how is it that some lasers that are 300mw can burn stuff?

[johnpoe12]

how is it that some lasers that are 300mw can burn a hole through paper or light a match, and some others that are also 300mw are safe to be used in light shows?
just wondering

[Eidetic]

The same reason that some cars that weigh 1000 pounds can drive at 100 miles per hour, and some others that are also 1000 pounds are safe to be used in parades.

[-bart-]

Better analogy:

The same reason you can stand under the shower and enjoy it, while loosing your fingers when holding them under a water-cutter.

[Eidetic]

Or like how one can poke their eye out with a wooden toothpick, while some other wooden toothpicks are safe to use to pick your teeth!

[norty303]

I think the main point is that the 300mW laser used in a light show is just as unsafe as the pointer burning stuff.
The difference is that the show laser will probably be scanned and modulated so the chance of it falling on a single point for long enough to cause damage is very small (unless failure of some kind occurs). Also consider that burning with a 300mW laser takes place at a few feet or so. At 20ft it's unlikely to burn much stuf due to divergence and other factors. The one in the light show is unlikeluy to have anything/anyone within a good distance of the aperture.
One other thing I've noticed is handhelds are often focussed to a converging point a small distance from the aperture to improve burning. With a show laser, the operator usually tries to have the best divergence possible, or uses higher than normal divergance to spread the power for crowd scanning.

[-bart-]

Unicorn-glitter-almost-patented-technology makes lasers safe too

[johnpoe12]

Awesome thanks guys! and i like the analogy's...

[smokeAndMirrors]

Or maybe it's just that they use a diverging lens on the beam. Or a diffraction grating. Sorry to be to specific

[Insanity]

Most importantly, a show operator (if doing things properly) will calculate the irradiance of the laser beam based on its distance to the audience, beam diameter and divergence, and attenuate the laser accordingly to suit the defined MPE levels...

So for example, a 3 watt laser may well be doing 3 watts in the safe zone (above the audience) and doing far less power (due to attenuation and other methods) whilst in the audience... Prior to the show, the operator will also measure irradiance in the audience area...

In the UK this is a requirement for all operators based on the document HS(G)95...

So, effectively the 300mw laser in your example will probably not be delivering 300mw when in the audience area...

Mark

[heroic]

The same way that a brick with 40 joules of kinetic energy goes "thud" on a wall, but a bullet with 40 joules of kinetic energy leaves a crater.