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From: sam@colossus.stdavids.picker.com (Sam Goldwasser)
Subject: Re: Broken CD Rom drives
In-Reply-To: rstevew@armory.com's message of Thu, 29 Sep 1994 17:40:54 GMT
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Date: Fri, 30 Sep 1994 02:33:05 GMT
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In article <CwwJs8.Hp4@armory.com> rstevew@armory.com (Richard Steven Walz) writes:

>   Here is the confirmation I am looking for. Then for a 3mW laser diode, the
>   beam at 1" is spread over 1/96th of 6" diameter, which is 6.25 x 10^-2
>   inches, or .15875 cm and .0015875 m. Now the area of that circle is
>   1.98 x 10^-6 m^2. Thus the W/m^2 is  3 x 10^-3 W/ 1.98 x 10^-6 m^2 =
>   1515 W/m^2 or the solar flux level. Given that these lasers are about 10%

10% efficient?  A 3 mW laser is a 3 mW laser - this rates the optical output,
not the electrical power in.  A typical CD laser uses 50 mA at 1.7 V - 85 mW.
If you mean the optical system, it is much more efficient than 10%.
However, you are correct that many CD players have about .3 mW optical output.

>   efficient, this becomes 150 W/m^2 or about like looking at a light bulb
>   while changing it with it on! And this is of course at the retina, an inch
>   back inside the eye. This seems unlikely to blind anyone anytime soon! It's
                                    ^^^^^^^^^^^^^^^^^
Unlikely or impossible?

>   It's not even probably THAT concentrated, because the beam as it emerges
>   from a junction laser diode has width! The cone does not even come to a
>   point at the source! Thus I am being slightly conservative! Couple that

Sure, but it can be focused to a defraction limited spot of less than
a micrometer!  The beam from a HeNe or ruby laser doesn't come from point
source either.  The beam can be sharply focussed because it is coherent and
monochromatic.  In order to read a CD, you need to distinguish features that
are on the order of .5 um (the pit width) on tracks that are spaced
1.6 um apart.

>   with not even presenting a large amount of heat to dissipate at the retina,
>   because it lacks size, and the retina can sink some heat to cooler adjacent
>   regions, and we find the equivalent of looking at perhaps a bright reading
>   lamp bulb at a several feet away. And THIS would require someone look
>   directly at this laser diode module's direct output with it virtually
>   touching the pupil!!! This does NOT present great danger from these

Is it inconceiveable that someone who did not see a bright viible beam would
get quite close to observe the faint red spurious emmission not realizing
that the main beam is IR?

>   calculations! Anyone with more info????
>   -Steve Walz   rstevew@armory.com

Again, the problem is that these lasers are IR and essentially invisible, NOT
that they are instantly blinding or inherently dangerous.  However,
experimenters WILL modify the optical systems for their needs, thus increasing
the potential risk.  Other types of optical drives may use higher power
laser diodes which could present more serious risk especially with modified
optics.  The beams will be just as invisible.

Used with the proper precautions and common sense, the risk is very low.

Would you rather be safe or sorry?

--- sam

Samuel M. Goldwasser
Technical Director, Visualization, Computed Tomography
Picker International, St. Davids, PA 19087
sam@stdavids.picker.com
