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From: rstevew@armory.com (Richard Steven Walz)
Subject: Re: Crazy? Legal? Fesable? POSSIBLE AT ALL???
Organization: The Armory
Date: Tue, 4 Oct 1994 08:05:12 GMT
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References: <CwuE1w.o1x@nyongwa.montreal.qc.ca> <1994Sep29.133901.22319@mksol.dseg.ti.com> <CwxDys.4Hv@nyongwa.montreal.qc.ca> <36i7k8$60t@nntp1.u.washington.edu>
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In article <36i7k8$60t@nntp1.u.washington.edu>,
Blake Hannaford <blake@u.washington.edu> wrote:
>Prof. Kelin Kuhn studies lasers optics and photonics at 
>the University of Washington.  She is also something of
>a safety expert regarding her laser lab.  Finally, she
>and I are co-designing a course on Consumer Electronics 
>Design so she is very familiar with CD player lasers. 
>
>I asked her to comment on this debate and people might 
>be interested in her response:
>
>From kuhn@maxwell.ee.washington.edu Fri Sep 30 11:06:50 1994
>From: Kelin Kuhn <kuhn@maxwell.ee.washington.edu>
>Subject: Resend of bounce using reply
>To: blake@uw-isdl.ee.washington.edu
>Mailer: Elm [revision: 70.85]
>
>> Well, I'll add my two cents worth here.
>> 
>> First, the comments by Sam Goldwasser are correct on the fact that what
>> you see coming off a CD diode is NOT the laser.  It is additional emission
>> by the diode and is usually very dim.  This makes the laser diode very
>> deceptive.  A 25 mW 780nm laser diode will look very innocent, but still
>> have enough power to burn your retina. 
--------------------
Indeed! But we are talking about 3mW laser diodes with lens systems that
diverge the beam after convergence quite close to the lens, e.g. < 1mm .
I am aware of the difference between 680nm and the 780nm you don't see!
-Steve

>> However, the second set of comments are correct -- as far as they go.
>> Most CD player laser diodes are less than 1 mW -- and are generally
>> considered eyesafe Class I lasers.  It is true that the majority have 
>> very wide thetas, often 10 degrees in one direction and up to 30 degrees 
>> in the other.  It is also true that they require an external collimating 
>> lens (either a cheap plastic lens, or a GRIN lens) to even get something 
>> that resembles collimated light.
>> 
>> However, there are a number of things that bother me about the second 
>> article.  
>> 
>> To begin with -- there is a technical error.  Although CD diodes are not
>> highly monochromatic -- they are still a hell of a lot more monochromatic
>> than a flashlight.  Most laser diodes have a center mode linewidth of a
--------------------
And what ISN'T more monochromatic than an incandescent source?
-Steve

>> few A -- not exactly an incandescent light. 
-------------------
A few "A"'s is close, but non-coherence dispersion is not the issue here.
-Steve

>> Second -- and more importantly -- laser safety is a one time only error.
-------------------
So's driving a car. Know many blinded by broken CD players? I have seen
even young children taking them apart and playing with the laser and
watching it "hunt" when it's out of alignment! I know plenty killed on the
streets! I don't know of any with retinal damage! Do you?????? Answer me!
-Steve

>> Remember that all laser diodes look much alike -- from the cheap 1 mW 
>> diodes in portable CD players to the 100 mW diodes used for optical 
>> spectroscopy.
--------------------
I'm sure we'll all go out and burn out our retinas right away!
-Steve

>> There are lasers in WORMS that are more than capable of burning your
---------------------
Lady, I can do that with a magifying glass! Of COURSE WORM-drives have to
be able to blow little holes in Al ! That's what they're for!!!
How many of us have one, even a broken one???
-Steve

>> retina.  High quality index-guided laser diodes coupled with a GRIN lens
>> and in the 5 mW power or greater can also burn your retina -- and there is
>> no visual way to tell if you have a cheap gain guided diode or a more
>> expensive index guided one.  Although the majority of laser diodes in
>> cheap portable CD players are indeed "eyesafe" (i.e., less than 1 mW
>> diodes with cheap plastic collimating optics) --- by considering all CD
>> laser diodes (or all laser diodes for that matter) eyesafe is to engage in
>> Russian roulette. 
--------------------------------
I can't disagree about that! But my comment was not a call to have parents go
buy up powerful laser diodes in the IR and hook them up to play with for
their kids! They weren't doing that with He-Ne's, why would they with
these? Oh, yes, of course, they're "INVISIBLE"!!! How horrible! I doubt
that any one with the know how also does not understand that these are
concentrated light sources!
-Steve

>> It seems to me that the individual who wrote the second set of comments has 
>> never suffered a retinal burn or known someone who has.
---------------------------
I know a couple kids who looked at the total eclipse in the 60's and burned
their retinas. One pretty badly! You might be a PhuD, lady, but I'm a 44
year old BA in physics who deserves some credit for brains!
-Steve

