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  • The interference filter of optical filters, in their simplest form, are substantially Fabry-Perot interferometers. Namely, they consist by two reflectors separated by a spacer layer.

    Without any other expedient, their transmission would be a periodic function and they would be able to transmit many wavelengths. In order to avoid this problem the spacer and the substrate on which are deposited the reflecting layers are constituted by a colored glass (i.e. an absorption filter).

    If the bandwidth of the interference filter is small and the absorption filter is not enough to eliminate the unwanted wavelengths, we resort to the expedient to superimpose severalinterference filters with different spacing, preventing , in this way, the transmission of unwanted waves.

    Sometimes, in order to make more squared the transmission function and increase the reflectivity outside the pass-band, several filters with the same spacing are overlapped.

    The reflective layers (usually dielectrics) are protected by a glass; the edge of the package, composed by the protective glass, the absorption filter and the layers which constitute the real interference filter, is preserved by a layer of material that prevents the penetration of the humidity (sealing ring).

    Fig. 20 – Section of an interference filter.

    The interference filters may have very narrow band pass widths (1 Å) and the peak wavelength is function of the temperature (since the temperature changes the spacer thickness) and of the radiation incidence angle.

    In particular it should be remarked that, unlike the absorption filters, the interference filters transmit a different wavelength for each incident angle. Thus varying the incidence angle, unlike the absorption filters where only a variation in the attenuation occurs, in the interference filters even the wavelengths change.

    As a consequence it is possible to obtain simple variable monochromators by rotating the filter around an axis parallel to its surface. In addition, if a divergent beamimpinges on an absorption filter, the transmitted beam has the same spectral band for the entire impinging angle. Conversely, if the same divergent beam impinges on an interference filters, to each incident direction will correspond its own spectral band.

    The total transmitted spectrumis only enlarged, but, by examining the spectrum separately in each direction, the spectrum will result composed by different bandwidths depending on the observation direction.

    When the beam incidence angle changes, also changes thepath which the beamtravels in the spacer: the central wavelength of the filter changes. Anyhow we move away from the filter normal direction, the transmitted wavelength decreases (i.e. it moves toward blue).

    This is the reason why, normally, it is required that the incident beam on an interference filter is collimated.

    Fig. 21– Behavior of the wavelength transmitted by an
    interference filter for different angles of incidence.


    Fig. 22 - Behavior of the wavelength transmitted by an absorption filter; for different incidence angles the wavelength does not change, but the transparency changes.

    It also should be noted that, while the absorption filters absorb, i.e. they convert into heat the radiation that is not transmitted, the interference filters reflect them(almost completely). Hence by inserting an interference filter in an optical system, it has to be taken into account the fact that the not transmitted radiation will be almost totally reflected within the optical system itself.

    One final note: in order to avoid an excessive warming of the absorber due to the not transmitted radiation, it is a good practice to impinge with the radiation from the side of the protective glass, which is the reflective side.

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