When the excitation light illuminates the sample, a small amount of excitation light is reflected off the optical elements within the objective and some excitation light is scattered back into the objective by the sample. In order to select the excitation wavelength, an excitation filter is placed in the excitation path just prior to the dichroic mirror. In order to more specifically select the emission wavelength of the light emitted from the sample and to remove traces of excitation light, an emission filter is placed beneath the dichroic mirror. In this position, the filter functions to both select the emission wavelength and to eliminate any trace of the wavelengths used for excitation. These filters are usually a special type of filter referred to as an interference filter, because of the way in which it blocks the out of band transmission.
Interference filters exhibit an extremely low transmission outside of their characteristic bandpass.
Thus, they are very efficient in selecting the desired excitation and emission wavelengths. The dichroic mirror is mounted on an optical block commonly referred to as a filter cube . The excitation and emission filters are usually affixed to the filter cube. This cube provides a convenient means to change the dichroic mirror without direct handling of either the mirror or filters. Figure 2 shows the light path through the filter cube in a fluorescent microscope.