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Really neat pic of the "pupil". One thing that comes to mind that uses the same technique is the traffic light sensor; it has a tube that points at oncoming cars, and only when the headlights are shining down the tube do the lights change.
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Inventor of the easy-to-DIY upflow scrubber, and also the waterfall scrubber that everyone loves to build:
http://www.reefcentral.com/forums/showthread.php?t=1424843
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