How does laser dissipation in air affect power and intensity?

[Energize]

What equations would I need for calculating this, say if I wanted to know what the power/intensity of an x watt, x wavelength laser would be reduced to after x metres?

 

[mgb_phys]

Simplest is just the beam divergence. Unless the laser is perfectly focussed it will have a beam divergence in mRad. A milli-radian is 1 part in 57,000
Over very long distances you might want to consider atmospheric absorption.

 

[Andy Resnick]

If you know the extinction coefficient 'k' for air (which varies with wavelength, humidity, density, CO2 concentration, etc) the intensity follows Beer's law:

I = I_0 exp(-kz)

 

[Vanadium 50]

It also matters whether you're talking about a laser pointer or an SDI-sized laser. For the latter, there's also something called "thermal blooming" - the laser heats the air, the air develops a changing index of refraction depending on the distance to the beam, and you've just stuck a lens in front of your laser, dispersing it.