Eye Safety with Pulsed Lasers

[tefeari]

If a laser has a duty cycle of something like 33%, does that mean that in terms of eye safety it is equivalent to a non-pulsed laser with 1/3 of the total power?

To make things simpler, let's assume that the laser doesn't pulse at insane rates (let's say less than 200 hz) nor does it pulse for insanely short periods of time (for example, 5 milliseconds on and 10 milliseconds off).

I've heard that laser safety on nanosecond or Q-switched lasers is much less straightforward, but I assume that here it is actually quite simple?

 

[FrothyChimp]

Turning the laser on and off is called modulation of a laser and must take into account the highest output the laser may be expected to reach. It is still calculated as a CW laser. Modulation only releases the maximum loss the output coupler is designed to release, leaving the rest of the energy in the resonator.

Q-switched lasers, on the other hand, have short but hugely intense output pulses and require a slightly different calculation to determine the OD requirements. The huge pulse is a result of release all the energy contained within the resonator in a very short period which is very different than the modulated CW above.

 

[tefeari]

So let's say we take a 15 mw laser pointer and modulate it at 60 hz with 5ms on and 10 ms off (33% duty cycle), is it now as safe as a 5mw laser?

 

[FrothyChimp]

No, it's still a 15mW laser.

 

[tefeari]

That feels a bit oversimplified. This is a 15 mW laser not a 15 mJ laser.

A LED pulsed at such a rate is going to need drastically less heatsinking for itself as the heating/cooling process is nonlinear. Similarly, the damage from such a laser would be thermal in nature, and 10ms would give the retina a fair bit of time to cool in between laser pulses (not to say that it is necessarily exactly the same as a 5 mW laser, however, this is the aspect of the topic I wanted to explore).

Obviously I'm not talking about shooting a laser into someone's eyes. However, I was hoping for a slightly more in depth discussion of the general topic.

 

[Bluefan]

With a 5ms on and 10ms on duty cycle a 15mW laser is not as dangerous as a CW 15mW laser but more dangerous than a 5mW laser. Any transient heating of the retina even of a few degrees can cause damage, so even though the power is on average 5mW it's not as straightforward to say it's as dangerous as a 5mW laser. At higher repetition rates the averaging can be applied better. The 5ms on period would then be shorter.

I do not know how to determine what powers at what duty cycles are dangerous and which are not, so it's best to err on the side of caution. That means applying the safety measures you'd normally do for a 15mW laser.

If I had to make a guess to the right approach I'd look up the MPE for both the pulse power and duration and for the average power and see if one of those is exceeded, which would be bad. But considering 5mW is already class 3A and bordering class 3B Any duty cycle averaging to 5mW will never be safer than a 5mW laser, so I woulnd't trust it to be safe.

 

[lasersbee]

If a laser has a duty cycle of something like 33%, does that mean that in terms of eye safety it is equivalent to a non-pulsed laser with 1/3 of the total power?

Basically.....

That would only apply if you tested the output of that modulated
15mW Laser on a Thermopile LPM.
The Thermopile would average out the pulse on/off time to give
an averaged power reading.

If on the other hand you used a fast optical sensor attached to
an oscilloscope you could easily see the Max pulse at 15mW..
The 15mW total is still there but for 1/3 the time...

Your eyes are optical sensors.... They would still get hit with
15mW spikes.

The duty cycle and repetition rate would also need to be considered..


Jerry

 

[ShortyInCanada]

So now, if you were to view a 15mw pulsed laser that had a 1/3 duty cycle and therefore was averaging only 5mw would the beam appear to be greater than an equivalent 5mw continuous cycle one ?
I don't have an example myself, but I think that the pulsed one would appear to be somewhat brighter. Although it isn't on for that 2/3 of the time, with the persistence of vision we would "see" it as brighter.
That perception would be correct in that the beam, when on, is still 15mw.
I would expect that it would still be just as dangerous to the eye as an unmodulated 15mw beam as there is no way for the human eye to dissipate the effects at those frequencies.
Regardless of the technical power average ratings.
The moment that you react to the first detectable exposure from something that powerful it would be too late already.

At least that is my take on it.

 

[tefeari]

Bluefan: Can you recommend any online resources for calculating the CW and pulsed MPE? Most references I find to such numbers often point to expensive offline books (in the many hundreds of dollars range).

ShortyInCanada: In my experience a 1/3 duty cycle essentially makes a light source appear 1/3 as bright. I haven't done a direct comparison, but qualitatively, there is a huge difference in perceived brightness from modulation.

 

[lasersbee]

I suppose that depends on if your duty cycle is 1 minute ON
and 3 minutes OFF... :undecided:

Jerry

 

[Guyfromhe]

I suppose that depends on if your duty cycle is 1 minute ON
and 3 minutes OFF... :undecided:

Jerry

Heh Really it only would matter if it was quicker than your eye / brain could detect and your brain had to do some post processing on it to bring it to "human time".

 

[ShortyInCanada]

I realize that a pulsed beam wouldn't appear to be as bright as the full power...
What I'm thinking is that it would appear to be slightly brighter than the average power.
And that it all would be due to the brain's perception and the persistence of vision.
-All being theoretical here now :
I'm thinking that the frequency of the cycles being fast enough that they aren't noticeable and that the brain would "experience" it as steady on would result in, for some reason I'm thinking a logarythmic manner. Much like a RMS power rating is calculated kind of thing. Your brain tricks you into thinking it is a bit brighter than the average because it can't determine the periods of darkness. So a mechanical measurement contravenes what your brain tells you is happening.
While the solid 5mw is obviously as bright as 5mw, the super-fast pulsed 15mw might be perceived as more like 8mw, theoretically. (If you could tell the difference between 5mw & 8mw in this example.)
Just my thoughts on this one. Because this sort of thing involving "perceptions" is always just what catches my attention. The way a person's mind can fool them despite evidence to the contrary. I just love that kind of stuff.