Polarizing Interference Filter

Polarizing Interference Filter 

a narrow-band monochromator used mainly in astrophysics to produce monochromatic images of the sun. It was invented in 1933 by B. Lyot of France and independently in 1934 by I. Eman of Sweden.

The operation of a polarizing interference filter is based on interference of two polarized beams generated by the passage of light through a birefringent crystal plate (quartz or spar) placed between two polaroid filters whose optical axes are oriented at an angle of 45° with respect to the optical axis of the crystal. A pile of several such elements having multiple thicknesses (Figure 1, a) exhibits transmission in narrow spectral bands that are far removed from each other (Figure 1, b). One of the bands is separated by a glass or interference filter. Polarizing interference filters are placed in a thermostat, in which the temperature is maintained to within a few tenths of a degree. The best polarizing interference filters have transmission band half-widths of up to 0.1–0.2 angstroms, 10–20 percent transmission, and field of view of 3°–4°.

Figure 1. (a) Diagram of a polarizing interference filter: (P) polaroid filters, (Q) quartz plates; (b) transmission of individual stages (1–6) and of entire filter (7). Wavelengths are shown at bottom.

REFERENCES

Evans, J.V. “Monokhromaticheskie fil’try.” In Solnechnaia sistema, vol. 1. Moscow, 1957. Pages 506–13. (Translated from English.)
Zirin, H. Solnechnaia atmosfera. Moscow, 1969. Pages 39–46. (Translated from English.)

E. V. KONONOVICH