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From: rstevew@armory.com (Richard Steven Walz)
Subject: Re: Broken CD Rom drives
Organization: The Armory
Date: Mon, 3 Oct 1994 12:43:12 GMT
Message-ID: <Cx3Ko3.2HJ@armory.com>
References: <35sr1e$gco@handler.Eng.Sun.COM> <SAM.94Sep27090926@colossus.stdavids.picker.com> <CwwI80.Gso@armory.com> <SAM.94Sep29205954@colossus.stdavids.picker.com>
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In article <SAM.94Sep29205954@colossus.stdavids.picker.com>,
Sam Goldwasser <sam@colossus.stdavids.picker.com> wrote:
>In article <CwwI80.Gso@armory.com> rstevew@armory.com (Richard Steven Walz) writes:
>
>   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
>
>Not the reason.  Example: a HeNe laser, which is inherently very well
>collimated would also produce such a cone shaped beam if put through a short
>focal length lens.  However, another lens will recollimate it just fine.
>
>   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!!! 
>
>--- deleted ---
>
>Just because the beam coming out of a bare laser diode is wedge shaped does
>not imply that it is either incoherent or non-monochromatic.  It is true that
>these characteristics of solid state laser diodes do not compare to other
>types of lasers such as HeNe.  However, in order to read the CD, the optics
>do generate a nearly defraction limited spot on the disc's aluminized layer.
>(The track pitch is 1.6 um and the pit width is on the order of .5 um.)
>A bit more than a glorified LED.
>
>>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!
>
>Was your professor an optics expert?
>
>Have you ever seen a pocket laser pointer?  Seems fairly collimated.
>And, quite efficient, too.  Of course, they are visible so it is obvious.
>The beam from the diode in a laser pointer is wedge shaped also before
>it is collimated.
>
>> 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
>
>At any reasonable distance, true enough.  I believe this has been posted
>before.  The power output printed on the CD laser assemblies typically
>are somewhere between .3 and 1 mW.  This is the power level specified
>at the output of the optical pickup - not the diode.
>
>>   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
>
>Yes, you can buy more powerful diodes, visible and IR, from the national
>electronics distributers - about $20 for a 5 mW (output) diode.
>
>>   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
>
>You can see the light from a 150 watt bulb and therefore take the appropriate
>precaultions!
>
>Also, as the popularity of magneto-optical and other read/write
>optical drives increase, laser assemblies from these devices will become
>available surplus.  The power levels are often much higher than your common
>CD since they need to heat the disk material during the writing process.
>You cannot tell the difference in power by looking at an invisible IR beam!!!
>
>>   magifying glass!!!! Of course they shouldn't! But in this case they would
>>   nearly have to grind their own special lens!
>
>And someone posted to this very group about using some grab bag lenses 
>from Edmund Scientific to collimate the beam quite effectively.
>
>OK, I am not suggesting that CD lasers are so dangerous that a casual glance
>will cause any kind of damage.  However, as pointed out in previous
>postings, safety around any kind of laser is important based on awareness of
>the potential for harm with the particular type of laser and associated
>optics.  Many people reading these postings are the type of experimenters
>who will modify the optics to build rangefinders or illuminators.
>
>The point about IR lasers is that there is almost no visible indication
>of whether the beam is on or off, or how powerful it really is.
>
>--- sam
>
>Samuel M. Goldwasser
>Technical Director, Visualization, Computed Tomography
>Picker International, St. Davids, PA 19087
>sam@stdavids.picker.com
-------------------------------------
Actually, yes, my professor for Optics in my Physics degree IS an optics
specialist!! And yes, I realize that we shouldn't encourage teens and small
children, let alone idiot adults, to go ahead and look in any old laser!
What *I* said is that the CD and CD-ROM makers have an interest in NOT
getting sued for retinal damage, and that a signal reader doesn't have to
be significantly powerful to read CD's. This is why I analayzed and
questioned the blind certainty which many on the net cautioned others
against it in the case of CD players! And yes, the lenses to collimate the
wedge output of a solid state laser are simple to make or acquire, but that
few know how to make them collimate a laser to high energy density over
area! And the cone coming out of a CD player is sufficiently large in its
diopter number that even the human eye will not refocus it so as to damage
the retina! They weren't STUPID in making all these lasers in such cheap
devices that virtually everyone has two or more broken ones laying around
by now!!! They would be on the rocks if it were not extremely HARD to weld
your cornea or retina with one of these little darlings! And if you read up
on collimating a solid state wedge, you might note that doing so DOES
destroy its coherence, strictly speaking, and that its Airy disk soars in
half-width at that point; as I say, it's amazing they get them to work at
all, which they don't for long, (usually)!
-Steve Walz   rstevew@armory.com

