2510332 - February 2009
There are tolerance buildups that should be addressed according to each manufacturer’s processes. For example, the device typically is drop-in mounted to the optical engine assembly with no adjustment, for maximum performance in the field (no adjustments that change during the lifetime of the product) and ease of assembly. The build-up of tolerances between the optical axis of the projection lens and the parallelism of the device plane, including device package tolerances, must be accounted for in the design of the projection lens for satisfactory MTF performance on the screen. The lens must have MTF margin to account for the apparent defocus of pixels caused by nonparallelism. These margins can be large in the design of the lens, allowing lower-cost/lower-precision mechanical processes in the engine parts. However, this tends to make the lenses larger due to more lens elements, and, therefore, higher in cost. This might be favorable, however, depending on the cost of the mechanical processes required to reduce the tolerance buildups. However, it may be more cost effective to have lower performance margins in the lens, but apply more precise processes to the mechanical tolerance buildups. Then, the lens will be smaller and less costly, and might meet a product size constraint that higher margin designs would not meet. The resultant MTF on the screen must be the same, regardless.
4.3.3 Throw-Ratio and Offset Optimization
In general, longer focal lengths for projection lenses result in smaller lenses, which lower costs. The lower magnifications of longer focal lengths typically reduce tolerance sensitivities, resulting in better and more consistent performance (tighter distributions), which can help with the tradeoffs mentioned in section 4.3.2 as well. Longer focal lengths mean longer throw ratios, and often there is a product requirement that sets some limit for this. If there is an option, it is generally better to go as long as possible for cost and performance reasons.
In the case of telecentric designs, it also is possible to consider offset as an independent variable. Projection offset is the amount by which the projected image must be raised above the optical axis of the projection lens. For example, 100% offset means that the bottom of the image is at the centerline of the projection lens, and 100% of the image falls above it. This is convenient for tabletop projectors, which must project without interference from objects in front of the projector (including the table). It also produces a keystone-corrected image on a flat screen, unless the projector itself is tilted to raise the image further up. This offset amount determines the field radius required to be imaged by the projection lens at the device plane, which is the single most influential parameter driving the cost, size, and complexity of most lenses. Minimizing field size pays many dividends.
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