New laser weapon works in fog rain

John Cuthber · September 26, 2014

I think it's a bit unfortunate that we don't have a lot of information about it,but that's only to be expected in the circumstances.

However, the vehicle, the nature of the beam steering and whether it's a a solid state laser or not don't make a difference.

Once the beam has left the truck, what stops it getting scattered every which way by the fog and rain?

We can make a few reasonable deductions, given the power of the laser it's probably IR.

It's more difficult to get high output power for shorter wavelengths.

Also, since the power is being focussed onto a small enough spot to do damage, the wavelength cant be more than 1 cm or so.

The scattering depends on wavelength, refractive index and the size of the droplets.

Well they can't choose the droplets.

For any given wavelength, the scattering will be what the weather God gives them.

They also can't change the refractive index of water.

So, all they can do is choose a wavelength where the scattering is lowest.

The simple way to do that it to see where the refractive index of water is most similar to that of air.

Since the index for air will always be much lower than for water, that's a matter of finding the wavelength where water has the lowest RI.

Well, here's the data

There are a couple of promising looking troughs at about 2 microns and 12 microns (or so- it's hard to read from the log scale).

But those correspond to absorption peaks so they are no use- the light isn't scattered, but soaked up.

The "hot spot" on the curve seems to be near 1.5, 4 or 8 microns,

At any of those wavelengths the RI is about 1.3

That's pretty near the same as it is at visible wavelengths.

So, from that point of view the reflectance of the water will be pretty close to what it is for visible light.

For most angles, that's about 2 or 3 %.

You don't need to scatter 3% very often before most of the beam is "lost.

Edited September 26, 2014 by John Cuthber

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