Basic definition of front and rear projection TVs:
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Front Projection
Front projection is the use of a source to bounce an image off a surface and back to the viewer. In this case, the surface should be highly reflective, in order for the audience to get the brightest image possible. "On equal-intensity, distance, and size-of-image bases a front projection is brighter than a corresponding rear projection. In short, it takes less wattage to project the same image from the front than from the rear". The source is normally hidden from the audience, but must be placed with a clear throw to the surface. If the throw is very long, a source with the appropriate wattage should be chosen.
This method is best for situations where the source is not very strong, as a highly reflective surface will maximize the amount of light being bounced back at the user. A front projected image maintains its intensity, clarity, and contrast through a wide angle of vision, meaning those sitting house right and left can see the image almost as well as those in the center of the audience.
Using a front projection surface has some disadvantages, however First, having a very light colored surface on the stage can be distracting. Some designers feel their audiences are left "waiting for the movie to start". The bright surface also reflects ambient or "bounce" light back to the viewer, which can cause the image to look washed out if care is not taken in lighting the areas adjacent to the surface. Backlighting and sidelighting actors and scenery becomes very important in this situation in order to keep the actors shadow off of the surface. Hiding the source in an extreme lighting position to avoid casting shadows can cause distortion in the image and requires correction in the slide to compensate.
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Rear Projection TVs
Rear projection is the use of the source to transmit the image through a screen to the viewer. In this case, the surface should transmit an image with true optical clarity and brightness. A higher intensity source is necessary as the light rays are forced through the surface to the viewer By definition the source is masked to the audience, but space must be left backstage between the source and the surface, in order to prevent the actors and crew from casting shadows on the surface. The darker colors included in the rear projection palette allow the surface to blend inconspicuously into the scenery. Actors can stand in front of the screen without blocking the image. Because the image is usually projected on an angle directly perpendicular to the surface, distortion is generally not a problem. If the surface is painted, varied and interesting effects can be achieved by projecting them from behind so that they blend with the painted picture. Day to night effects are a perfect example of this.
Rear projection also has its disadvantages. In order to allow the image to pass through it, and because the viewer is usually looking almost directly at the source through the screen, a bright spot ("hot spot") is often visible at the center of rear projected images. Because of the density of the material required to diffuse the image and prevent hot spoiling, the image quality falls off sharply when the viewer is seated outside the 60 to 110 degree viewing cone. The actual size of the viewing cone is dependent on the color of the surface chosen, with darker (and therefore denser) surfaces "falling off" fairly close to the center axis.
Common RPTV Problems
Burn in is a problem that may be more common with rear projection televisions than with other types of projection. But what is burn? Burn in is a damaged pixel, whose phosphors have been prematurely aged and so do not shine as well as before. In simple words the screen develops a 'memory' of the image that has been fed to it repeatedly. The quality of image shown on such a screen thereafter will not be optimal.
User habits can help keep burn in away. For example, switching off the set when not being watched, not pausing DVD for more than twenty minutes. New plasma televisions burn more easily so makes sure to switch it off when not in use, set the contrast setting to less than 50 % as this will be more kind on the phosphors. When playing video games or other applications with static images; switch on your burn in tools to save the screen. Most of these actions will save you the need of an early replacement of a screen.
Television maintenance should not be neglected. Keep your set dust and lint free as this prolongs the life of the lamp and the cooling of the lamp. Wipe the screen and body with a soft cloth with a very mild brand of window cleaner or just a soft cloth. If you are ever in need of replacing any parts our AllTV specialist is your best first bet.
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