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In article <5632@homxc.ATT.COM>, h16@homxc.ATT.COM (D.JACOBOWITZ) writes:
> In article <1895@tank.uchicago.edu>, kean@tank.uchicago.edu (Keane Arase) writes
> > 
> > The laser beam will spread 3.5 feet in 1000 ft (1/5 mile!) and register
> 
> 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
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
a single frequency, discounting some small amount of higher and lower energy
photons arising from trap states within the energy gap and higher-energy
radiative recombinations.
Unlike a laser, an LED is not COLLIMATED.  I.e., it's output is not even
close to being aligned.  Photons are emitted in many directions.
But even a laser is not perfectly collimated.  If you examine the near-field
and far-field patterns of lasers, you quickly come to the conclusion that
they do possess some spread.  A collimating lens minimizes the spread.

(Now gas lasers?  That I'm not sure of.  My field is semiconductors.)


Norman Kluksdahl              Arizona State University
            ..ncar!noao!asuvax!enuxha!kluksdah

standard disclaimer implied

Useful criticism always appreciated.  Senseless flames always discarded.