Abstract: Disclosed is an optical filter for transmitting light in a narrow bandwidth centered about the wavelength .lambda., including a birefringent half-wave crystal defining an optical path through the crystal and normal to the optical axis of the crystal, the thickness of the crystal effecting a 180.degree. phase change between the fast and slow components of light of wavelength .lambda. along the optical path; a first polarizer preceding the crystal in the optical path and having a first polarizing plane rotated 45.degree. about the optical path from the optic axis of the crystal; a first reflective surface preceding the first polarizer in the optical path and normal to the optical path; a second polarizer following the crystal in the optical path and having a second polarizing plane rotated 90.degree. about the optical path from the first polarizing plane; and a second reflective surface following the second polarizer in the optical path and normal to the optical path.

Abstract: Disclosed is an optical filter, including a plurality of adjacent layer pairs, each pair having an incident and an emergent surface and including a first dielectric layer having a thickness t and a refractive index n, a second dielectric layer, adjacent to the first layer, having a thickness t and a refractive index n, and a control electrode disposed between and in electrical contact with the layers. The filter further includes a plurality of ground electrodes disposed on the layer pairs to electrically contact each incident and emergent surface, a source of electric potential, and a switch for connecting the source between the control electrodes and the ground electrodes, whereby light having a wavelength .lambda.=4nt will be reflected by the filter upon closing the switch and thereby applying the electric potential in opposite directions across the first and second layer pairs. The plurality of layer pairs may include m layer pairs, with the thickness of each dielectric layer in the ith pair (i=1, 2, 3 . .