[Cyborg] 200 laser diodes

Mitch Altman maltman23 at hotmail.com
Sat Jun 5 19:25:15 UTC 2010

> Date: Sat, 5 Jun 2010 11:21:51 -0700
> From: mrericboyd at yahoo.com
> To: maltman23 at hotmail.com
> CC: adi at hexapodia.org; cyborg at lists.noisebridge.net
> Subject: Re: [Cyborg] 200 laser diodes
> Mitch Altman wrote:
> > > I have an interesting safety question: do you think that PWM controlling
> > > the lasers would actually make them eye safe? For instance, let's say
> > > we discover that the true optical power of the lasers is something like
> > > 10mW. Would it actually make them any eye safer to PWM control a laser
> > > to 50% duty cycle? Theoretically that reduces the "power" to 5mW, but I
> > > expect that in practice it's the peak intensity on your fovea that
> > > causes the damage, and PWM will not reduce the peak intensity. Thoughts?
> >
> > PWM definitely can reduce the effective light output. But, will these
> > laser diodes pulse at a high enough frequency to allow for PWM? We'll
> > just have to try and see.
> Actually, it's precisely the fast response of the lasers that I am 
> worried about. PWM can definitely control the average 
> electrical/optical power, but my understanding it that is isn't 
> necessarily the average that is dangerous. If you had a 1W laser, but 
> PWMed it to 5mW (so 0.5% duty cycle), I still wouldn't want to shine 
> that in my eye, I think it would be hella dangerous, much more dangerous 
> than a constant-strength 5mW laser. It's the difference between a bed 
> of nails and a nice mattress. It gets more dangerous the slower the PWM 
> is, of course. So maybe the trick is that we need to PWM as fast as 
> possible? Arduino PWM is only like 500 Hz, which isn't very fast.

Yeah, exactly.  Look at the extremes.  If the PWM is at 0.5Hz, that certainly wouldn't be OK, since 1/2 second of laser light focused on your retina would make most people very unhappy.  But if the PWM were at 1MHz, then it would be fine, as no amount of laser light we'll be obtaining can cause any damage in 1usec.  Of course, our PWM will be somewhere within those extremes.  Conceivably, one could do the math to determine the minimum amount of time for PWM period for a given amount of laser light that is safe (i.e., the amount of time it takes for the given amount of laser light to heat up your eye too much, and the amount of time it takes for you eye to cool down enough once exposed to that amount of laser light).  If this were PWM for an IR diode rather than an eyeball, the math is pretty easy (I over-drive the "safe" limit for IR emitters in my TV-B-Gone and have never had an IR emitter fail me yet).  But I don't know how to do the math for safety for eyeballs.  And the consequences are a bit more upsetting with eyeballs than a $0.14 IR emitter.  Anyone know how to do the math for eyeballs?  Or, are there studies that show the safety limits for given amounts of laser intensity?



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