Path: utzoo!attcan!uunet!lll-winken!ames!mailrus!tut.cis.ohio-state.edu!ucbvax!decwrl!labrea!siegman@sierra.Stanford.EDU
From: siegman@sierra.Stanford.EDU (Anthony E. Siegman)
Newsgroups: rec.autos,sci.electronics,misc.consumers
Subject: Re: Correct Terminology (was Re: Radar Detectors (Ka band) ...)
Keywords: radar, laser, lidar?
Message-ID: <71@sierra.stanford.edu>
Date: 18 Mar 89 19:32:25 GMT
References: <603@icus.islp.ny.us> <7944@netnews.upenn.edu> <1895@tank.uchicago.edu> <5632@homxc.ATT.COM> <470@ispi.UUCP>
Sender: siegman@sierra.STANFORD.EDU (Anthony E. Siegman)
Reply-To: siegman@sierra.UUCP (Anthony E. Siegman)
Organization: Stanford University
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Re: The safety of a police laser radar

Laser surveying instruments that are in essence laser radars or lidars
have been in common use for some time, and do not pose any eye (or
other) safety problems.

Re:  Coherence and beam spreading

Laser beams must be characterized both by their _temporal_ coherence,
which means in essence how monochromatic, or spectrally pure, or
"single frequency" the laser output is; and by their _spatial_
coherence, which means how much coherence there is between the optical
signal, or optical wave, at different pairs of points across the
output beam, or the output aperture, of the laser device.  The two
types of coherence are more or less independent of each other.

A laser beam with high spatial coherence can be transmitted as a very
narrow "diffraction-limited" beam, or focused into a very narrow spot,
on the order of a single wavelength of light in diameter, using
suitable optics.  The smaller the starting diameter of a collimated
beam at the transmitting aperture, the more rapidly it spreads in the
"far field", which is why you use a large transmitting telescope if
you want the smallest spreading anlge in the far field.