View source for Lasers and the electromagnetic spectrum

The color of light you choose will have a big effect on what your laser beam can do.  Let’s discuss various colors of light, both visible and invisible, to get an overview of what each is good for and what limitations each may have.  After all, choosing the right color is the key to really making your laser <i>shine</i>.

==Radio==
Radio waves are essentially useless for lasers.  Sure, they get through the air okay, but diffraction makes them almost impossible to focus enough to cause damage.  So without further ado, we’ll move on.
<table border=1>
<tr><td>Color <td>Frequency <td>Wavelength <td>Energy
<tr><td>Radio <td>30 Hz - 30 MHz <td>10,000 km - 1 m <td>&lt; 1.25 μeV
</table>

==Microwaves==
Microwaves, at least, focus better than radio waves.  They still don’t focus well enough to make practical weapons - at least not if your intent is cooking or burning or blasting your enemy.  They are useful at projecting destructive currents into electronic circuits.  Such high power microwave devices operating in a counter-electronics role are usually considered a different class of weapon than lasers, so we will leave them for now.
<table border=1>
<tr><td>Color <td>Frequency <td>Wavelength <td>Energy
<tr><td>Microwave <td>30 MHz - 300 GHz <td>1 m - 1 mm <td>1.25 μeV - 1.25 meV
</table>

==Terahertz waves==
Terahertz waves are all the rage these days for remote scanning and a new window for material spectroscopy.  Unfortunately, they are absorbed by the air within a few tens of meters, so they are a poor choice for a weapon.
<table border=1>
<tr><td>Color <td>Frequency <td>Wavelength <td>Energy
<tr><td>Terahertz wave <td>300 GHz - 10 THz <td>1 mm - 30 μm <td>1.25 meV - 40 meV
</table>

==Far infrared==
Far infrared is a sort of orphan band of the electromagnetic spectrum, because it is hard for us to get sources and detectors in this range.  As a result, we don’t have a lot of experience with what it can do.  In principle, your high-tech sci-fi society could be able to make fiercely high power far infrared lasers.  But they’re still not a good choice for a weapon because the diffraction limit makes them hard to focus to damaging intensities without huge focal apertures and they’d have bad issues with cascade breakdown of the air.
<table border=1>
<tr><td>Color <td>Frequency <td>Wavelength <td>Energy
<tr><td>Far infrared <td>10 THz - 20 THz <td>30 μm - 15 μm <td>40 meV - 80 meV
</table>

==Long-wave infrared==
This is the electromagnetic band where most thermal radiation from room-temperature and body-temperature objects occur.  When someone is seeing heat with infrared vision, this is the color they are seeing with.  The atmosphere is quite transparent to long-wave infrared radiation.  Unfortunately, these long wavelengths are still difficult to focus at any useful distance, and they tend to cause cascade breakdown in the air when high powers are put into tight focal spots.  The main reason they pop up in talking about lasers is that one of the first kinds of high power lasers, and still one of the cheapest and simplest to build, is the carbon dioxide laser which operates in the long-wave infrared.
<table border=1>
<tr><td>Color <td>Frequency <td>Wavelength <td>Energy
<tr><td>Long-wave infrared <td>20 THz - 38 THz <td>30 μm - 15 μm <td>80 meV - 160 meV
</table>

==Mid-wave infrared==
A lot of the mid-wave infrared spectrum is absorbed quickly by air.  However, there is a “window” between 3.5 and 4 μm where the light can get through.  This window was investigated by early laser weapon designers, using chemical deuterium fluoride lasers.  Chemical laser weapons were nasty, toxin-spewing, noisy monstrosities of machines with abysmal beam quality and long logistics chains to supply their highly toxic, corrosive, flammable, and explosive chemicals.  And even when they were the only game in town, the deuterium fluoride laser was replaced as soon as they could by chemical oxygen iodine lasers that at least operated in the near infrared and so could be focused three times as far.  Today we have far better choices, so don’t expect mid-wave infrared lasers to get much love.
<table border=1>
<tr><td>Color <td>Frequency <td>Wavelength <td>Energy
<tr><td>Mid-wave infrared <td>38 THz - 100 THz <td>8 μm - 3 μm <td>160 meV - 410 meV
</table>

==Short-wave infrared==
Short-wave infrared is a good choice when you are looking for a color of light that focuses well, can get through air, can maintain a tight focus without two-photon ionization messing it up, and that won’t pose a severe blinding hazard to anyone nearby.  It is the shortest wavelength (and thus longest ranged) color of infrared that is eye-safe.  You won’t get eye-safe colors again until you are up into the ultraviolet.

Some modern lasers can output high power short-wave infrared beams, primarily fiber lasers.
<table border=1>
<tr><td>Color <td>Frequency <td>Wavelength <td>Energy
<tr><td>Short-wave infrared <td>100 THz - 215 THz <td>3 μm - 1.4 μm <td>410 meV - 900 meV
</table>

==Near infrared==
This is the color that almost all modern combat lasers operate.  The air is nicely transparent to light at this color, the beams focus well enough, and you can get crazy high powers out of fiber lasers these days.  Sure - look at the thing they are shooting and you might go blind, but there are bigger hazards in the military.
<table border=1>
<tr><td>Color <td>Frequency <td>Wavelength <td>Energy
<tr><td>Near infrared <td>215 THz - 430 THz <td>1.4 μm - 0.7 μm <td>900 meV - 1.8 eV
</table>

==Visible==
Yeah baby!  Now we’re talkin’!  Flashing beams lighting up the sky for stunning visual effects, strobing flashes where the beams hit.  These are the beams you can really see!  Except for the minor detail that if you ever do actually see one in person, there’s a good chance you won’t ever see anything again.  But we’ll ignore that for the sake of a good special effects loaded sci-fi extravaganza.

The air is very transparent to visible light.  It turns out that water is also at around its optimal transparency to visible light in the green, blue, and violet colors, so lasers of these colors might be the choice for underwater combat (or at least, underwater combat where you are close enough to see your enemy).  If you want maximum range in air without two photon absorption making your life miserable when your beam reaches close focus, visible light is the color choice for you.

Visible light is where you start having significant losses due to Raighleh scattering for light going all the way through an Earth-like atmosphere.  Shorter wavelengths (the closer to violet) are scattered more than longer wavelengths (closer to red).  But still, the shorter wavelengths focus better because of diffraction.  When shooting straight down through Earth’s atmosphere, you still get the best laser intensity with violet light (even though your beam will have less energy, that energy will still be more concentrated).  But as you shoot at more and more of an angle, the best choice goes toward longer and longer wavelengths.  And for alien planets, all bets are off.
<table border=1>
<tr><td>Color <td>Frequency <td>Wavelength <td>Energy
<tr><td>Red <td>430 THz - 480 THz <td>0.7 μm - 0.62 μm <td>1.8 eV - 2 eV
<tr><td>Orange <td>480 THz - 510 THz <td>0.62 μm - 0.59 μm <td>2 eV - 2.1 eV
<tr><td>Yellow <td>510 THz - 530 THz <td>0.59 μm - 0.57 μm <td>2.1 eV - 2.2 eV
<tr><td>Green <td>530 THz - 610 THz <td>0.57 μm - 0.49 μm <td>2.2 eV - 2.5 eV
<tr><td>Blue <td>610 THz - 670 THz <td>0.49 μm - 0.45 μm <td>2.5 eV - 2.7 eV
<tr><td>Violet <td>670 THz - 750 THz <td>0.45 μm - 0.4 μm <td>2.7 eV - 3.1 eV
</table>