An optical filter selectively transmits light according to properties such as wavelength or polarization of waves.
An interference filter uses wave interference in a layered optical structure to transmit or reject selected wavelengths. The accumulated optical path length makes reflected contributions reinforce or cancel differently across the optical spectrum.
A narrow-band example can isolate emission before a photodetector or reject an excitation beam in fluorescence microscopy. Tilting the filter changes the propagation angles and optical paths inside its layers, shifting the transmitted band. A filter characterized in a collimated beam may therefore behave differently in a strongly converging beam; temperature and polarization can also affect transmission.
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