2510332 - February 2009
Therefore, for a given fixed image size (typically screen limited) and height from the floor, there is an opportunity to trade offset with tilting of the projector by increasing throw ratio, still resulting in acceptable keystone distortion. For example, if a 2-m wide image is desired to be 1.5 m from the floor at the bottom edge, a short-throw projector on a <1-m typical conference-room tabletop will have to be tilted more to raise the image the desired amount than a long-throw projector would, at the same offset. Therefore, the short-throw projector introduces more keystone distortion of the image than the long-throw projector because more angle is required to raise it any given distance. Therefore, for any acceptable keystone distortion specification and image size, the longer-throw projector can have less offset and can be tilted the same amount as the short-throw projector with more offset. This can reduce significantly the cost and size of the projection lens. An example comparison for various throw ratio and offset combinations is shown in Figure 14.
XGA Keystone at 3 m Throw Projector tilted as required to raise image bottom to lens center
THROW RATIO (ANSI)
KEYSTONE % @ 100% offset
KEYSTONE % @ 85% offset
KEYSTONE % @ 75% offset
31.00 Focal Length
Figure 14. Keystone Distortion for 0.9 XGA by Throw Ratio and Offset
For example, a 2:1 throw-ratio lens for this 17-micron XGA device induces only 2% keystone distortion in the image (top will be 2% wider than the bottom) when the projector is tilted to bring the image bottom up to the center height of the lens, if field is reduced to 75% offset. Conversely, if 1% keystone distortion can be tolerated under these conditions, a 1.8:1 throw-ratio lens could be used with only 85% offset field. Additionally, it can be seen that the keystone impact is nonlinear, indicating that longer-throw ratios increase keystone more slowly with increasing projector tilt than shorter-throw ratios. While the percentage reductions possible with these trades seem relatively small, they can have a significant impact on lenses because they affect area of the elements, which affects coating costs dramatically.
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