Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!uflorida!haven!adm!cmcl2!ccnysci!sukenick
From: sukenick@ccnysci.UUCP (SYG)
Newsgroups: sci.electronics
Subject: Re: Correct Terminology
Summary: Correction for the correction
Keywords: radar, laser, lidar?
Message-ID: <1310@ccnysci.UUCP>
Date: 23 Feb 89 17:06:40 GMT
References: <603@icus.islp.ny.us> <7944@netnews.upenn.edu> <5632@homxc.ATT.COM> <55@enuxha.eas.asu.edu>
Reply-To: sukenick@ccnysci.UUCP (SYG)
Organization: City College Of New York
Lines: 35

>> A laser will spread 3.5 feet in 1/5 mile? That doesn't sound like coherent light.
>
>Beware the trap of incorrect terminology!  There is a great difference

indeed!

>between COHERENT light and COLLIMATED light.  A laser is pretty much
>coherent light in that the spectrum of output is nearly delta-function like.
>However, LED's are also nearly coherent in that their output is at nearly


Ahem!

No - No:

MONONCHROMATIC: narrow frequency range ( The minimum width of a line
		depends upon temperature, etc. (doppler shift
		uncertainty principle, etc)
		Also lasers may lase at different frequencies at the same time
		other than the advertised line (harmonics, etc) and don't
		forget dye lasers which have a BIG wavelength spread
		when used without a grating......

COHERENT: All them photons are in phase with each other for a while in and
	  as the stuff leaves the laser
	(as in: when  Bertha jumps up,  Fred jumps up  at the same time)
	Thats what them fangled lasers do. (then there's spatial & there's
	temporal coherence, let them doggies lie for now) )

Collimated: the output is directional, and there is no such thing as perfectly
	collimated... (Dr. Heisenberg wouldn't let you get them perfectly
	straight and even if you did your best, the photons'll feud
	amongst themselves and spread out)

LEDs are pretty much monochromatic but not very coherent at all.....