Filter and method of making an optical device

Abstract

A cholesteric film is formed into an optical device comprising a plurality of layers. The cholesteric film is initially placed in a first predetermined state, for instance heating to a first predetermined temperature, and is then irradiated with narrow band UV radiation. The properties of an upper layer of the film are fixed, for instance for reflecting normally incident infrared radiation of a particular handedness. This process is repeated for different wavelengths of UV fixing radiation so as to penetrate to different depths from the surface of the film in order to form the layers. This method may be used to produce a filter of reduced sensitivity to angle of incidence. In addition to the infrared reflecting layer, red and green reflecting layers are provided. For normally incident radiation, the red and green layers reflect red and green parts R and G, whereas the blue part B of the spectrum is transmitted. For off-axis illumination or viewing, the red part R is reflected by the infrared layer and the green part G is reflected by the red layer. The birefringence of the infrared layer and the red layer reverses the direction of circular polarisation of the blue part B, which thus passes through the green layer.