Originally Posted by
heroic
Well, there's power and there's power.
It's important to remember that power is energy delivered divided by the time taken to deliver it- one watt is one joule per second. But when you're talking about physical effects, what's more important than the total energy delivered or the rate of doing it, is the energy delivered to any one spot. This is what we call "power density", and is measured in watts divided by the area across which they are spread.
Now, remember that we're dividing these numbers out, which means we're talking about averaging. Put yourself in the position of one particular spot on the wall, and imagine what happens to you when you're being scanned. Let's say that you're being scanned ten times a second, each time for a tenth of a second, with a laser that has a power of one watt. That means that nine tenths of the time, the laser is pointing at something else; you only get exposed to one tenth of the power. That's your average exposure. However, during the other tenth of the time, when the beam is pointing directly at you, you're not being exposed to 0.1 watts- you're being exposed to the full watt!
What happens at this point depends on what kind of material you're made of; if you're made of some relatively inert, conductive material, the heat won't build up much during that time and the average power will give a good indication of the effects. If you're made of something that's sensitive to light- say a camera sensor, a projector DLP or a human retina- then the peak power is more important than the average power!
If you're scanning really really fast, then you might reach the point where the delivered energy to any one point during the scan interval is safe even for sensitive materials, and then the average power delivered is again the more appropriate number to describe what's going on. (You would need to be scanning very very very fast to do this with an eye, though; somewhat less so with a camera.)
So that's the story with scanning. However, there's another effect, and that's power density again; the size of the beam affects the power density, as I said earlier, and even laser beams gradually spread out (that's what divergence is). Divergence is measured in radians, which you may remember from high school are a measure of angle. The way this works is, at a divergence of one radian, one metre out from the source, the beam will be one metre wide. Obviously a radian is a pretty big unit, so we tend to use milliradians for lasers- 1/1000th of a radian. A decent DPSS will give you a divergence of 1 milliradian or so. So this is another way of saying, ten metres away from the projector, the beam will be one centimetre wide. Obviously as the beam spreads, the energy in it spreads too; the power density reduces. This is how safe audience scanning is achieved with powerful lasers- you use diverged beams so that the pupils of the audience don't catch more than a safe amount of the energy.
Hope this helps to explain the principles behind the equations!
-J.
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djmatt
I just printed this out so i can study it more.I would like to try to figure out how to do all this and see what I can come up with for mine. This stuff is just too cool to me!!!
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