Lyot filter

[4]: 2.5.2 [5]: 163 [6]: 5.8.3 [7]: 202 [8]: 327 [9]: 14 This section describes how the Lyot filter's wavelength dependent transmission of light arises from birefrigence.

[10][11]: 95–96  Linearly polarized light travels fastest when aligned with the waveplate's fast F direction, and slowest when aligned with the waveplate's orthogonal slow S direction.

For example, at one wavelength, if the light exiting the waveplate is horizontally polarized, then the light passes through the second horizontal polarizer fully, exiting the optical filter with no attenuation.

[10] Using this design, a graph describing the transmitted light intensity at each wavelength will show sharper major peaks (narrower bandwidth) of transmitted light and a greater wavelength interval between the major peaks of transmitted light (free spectral range).

[11]: 95–96 [1]: 108  As an example, extending the single-plate equation to a 3-plate optical filter with maximum waveplate thickness

[4]: 2.5.2  Splitting the crystals in half and adding a 1⁄2 waveplate in the middle increases the filter's field of view.

[10] The separation and narrowness of the transmission peaks depends on the number, thicknesses, and orientation of the plates.

[1]: 109 An electrically tunable Lyot filter contains tuneable electro-optic or liquid crystal birefringent elements.

[5]: 167  Often these filters are based on the original Lyot design, but many other designs exist in order to achieve other properties such as narrow or broad band transmission, or polarization selectivity.

[6]: 5.8.3  Poor transmittance occurs due to the large number of highly absorbing polarizers and imperfect waveplate action.

[10] In solar astronomy, viewing the sun's chromosphere, the sun's second atmospheric layer, requires narrowband optical filters (spectroheliograph), such as a Lyot filter, using wavelengths for viewing solar flares, prominences, filaments, and plages arising from calcium and hydrogen.

[11]: 96 Although their mechanisms are different, modelocking lasers and Lyot-filter lasers both produce a comb of multiple wavelengths which can be placed on the ITU channel grid for dense wave division multiplexing (DWDM) or used to give each suburban home its own return-signal laser wavelength in a passive optical network (PON) used to provide FTTH (Fiber To The Home).

[15]: 205 Other applications have been in microspectrometer and hyperspectral imaging devices and biomedical photonics.

The light's path (red dashed line) through a single-plate Lyot filter. The first polarizer transmits horizontally polarized light to the waveplate. The waveplate's fast (F) and slow (S) transmission directions make a 45-degree angle to the horizontal. For the wavelength in this example, the waveplate transforms the light's horizontal polarization to a circular polarization. The second polarizer transmits only part of the light, the horizontal component of the circularly polarized light, leading to a wavelength dependent amount of light transmission.