This brings to mind the Centronics QD7-5 quadrant photodetector that
http://www.rs-components.com sell as part number 652-027.

This is a windowed hermetic TO5 package with foursensor elements
arranged as quadrants. It seems rather well suited to measuring very
small angular defelections in one or two dimensions when illuminated by
a cheap diode laser. Sadly the detector itself is not cheap.

have fun folks

Mark
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