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In article ,
says...
Mark Borgerson wrote:
If your boat is moving, you could well be on a collision course that
would allow you to see only the red or only the green light. That
makes the problem more difficult.

I've described the colour discrimination method idea poorly. I'll try
again.

Imagine two photosensors side by side, identical except that one has a
green filter in front of it and the other has a red filter in front of
it.

Imagine a cylindrical lens in front of these photosensors, positioned
such that light from a distant source at a particular azimuth falls on
both photosensors, and light from any other azimuth falls on neither.

Imagine an electronic circuit which calculates the output from the red
photosensor minus the output from the green photosensor. Call this
result the "difference signal".

If light from a red source falls on both the photosensors then the
difference signal will be positive and similar to the signal strength
from the red sensor, as the green sensor will produce little output.

If light from a green source falls on both the photosensors then the
difference signal will be negative and similar to the signal strength
from the green sensor, though opposite in sign, as the red sensor will
produce little output.

If light from a white source falls on both photosensors then, (with the
relative sensitivities of the two sensors set correctly) the difference
signal will be zero since the red signal equals the green signal.

Thus a non-zero difference signal is indicative of a red or green light
(alarm!), and a white light produces a zero difference signal (no
alarm).

Sounds much closer to workable. Now all you need is some hardware
or software to discriminate against those beautiful tropical
sunsets, the green flash, and the occasional green tropical island
with floodlights over the beach resort. ;-)

I guess that could be done with an imaging sensor set to respond
only to point sources.

Depending on the sensitivity you desire, you might also have to
worry about your own running lights reflecting off waves or nearby
flotsam. To avoid that you may have to make the sensor auto-leveling
so it can ignore stuff near the boat. Hmm, it's getting a bit more
complicated now.

Mark Borgerson