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LIGHT VALVE PROJECTION

THEORY OF OPERATION

 

          The following is a simplified description of how light valve projection operates and the advantages of using lasers for the illuminating light source.

 

         A light valve is a thin rectangular glass envelope containing liquid crystals. The front of the light valve looks like a mirror with specific polarizing properties. A low intensity image is scanned onto the rear side of the light valve; this causes the orientation of the polarization of the reflected image on the front side of the light valve to correspondingly rotate between 0 and 90 degrees. 0 degrees projects black (off) and 90 degrees projects white (on) with shades of gray in-between these angles of rotation. By using 3 light valves illuminated by red, green, and blue light a full color image can be projected. A polarizing beam splitting cube is placed in front of each light valve. Polarized light is columnated into the top of the beam splitting cube reflecting on to the front of the light valve. Depending upon the orientation of the polarization of the liquid crystals, at any point on the light valve, an image will be passed thru the front of the beam splitting cube and be projected on to the movie screen. It should be noted that the liquid crystals fill the entire area of the light valve and do not project pixels. This does not limit the resolution of the image, 2000 lines and higher can be obtained depending upon the source material.

 

         Most high lumen output video projectors use an arc lamp for the illuminating source. This white light is separated into red, green, and blue colors using dicroic color separating optics. The colors that can be obtained using this method have a limited available color spectrum. The laser light used in the Metatron Projector has purer and deeper colors that project a film like appearance. Arc lamp separated colors must pass thru a polarizing filter to work with the polarizing beam splitting cubes. This method does not create the best polarization reducing the contrast ratio of the image. Laser light is naturally polarized with a very tight orientation. This is a perfect match for the polarizing beam splitting cubes and light valve maximizing the contrast ratio.

 

         The light illuminating the front of the light valve from a standard projector is spread out flooding the entire surface of the light valve. The image on the front of the light valve is formed by a corresponding raster scanned image on the rear side of the light valve. This image rapidly decays or fades as the scan progresses down the face of the light valve. Wherever the image is not being scanned, which is most of the time, black or no image should be projected. By flooding the entire surface of the light valve with light some residual reflection of this black area leaks thru further reducing the contrast ratio. In the Metatron projector the laser light is concentrated into a narrow band of light that is synchronized to follow the scanned image on the light valve. This takes full advantage in transferring video information to the screen further increasing the contrast ratio.

 

         The arc lamp used in standard projectors tends to flicker, reduces its output with age, and is not a true point of light source. Consequently expensive focusing optics are required and the image is only in focus on a flat screen at a specific distance. Laser beams are a true point light source. This produces an image that is always in focus at any distance making it ideal for curved screens.

 

 

         BRADLEY NELSON,  223 LAS MIRADAS DR.  LOS GATOS,  CA.  95032  818-795-2407