That question has a million answers so you have to be a bit nore
specific. The range of any system depends on both a) the transmitter, b)
the receiver and c) the transmission medium. If it's short range, (and
air without smoke!) you can forget the latter. If if it's a laser pointer
application, the receiver is your eye in combination with the
reflectivity of the target. If as I suspect you are probably interested
in an short range altimeter the answer is dependent on some heavy
technical design considerations e.g. modulation of the beam, its
collimation (one of the advantages of lasers), receiver aperture and
sensitivity, range gating, noise cancellation etc. etc. If you want
something buyable, there are digital readout devices available for tens
of pounds which work on unambigous targets (like flat walls) at ranges
5-10m max. but they tend to get their sensitivity by methods which result
in several seconds of response time. I tried one for backing the car !
gemin
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