Optical filters play an important role in enabling applications such as fluorescence microscopy and Raman spectroscopy. In these applications there are two distinct types of beams: the illumination (or excitation) beam and the signal (or emission) beam. Not only are these beams spectrally distinct, but also they differ significantly in their intensity – the signal beam can be a million times (or more) weaker than the illumination beam. Therefore, the ability of filters to selectively transmit desired wavelengths of light while blocking unwanted light is critical.
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