Curved Focal-Surface Imagers

Many optical systems naturally (for example, the eye) produce a curved rather than planar focal surface. Optically correcting this curved surface to become planar often limits some performance attributes of the system (larger, heavier, smaller field of view, greater reflection losses, etc.). During the era when film was used extensively for image capture, it could be curved to conform to some non-planar surfaces so that these trade-offs did not have to be made.

Figure 1(a) shows the contrast (modulation-transfer function [MTF]) for a small lens suitable for a micro-air vehicle that must be very lightweight. If a spherical focal surface is used instead of the conventional planar surface, the MTF is improved dramatically.

MTF for a small lensFigure 1(a).


Thin silicone membrane Figure 1(b).
Silicon membrane cut into petalsFigure 1(c).

Conventional solid-state imagers are made with a planar surface. However, we have extensive experience in producing back-illuminated devices, in which the silicon is thinned to several tens of microns. We developed methods to accomplish this thinning to entire wafers (currently 200 mm diameter) and to handle the thinned membranes.

Thin silicon membranes are flexible and robust, as demonstrated in Figure 1(b).

Figure 1(c) shows a silicon membrane cut into petals and formed into a spherical surface over a mandrel. The model shown represents a solid angle of one steradian. We have designed and built a novel CCD to fit this petal format. In this device, charge is clocked radially out along each petal into an output register located at the outer edge of each petal.

The non-planar focal surface technology has been applied to special devices we have designed and fabricated for a large optical system (see the page for Space Surveillance Telescope).













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