🚀 go-pugleaf

There are a number of techniques for measuring distance with lasers,
interferometer techniques are common in the laboratory as well
as for very accurate longer distance measurements. Interference between
a beam and the  reflected beam from a target results in fringes
that can be seen and counted with simple photodiode set-ups. Accuracy
in the the micro-inch range.

We have a couple of laser scanners a CMU for robot navigation and
they use a modulated beam to measure distance over a wide range fairly
accurately. The outgoing beam is modulated by a carrier frequency, and
the phase shift between that and the return signal is proportional to
distance. A fast clock gives the phase shift between outgoing and 
incoming signals. Additionally the amplitude of the incoming signal
can be used to give reflectance. In our set the laser scanner information is 
sent to a frame buffer where the brightness on the screen corresponds 
to distance. Looks neat cause close objects are dark and objects farther
away get lighter and lighter. This mapping of distance to gray level
plus the reflectance image results in a 'camera' style view plus
depth information at every point. (2 axis raster style scanner)

As the distance increases the phase shift will become complete (one full
wavelength) and this distance is called the 'ambiguity interval' This can be
changed or detected in software.

Surveying instruments use laser ranging techniques too. The beam is
pointed at a corner cube held by an assistant and the beam
is reflected back to a detector in the unit. I'm not sure what
technique they are using for this but I suspect a phase shift
method is used.

nivek
Aka :	Kevin Dowling		Bell:	(412) 268-8830
Arpa:	nivek@rover.ri.cmu.edu	Mail:	Robotics Institute
				Carnegie Mellon University
				Pgh, PA 15213-3890