Power measurements and the wavelength?

[norty303]

Whilst I understand that different wavelengths have significantly different perceived brightness to the eye, do you need to take this into account when doing MPE calcs? I know from one of the tables in HSG95 that they do get grouped differently in one of the tables but most of the visible wavelngths are together in the same group.

The reason I ask is because I have the free version of an MPE calculator that is restricted to only 532nm. If I want to use for 650 as well then my thinking is that it should be fine right? Although less visible than 532nm, it still presents the same amount of energy and therefore the same math should apply yes?

Interestingly, my 1.1w 650nm is actually less dangerous than my 400mw 532nm because of the beam size and divergence.

[buffo]

different wavelengths have significantly different perceived brightness to the eye, do you need to take this into account when doing MPE calcs?

No. As far as the regulations are concerned, power is power, whether it's red or blue.

Now, you *could* make the argument that the diffraction limit for shorter wavelengths is smaller than it is for longer wavelengths, which would make blue more dangerous than red when focused to a spot on the retina, but this has nothing to do with the eye's sensitivity to a particular wavelength, and in any case it is not something that the regulations require you to take into consideration. They treat all visible light the same.

I have the free version of an MPE calculator that is restricted to only 532nm. If I want to use for 650 as well then my thinking is that it should be fine right? Although less visible than 532nm, it still presents the same amount of energy and therefore the same math should apply yes?

Yes. So long as you have the correct beam diameter, power, and divergence measurements, the calculation is the same.

Interestingly, my 1.1w 650nm is actually less dangerous than my 400mw 532nm because of the beam size and divergence.

This is actually quite common. Most DPSS green lasers have very good beam specs, while higher-power direct-injection diode lasers frequently have lousy beam specs. (Very wide beam width in the fast axis, and horrible divergence to boot.) But while that may make them less desirable for use in a laser projector, it does offer increased safety in the case of an eye exposure.

Adam

[norty303]

Ok thats great.

One other thing that has occured to me, how do you work out an overall MPE for solid state lasers where they have very different beam specs? In trying to establish a worst case scenario, (projecting orange using the previously mentioned 532 and 650 lasers) would you simply work out your safe limit for 650 being 5 times MPE and 532nm being 5 times MPE and add them together? (obviously I'm talking about 'new' type of limits in UK - transpose 0.5x MPE as you wish)

[Xytrell]

The retina is red, therefore it reflects a lot of the red that is beamed at it, right? Does that play a factor?

[mixedgas]

Ok thats great.

One other thing that has occured to me, how do you work out an overall MPE for solid state lasers where they have very different beam specs? In trying to establish a worst case scenario, (projecting orange using the previously mentioned 532 and 650 lasers) would you simply work out your safe limit for 650 being 5 times MPE and 532nm being 5 times MPE and add them together? (obviously I'm talking about 'new' type of limits in UK - transpose 0.5x MPE as you wish)

The charts are in here:

http://www.army.mil/usapa/med/DR_pub...f/tbmed524.pdf

Where color really matters is outdoors in aerospace paperwork.

steve

[norty303]

Sorry, I wasn't really referring to the colour, I was referring to the fact I'm trying to evaluate the combined output of a 400mW 532nm at 2mrad and 2mm dia and a 650nm 1.1W at 5mrad and 5mm dia. Clearly as a single beam with both wavelengths we're talking about a beam with a hot spot.
In lieu of a power meter I do know how much power the lasers do at CW at various input voltages, therefore I can work out a worst case scenario for power in any given situation, and this also has a built in safety margin as we know that modulating causes an average decrease in power.

[Doc]

I don't know specifically how you would do this with the software that you have, but you would calculate the power density in mWcm2 for each colour at the closest distance that eye contact is possible, then sum all three density figures together.

The summed value for a full power white beam is the figure you need.

You then implement your attenuation map if needed and re measure.

Repeat until you hit the value you're aiming for be that 5MPE or whatever.

I aim for 8MPE in the front zone but then keep the show overhead for most of the night with only a few minutes total for the whole night in the front zone.

Audience scanning is a fantastic experience for the crowd, but like anything else; it becomes boring if over done, so save it for the show highlights.

[Doc]

Have a peep at this

Its a first hash, but I'm sure that between us on this forum we can make it more accomplished and usefull.

[buffo]

how do you work out an overall MPE for solid state lasers where they have very different beam specs?

The quick and dirty method is to assume that all your lasers have the same divergence. Then you work the numbers based on the one that has the best (that is, least divergent, which is the most dangerous case) beam specs. And as Doc already mentioned, you measure the power for all three lasers and then sum them to get the total power. Measure the beam size for the best laser, calculate the area, and divide that into total power and you get irradiance.

But really, the easiest way to do it is to measure the irradiance directly with a calibrated sensor of a known size. So long as the beam completely fills the detector (spilling over the edges is OK), you will get an accurate measurement. Then you assume minimal divergence (use the specs from your best laser) and plug the numbers into your calculator.

Adam