Thin-film optical filters are optical filters which allow light of specific wavelengths to pass through unencumbered, while blocking other light from passing (to some extent).
This phenomenon is achieved by creating an optical coating with thin-film layers such that interference effects magnify the reflection of some wavelengths, while canceling the reflection (thus increasing the transmission) of others. The more alternating thin-film layers stacked up, generally the better the magnitude and waveband specificity of reflection; how this will affect the waveband that is transmitted is a more complicated question. The waveband reflected can also be narrowed by increasing the difference in refractive index of the alternating thin-film layers.
See also