Patent EP-1074863-A1 (EP)
Inventor: GRUPP JOACHIM
IPC: G02B5/30
Filed: 1999-08-04
Abstract: The optical device for use as a filter (20) reflects at least one predetermined wavelength band present in the incident light, eg. red, and comprises at least one film (23) of cholesteric liquid crystal with a birefringence gradient in at least one part of thickness, where the gradient is a function of depth (z) in the film from upper surface (26) to lower surface (25) is contact with a substrate (22). The film (23) is stratified, that is formed by a set of polimerized layers (24a-24j) of cholesteric liquid crystal material, where the coefficients of birefringence are constant within each layer, and change gradually from layer to layer forming the gradient of birefringence. The gradient of birefringence is negative, the birefringence (delta)n, that is the difference between the extraordinary (ne) and ordinary (no) indices of refraction, decreases as a function of depth (z) counted from the face of film receiving the incident light. In the second embodiment, the optical device for use as a Bragg reflector comprises a cell containing liquid crystal film between two substrates and sealing parts defining a cavity containing the film. The film is made as a mixture of two liquid crystals having different birefringent properties, where one is fixated by polymerization so that the concentration varies as a function of depth (z). The device also comprises two sets of electrodes on faces of film, and a control circuit connected to the electrode for a selective application of control voltages for switching between two states; the first state of reflecting light of specified band, and the second state of transmitting light. The extraordinary index of refraction decreases with depth, and the ordinary index of refraction increases with depth, so that the birefringence decreases with depth. Five additional profiles of birefringence are proposed, including the cases of decreasing extraordinary and constant ordinary index of refraction, increasing extraordinary and decreasing ordinary index, positive and negative sinusoidal change of extraordinary and ordinary indices, respectively, with finite differences at limiting surfaces, positive sinusoidal change of extraordinary and constant ordinary index, and sinusoidal changes with coincident values at limiting surfaces. The birefringence is maximum at the middle thickness, and minimum or nul at the faces of film. The absolute value of gradient of birefringence is greater than a minimum value, which is 0.01 per micrometre of film thickness, or 0.1 for a film of 10 micrometre thickness. A claim is also included for a device comprising a set liquid crystal films, each reflecting a different wavelength band. The device also comprises a reflecting layer on the side opposite to that of incident light. The manufacturing method includes the deposition and polymerization of cholesteric liquid crystal layers in succession, the first (24j) on the substrate, to form the film. In the second embodiment, the mixture contains an agent filtering UV radiation in addition to two choelsteric liquid crystal materials, and polymerization of the second liquid crytsal is carried out by UV radiation directed from the upper surface so to fixate a concentration decreasing with depth.