Switchable viewfinder display
A compact high quality and lightweight symbol generator for projecting symbolic information into the field of view of a viewfinder is provided. The symbol generator comprises at least one ESBG device sandwiched between a pair of transparent plates which together function as a total internal reflection lightguide, switching electrodes and means for coupling illumination into the lightguide. Each ESBG device contains information encoded in a multiplicity of separately switchable grating regions. A plurality of independently switchable transparent electrodes elements, substantially overlay the separately switchable grating regions. When no electric field is applied, the ESBG device is in its diffracting state and projects images of said information towards the viewer. The projected images are surimposed onto an image of the external scene. When an electric field is applied the ESBG no longer diffracts and hence3 no information is displayed. In a further embodiment of the invention, the symbol generator could be configured to provide symbols of different colors by arranging for different symbols to contain ESBGs optimized for the required wavelengths and LEDs of appropriate spectral output. In a yet further embodiment of the basic invention several ESBG panels could be stacked such that by selectively switching different layers it is possible to present a range of different symbols of differing colors at any specified location in the field of view.
1 . A symbol generator for presenting information in an optical viewfinder comprising:
a first ESBG device having a front side facing towards the viewer and a rear side;
wherein said ESBG is sandwiched between first and second transparent plates;
wherein said transparent plates together function as a light guide;
wherein each said ESBG device contains information encoded in a multiplicity of separately switchable grating regions;
a plurality of independently switchable transparent electrodes elements, said independently electrodes substantially overlaying said separately switchable grating regions; and
means for coupling illumination into said transparent plates;
said ESBG being operative to project the images of said information towards said viewer when said ESBG rear side is illuminated using light of a first wavelength and no electric field is applied to said ESBG.
2 . The symbol generator of claim 1 , wherein said ESBG device provides a grating within each of said separately switchable regions and is clear elsewhere.
3 . The symbol generator of claim 1 , wherein said illumination means provides linearly polarized light.
4 . The symbol generator of claim 1 , wherein said illumination means is a Light Emitting Diode.
5 . The symbol generator of claim 1 , wherein said illumination means provides light having a limited bandwidth centered about a wavelength, and the maximum diffraction efficiency of said ESBG device occurs at approximately the same wavelength.
6 . The symbol generator of claim 8 , wherein said wavelength is about 620 nanometers.
7 . The symbol generator of claim 1 , wherein said separately switchable grating regions provide images of symbols.
8 . The symbol generator of claim 1 , further comprising an external diffuser.
9 . The symbol generator of claim 1 , wherein said separately switchable grating regions are configured to diffract light at different wavelength provided by a multiplicity of light sources of appropriate spectral output.
10 . The symbol generator of claim 3 , further comprising a third transparent plate and a second ESBG sandwiched between said second and third transparent plates;
wherein said first, second and third transparent plates together function as a light guide;
wherein each said second ESBG device contains information encoded in a multiplicity of separately switchable grating regions;
wherein said switchable grating regions of said first and second ESBGs substantially overlap;
said second ESBG being operative to project the images of said information towards said viewer when said ESBG rear side is illuminated using light of a second wavelength and no electric field is applied to said second ESBG.