>> 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
>
>This can be replaced by two solar cells plus either a small incandescent or
>laser diode
>projected through a hole which covers half of each cell at a cost of about
>$5.

The particular advantage of this part is that the two differential
elements are separated by a submillimetric distance allowing much higher
resolution.

Also, making a small spot at the end of a long throw beam is really easy
if you put a short focal length lens at approximately it's focal lens
over the detector.


regards
Mark
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