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From: sleat@sim.ardent.com (Michael Sleator)
Newsgroups: comp.graphics
Subject: Re: Color desktop scanners (a technical
Summary: continuous-spectrum != multi-line
Message-ID: <7970@ardent.UUCP>
Date: 26 Aug 89 01:08:12 GMT
References: <1869@ucsd.EDU> <46900036@uxe.cso.uiuc.edu>
Sender: news@ardent.UUCP
Reply-To: sleat@sim.ardent.com (Michael Sleator)
Organization: Ardent Computer Corp., Sunnyvale, CA
Lines: 93

In article <46900036@uxe.cso.uiuc.edu> mcdonald@uxe.cso.uiuc.edu writes:
>
>
>>I have been told a white light laser does not exists because
>>cramming all those frequencies together they start cancelling
>>out and moving in different directions after some distance.
>

The participants in this discussion would do themselves, eachother,
and all of the observers a favor by defining their terms.  "White light"
is a very ambiguous term in this context.  We seem to have at least two
conflicting interpretations in progress.  One seems to be "radiation with
a more or less continuous spectrum", and the other seems to be "radiation
with sufficient spectral components that it appears white", or something
like that.  The indirect quote above, referring to "all those frequencies",
suggests a continuous spectrum.  The following paragraph seems to be
referring to a "multi-line source"; one that produces multiple
simultaneous (or nearly so) narrow-band outputs that viewed together
appear white.

From my limited knowledge of lasers, I can well believe that there are no
continuous-spectrum lasers.  How would such a beast work?  In what sense
would it even be a laser?  (In my understanding of them, lasers inherently
involve resonance, a notion rather at odds with a continuous spectrum.)
(These are rhetorical questions.  If you want to answer them, I'm quite
sure this is *not* the place to do it.  If you do it somewhere else, send
me a pointer, please.)

>White light lasers, very genuine ones, really do exist. There is one
>running right now about 30 feet above my head. The different colors do 
>come out at different TIMES - but, do you care that one color
>comes out 0.0000000000001 second before another? In any case, further
>work is expect to get them all coming at out once anyway. Another
>factor of three faster pulses and there would indeed be genuine,
>all at once, white light. All the colors would come out in 
>one pulse 0.0000000000000025 second long. 
>
>I should add that these lasers are really something to behold. They
>generate lots of oohs and aahs from the non-jaded person - laser 
>tables 5x14x2 feet weighing 5 tons crammed full of hundreds of optical
>parts. (This include amplifiers.) 

I'm sure they are, but it doesn't sound as if they're particularly
appropriate for the original application: an optical scanner.  To
corroberate your point that multi-line lasers do exist and to bring
it a little closer to home, I offer the following:

In LASER FOCUS/ELECTRO-OPTICS, vol. 24, no. 12, Dec. 1988, p. 62
there is an advertisement by Nihon Dempa Kogyo Co., Ltd.  ("NDK") for
their "RGB LASER(tm)".  This is a linearly polarized Helium-Cadmium laser,
model RGB300, with the following specs:

	output wavelength	RED	GREEN	BLUE
	(in nanometers)		636.0	537.8	441.6
				635.5	533.7

	output power		> 25mW (total)
	beam diameter		approx 1.0mm (1/e^2)
	beam divergence		approx 0.6mrad (full angle)
	optical noise		<1%

The tag says, "NDK introduces its hollow cathode He-Cd laser commercially
for the first time in the world.  RGB LASER(tm) emits three primary colors,
red, green and blue light, simultaneously and coaxially by a single laser
tube."

Model RGB400 is also available with 100-150mW output.  They don't give
any indication of the relative power outputs at the different wavelengths
for either model.

Note that these are CW lasers, not pulse lasers like the monster described
above.

It doesn't seem that the fact that the red and green outputs are twin lines
would be a significant problem in a scanner application, since the pairs
are quite close together.  In the acompanying photo, they show the end of
the enclosure, which appears to be a table-top box roughly the size one
might expect for a comparable power He-Ne laser.  The beam is shown being
split into red, green, and blue components with a series of dichroic mirrors.
It looks pretty good in the picture...

No mention is made of cost.  Nevertheless, it appears that the technology
is there, and if a market exists, it can probably be made cost effective.
(A thousand bonus points to the first person to report back with news of
a scanner product actually using one!)


Michael Sleator
Ardent Computer
880 W. Maude Ave.
Sunnyvale, CA  94086
408-732-0400
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