Abstract:
A light source may enhance the generation of circularly polarized light of a desired polarization state. The light that is produced by a lamp and initially rejected by a circular polarizer may be subjected to polarization reversal. The polarization reversed light may again impinge on the circular polarizer. A substantial percentage of the previously rejected and then polarization reversed light is passed by the circular polarizer.

Description:
BACKGROUND 
     This invention relates generally to lamps that emit polarized light for example for use with reflective spatial light modulators. 
     Spatial light modulators may use a liquid crystal light valve to modulate light for display or projection of images. Such modulators may use reflective or transmissive technologies. Spatial light modulators may be formed on integrated circuits together with logic circuitry. Thus, integrated displays with integrated drive electronics may be formed. 
     As a result, spatial light modulators may be formed in a relatively cost effective fashion. Ultimately, such displays may be competitive with conventional displays such as cathode ray tubes. Generally, spatial light modulators utilize circularly polarized light, which is reflected from a liquid crystal surface. That surface has its reflective properties modulated by underlying electrodes. The resulting displays may be able to modulate large light powers, without excessive heating, with reduced box sizes for the same screen size as compared to cathode ray tubes. 
     Reflective spatial light modulators need a bright source of circularly polarized light. Conventionally, a high pressure discharge source, such as a weakly ionized plasma, produce unpolarized light. The unpolarized light is then filtered through a circular polarizer. The polarizer transmits the circularly polarized photons and rejects photons of the opposite polarization state. 
     Ultimately some of the light from the light source passes outwardly for reflection from the spatial light modulator. The remaining light is trapped and absorbed inside the light source as wasted heat. Ideally, fifty percent of the amplitude of the initial light produced by the lamp can be emitted by the lamp cavity to the outside optics in the appropriate circularly polarized state. The other half of the light produced by the lamp is wasted. Thus, the light source must generate twice the amplitude that is actually used. This may result in unnecessary heating, unnecessary expense, and increased component size. 
     Thus, there is a need for better ways to produce a circularly polarized light source, for example in use in connection with reflective spatial light modulators. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic depiction of one embodiment of the present invention; and 
     FIG. 2 is a depiction of the embodiment shown in FIG. 1 used in connection with a reflective spatial light modulator. 
    
    
     DETAILED DESCRIPTION 
     Referring to FIG. 1, a lamp  16  may be a high pressure discharge source such as a mercury arc lamp that may use a weakly ionized plasma to produce unpolarized light. The resulting light is filtered in a circular polarizer  18 . The circular polarizer  18  may pass only one of two circular polarization states. In one embodiment, the circular polarizer  18  passes the left polarization state and prevents the passage of the right polarization state. Thus, the light  22  of the appropriate polarization is passed from the circular polarizer  18  for use in a suitable optical system such as a reflective spatial light modulator. 
     Ideally, fifty percent of the amplitude of the light produced by the lamp  16  that impinges on the circular polarizer  18  is passed in the appropriate polarization. The remaining portion of the impinging light is rejected by the polarizer  18 . The rejected light, indicated at  24 , passes through a quarter wave plate  14  that converts circularly polarized light (either right-handed or left-handed) into linearly polarized light at 45° to the fast axis of the plate  14 . The light that passes through the plate  14 , indicated at  26 , is reflected from a reflector  12 , which may be a simple mirror. 
     The reflected light, indicated at  28 , passes outwardly through the plate  14 . The plate  14  takes the reflected linearly polarized light and converts it into oppositely, circularly polarized light. For example, if the rejected light is right-handed, circularly polarized light, the plate  14  initially converts that light into linearly polarized light. That linearly polarized light is reflected by the reflector  12  and the plate  14  converts the light passing through it to left-handed, circularly polarized light. 
     Birefringent materials may be utilized as the plate  14 . Birefringent have two dielectric constants that are aligned along crystalline space axes. As a result, light that is polarized in the direction of one of the optically active axes propagates through the material at constant velocity. The transmission velocity varies according to the axis used. The two polarization directions are the “slow” and “fast” axes. 
     Circularly polarized light can be analyzed into the vector sum of two components of light with perpendicular linear polarizations, with the two components offset by a constant phase factor of η/2. This phase shift results in an electric field vector that rotates around the axis of propagation at the frequency of light. If the circularly polarized light is transmitted through a Birefringent material thick enough to shift the two components&#39; phase offset by a quarter wavelength, then linearly polarized light will exit the material with the direction of linear polarization being 45° from the slow and fast axes. Thus, quarter wave plates can turn circularly polarized light into linearly polarized light and vice versa. 
     In accordance with one embodiment, the integrating cavity around the light source and/or the output filter may be coated with a quarter wave layer of birefringent material. That is, a coating of birefringent material may be applied to a sufficient thickness to create a quarter wave plate  14 . After propagating through the birefringent material, reflecting from the integrating cavity interlayer and finally propagating back through the birefringent layer, the net result is again circularly polarized light, but with the opposite sense from a simple reflection. This creates the correct polarization to pass through the exit polarizer  18 . 
     Birefringent coatings of controllable thickness are available from Measurements Group, Inc., Raleigh, N.C. 27611. These coatings may be sprayed on coatings of polycarbonate or epoxy, as two examples. 
     The light that passes through the plate  14  and is reflected by the reflector  12  together with the light that was rejected by the plate  14  forms the light indicated at  30 . This light again impinges on the circular polarizer  18 . Again, ideally all of the incident light is passed by the circular polarizer  18  because it is of the appropriate circular polarization state. Any remaining, unpassed light is again reflected and undergoes the same processing described previously. Ultimately, substantially all of the light produced by the lamp  16  may eventually pass through the circular polarizer  18  in some embodiments. 
     The action of the reflector  12  and the plate  14  effectively reverses the circular polarization imposed by the circular polarizer  18 . Thus, each time half of the light is rejected by the circular polarizer  18 , its circular polarization state is reversed. The rejected light, whose polarization is reversed, passes through the circular polarizer  18  on the next cycle. 
     Referring to FIG. 2, the light source  10  may emit the circularly polarized light of the appropriate state to illuminate the reflective spatial light modulator  32 . The incident light, indicated by A, is then modulated to form light beam B which may be viewed by a user or provided for projection in a projection display system. 
     While the present invention has been described with respect to a limited number of embodiments, those skilled in the art will appreciate numerous modifications and variations therefrom. It is intended that the appended claims cover all such modifications and variations as fall within the true spirit and scope of this present invention.