>> I generally suggest that students wear laser goggles for any laser diode
>> over 1 or 2 mW.  (Remember that we have some laser diodes in my research 
>> lab -- which by the way, look just like the 1 mW ones -- that are 5 watts!)
---------------------------
Hear the one about the lab group that wore the wrong spectrum goggles for
Argon? Their retinas are Ar Gon!!!;-(
-Steve

>> For lasers less that 2 mW, I suggest that students avoid staring at the
>> diode.  Remember that 1 mW Class I lasers are NOT eyesafe when stared at. 
>> The class I laser distinction assumes a blink response -- not flat out
>> staring.  This may sound silly, but there IS a VERY strong temptation to
>> stare at the diodes.  The reason is that they are putting out a very dim
>> signal at 680 nm and students will tend to lean close to the laser and
>> stare at the dim red light in order to to see it better. 
>> Kelin
---------------------------------
How about if the light is dispersed in a huge cone, as in the lenses in a
CD player? Has it dawned on you yet that people aren't really as stupid as
you seem to wish they were for your self aggrandizement? Safety is one
thing, assininity is another. I suggest you research these optics if you
don't know that the beam disperses down to piffle at a foot! You'd have to
stick your pupil right against that lens to get the W/m^2 you're talking
about!!!

>> > Kelin,
>> > This is part of a raging debate on the saftey of CD lasers 
>> > going on now in comp.robotics.  Your expertise would be 
>> > welcome.
>> > 
>> > Blake
>> > ...........................................................
>> > In article <SAM.94Sep27090926@colossus.stdavids.picker.com>,
>> > Sam Goldwasser <sam@colossus.stdavids.picker.com> wrote:
>> > >RE: red light from CD Lasers.
>> > >
>> > >There is a small amount of red light emitted by the Lasers in CD players.
>> > >However, if you look into this thinking that you are seeing the main
>> > >beam, you are mistaken.  DON'T DO THIS!!!  The main beam is invisible.
>> > >Just compare that red light intensity to a Laser pointer or 1 mW HeNE
>> > >Laser.  It is much much less.  The power output of a CD Laser is in
>> > >the .3-1.0 mW or so range.  I have a Sony Discman Laser that
>> > >says .3 mW, for example.  WORM and MO drive Lasers are higher power since
>> > >they need to heat the disk material to record data.
>> > >
>> > >CDs are designed to work best with IR, usually 780 nm.  If my memory serves
>> > >me correctly, the depth of the pits on the CD are 1/4 wavelength of
>> > >780 nm light in the polycarbonate plastic so that you get destructive
>> > >interference at the pit edges and enhanced SNR for the returning data.
>> > >--- sam
>> > -----------------------------------
>> > Oh boy have people got this issue screwed up!!!
>> > 
>> > The solid state lasers are so poorly collimated that they barely work in
>> > CD's!!! Without the plastic lens which focuses them on the CD at the
>> > aluminum layer, they are usually so spread out that 3mW of anything
>> > couldn't hurt you at any reasonable range, and by the way, they are this
>> > way because they are not very coherent either or monochromatic! They have
>> > lines in the visible as well as the IR, and when the CD focusing lens is
>> > over them, they cone all their output to about a two foot circle at a one
>> > foot distance!!! If you do the math, you will find that it would be damned
>> > hard to hurt anything with them! The W/m^2 is simply minute!! Even the best
>> > solid state laser diodes issue light in a wedge shaped output from the
>> > substrate junction that is hard pressed to have much of it reasonably focus
>> > by lens into anything like a parallel beam! I would love to see how little
>> > these actually put out, but I bet they are below all safety limits and not
>> > sufficiently concentrated without their conizing lens to harm tissues even
>> > on the retina!!! Any solid state laser people to confirm this? This is what
>> > I learned in a good solid state device course! In other words, mere
>> > amateurs are ruling on the safety of basically glorified LEDs!!!!!
>> > -Steve Walz   rstevew@armory.com
>> > P.S., And yes, I DO know that they CAN and DO make solid state lasers that
>> > can be harmful. My contention is that these are NOT THEM!!!! Unless someone
>> > had a special focal length lens made to focus this spread out light precisely
>> > on their retina, harm to the eye from them would be impossible!!! But this
>> > is as dumb as warning college students not to look at a 100W bulb with a
>> > magifying glass!!!! Of course they shouldn't! But in this case they would
>> > nearly have to grind their own special lens!
>> > -Steve
----------------------------
-Steve Walz   rstevew@armory.com
-see below:

\                                                                 /
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    \                            PEOPLE                       /
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                                 x   <---- CD
                                / \
                              --------   lens

-Steve
----------------------
