MPE measurements

[Xytrell]

I also heard the laser check is not that accurate.

Perhaps, but it's not off by enough to give someone the reading "hey, it's eyesafe. Go for it" and then everyone's retinas explode.

[sk_1810]

Perhaps, but it's not off by enough to give someone the reading "hey, it's eyesafe. Go for it" and then everyone's retinas explode.

There is just one thing I don't understand... Let's say I'm using the Coherent Lasercheck which has an aperture of 8mm(I put it where the eyes of the closest member of the audience would be), and I need the result of laser power per square centimeter. 8mm diameter is not one square centimeter!

Maybe my logic is off, but please explain...

[mccarrot]

here is a example:

you have 100mW in a 4mm beam.


A= πr²

=π(2mm)²

=4πmm²

≈12.56mm²

If you want /cm² instead:

12.56mm² × 1cm/10mm × 1cm/10mm

= 0.1256cm²

So you have an average power density of 100mW/0.1256cm² or simplified: 796mW/cm² If you need a formula:

I = p/(π[d/20]²)

Where I is the average intensity in mW/cm², p is power, and d is the diameter of a round spot in mm.

[sk_1810]

here is a example:

you have 100mW in a 4mm beam.


A= πr²

=π(2mm)²

=4πmm²

≈12.56mm²

If you want /cm² instead:

12.56mm² × 1cm/10mm × 1cm/10mm

= 0.1256cm²

So you have an average power density of 100mW/0.1256cm² or simplified: 796mW/cm² If you need a formula:

I = p/(π[d/20]²)

Where I is the average intensity in mW/cm², p is power, and d is the diameter of a round spot in mm.

McCarrot, you'll gonna have to charge me for consulting services...

Thanks!

[mccarrot]

No problem, thats were a forum is for right

To complete this topic:

If you consult the ANSI Z136.1 (and the IEC 60825.1), one finds that the MPE is directly related to the time of exposure, using the formula of

1.8 x 10^-3 x t^3/4 Joules/centimeter squared.

If "t" is 0.25 seconds, we arrive at the common 2.5 mW/cm^2, which relates to the aversion response (1/4 second).


If "t" is 1 millisecond (lasershow scanning), then the MPE becomes about 10 microjoules/cm^2, which when divided by the 1 millisecond of exposure is about 10 milliwatts/cm^2.

going from an exposure time of 0.25 seconds down to 1 millisecond, our MPE goes from 2.5 mW/cm^2 to 10 mW/cm^2, a increase by a factor of FOUR! In other words, even with fast scanning, the increase in the MPE is rather small.

If the beam's irradiance is measured four times more powerful, at 10 mW/cm2, then the beam is at the MPE for a scanned beam. This beam could be aimed into the audience and -- AS LONG AS IT IS SCANNING CONTINUOUSLY WITH NO "HOT SPOTS" -- it would be OK (not exceed the MPE). The reason is that if a beam is scanned so pulses are 1 millisecond long, the MPE for such a pulse is 4 times the MPE for a 1/4 second exposure. That means we are allowed to have four times as much light if the beam is in short, 1 msec pulses, than if it relies on the 1/4 second blink reflex.


(I should also note that if the scan is even faster -- with pulses up to 18 microseconds -- the MPE for such a pulse is 10 times the MPE for a 1/4 second exposure. That would mean that scanning allows a 10 times increase over the 1/4 second MPE. However, since scan speeds can vary, it is safer to use the slower assumption of a beam causing a 1 millisecond pulse, which allows a 4 times increase over the 1/4 second MPE).

So this is ILDA's first recommendation: If you want a scanned show which is at the MPE, adjust a static beam until it's irradiance is 10 mW/cm2. During the show, the beam must be kept moving, scanning continuously with no hot spots. I should note it is generally better to increase the beam's divergence, in order to reduce the irradiance, rather than to simply lower the power of a tight beam. A fatter beam can be brighter since some light is beyond the detector's aperture.

In europe we could use the 10XMPE approach (100mW/cm2) for higher risk locations like raves and disco's.

Keep in mind that this's technique uses the worst-case beam: "all white" (all lines at maximum) with no blanked points. A real show will have a lot of colored beams where the beam power could be 2/3 to 1/3 less than the maximum "all white". During the colored sections, the 10x beam really could be 6.6x or 3.3x so the increase in MPE is not as bad as it might seem at first.

Credits goes to Greg Mahkhov and Patrick Murray

[Adrian Cavaleru]

@mccarrot...

did sk 1810 told you where is his location?

[mccarrot]

No, but MPE is the same over the world. but 10MPE is not accepted (yet) in the US.

In Europe where we have no rules about audiance scanning its best to use the 10xMPE. If you have a problem ILDA can back you up.

[White-Light]

Not sure...

But the lasercheck can only measure one wavelength so it's not usable for RGB laser MPE calculations. (or you must measure every color independent and do more math)

I also heard the laser check is not that accurate.

Also not sure what the safety officer will say if you tell him you did measurements with a pen

Jem's got one and it measures RGB - he used it to test my system. You can cycle through wavelengths.

As for accuracy, I can't comment.

[Adrian Cavaleru]

How close am I from reality when I say it seems that high power projectors will become "for own use" or "not for public areas" in the future?

[Jem]

Jem's got one and it measures RGB - he used it to test my system. You can cycle through wavelengths.

As for accuracy, I can't comment.

ahh... But you can only measure one wavelength at a time with the Lasercheck. With a proper thermopile head such as the PM3, you are measuring all wavelengths at once. This combined measurement makes calculations so much easier.

Cheers

Jem