Note on interference filter

Figure 7.8(II) Schematic drawing of an interference filter. The dielectric film thickness h plays an important role in fixing the wavelength that passes through the filter.

The interference filter is fabricated as a thin solid state Fabry Perot interferometer (of thickness h ) in which the working (reflecting) surfaces are covered by a colored glass plate, Figure 7.8(II). For a given reflectivity, the intensity distribution of the transmitted rays may be treated as having maxima at certain wavelengths given by

At normal incidence, and . For example, let correspond to maximum, let correspond to m^th maximum and correspond to the maximum of the transmitted rays. Thus

The wavelength difference between and is now obtained as

Assuming the wavelengths to be only slightly different from we can write

Clearly, the colored glass filter should be transparent for wavelengths between and . A combination of colored glass filter and interferometer will possess the transparency of a Fabry Perot interferometer but will have only one certain interference maximum (at ).

Interference filters can be interpreted as low order Fabry Perot etalons with solid spacers of thickness h and refractive index greater than unity. Another difference is that the tuning of the filter is effected by inclining or tilting the interference filter relative to the optic axis of the instrument.

Interference filter using this principle can be obtained by modern vacuum deposition techniques. A thin metallic film is deposited on a substrate by vacuum deposition technique (Figure 7.8(II)). A thin layer of a dielectric material such as cryolite with a known refractive index is deposited over it. This structure is again covered by another metallic film. To protect this film structure from the damage, a glass plate is placed over it. By varying the thickness of the dielectric film, one can filter out all except a particular wavelength.

Interference filters may be designed to pass very narrow bands, but usually sacrifice peak transmittance in that case. A typical interference filter for visible light may have a 7 nm wavelength window with 70% peak transmittance or a narrower window and a lower transmittance. Multilayers can also be designed to transmit or reflect relatively broad bands or reflect infrared while transmitting visible light.