then gathers all the light that its NA allows it to and then the dichroic mirror passes the fluorescent light up to the barrier filter and reflects any excitation light back towards the light source. The barrier filter screens out any left over exciter light and any fluorescent wavelengths that we don't want to see.