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According to this description, class II lasers have a power output of less than 1mW, class IIIa lasers have a power output of 1-5mW, and class IIIb have a power output of 5-500mW.

Can a class IIIb 25mW 5V laser be turned into a class II 0.5mW laser simply by powering it with a PWM signal at a 2% duty cycle?

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closed as off-topic by CuriousOne, John Rennie, ACuriousMind, Martin, Danu Sep 10 '15 at 15:59

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    $\begingroup$ Probably not - one reference says: "Class 2 lasersare CW and repetitively pulsed lasers with wavelengths between 0.4 µm and 0.7 µm that can emit energy in excess of the Class 1 AEL, but do not exceed the Class 1 AEL for an emission duration less than 0.25 seconds and have an average radiant power of 1mW or less." By pulsing you reduce the average power, but do not reduce the peak during the pulse. Sorry. $\endgroup$ – Jon Custer Sep 9 '15 at 15:25
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    $\begingroup$ If this is important to you, then you need to ask a certification consultant. None of the answers you will get on this site have any meaning in a case where you will have to defend your classification. $\endgroup$ – CuriousOne Sep 9 '15 at 15:25
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    $\begingroup$ To amplify @CuriousOne's comment - get with your institution's Laser Safety Officer - they should know what to do. I personally know people who screwed up and have permanent eye damage. Don't do something stupid. Trying to skirt the rules is stupid. $\endgroup$ – Jon Custer Sep 9 '15 at 15:29
  • $\begingroup$ @Jon Custer, What if I used a capacitor in parallel with the laser to smooth out the pulse into a voltage with a constant reduced peak? $\endgroup$ – Cerin Sep 9 '15 at 20:18
  • $\begingroup$ @Cerin, you might also be interested in a similar old question on ee.se: Laser - Damaging to your eyes? $\endgroup$ – The Photon Sep 9 '15 at 23:46
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Can a class IIIb 25mW 5V laser be turned into a class II 0.5mW laser simply by powering it with a PWM signal at a 2% duty cycle?

One of the requirements for laser safety is that the power output must be limited under foreseeable single fault conditions.

One foreseeable fault condition would be if the PWM signal were somehow stuck at the high level. For example, this could happen due to a software fault causing the PWM to stop switching while in the high state; or due to a physical fault like a bit of wire falling between the pwm signal wire and a power supply wire.

If you added sufficient safeguards (maybe fuses or clamp circuits) to ensure that such faults couldn't result in an unsafe laser output power, you could probably get your system certified as class 2.

(Note that current practice uses "class 1", "class 2", etc. The old ratings in "class I", "class II", "class III" are obsolete).

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