Laser Weapons

A laser is a device that amplifies light of a certain frequency into a coherant beam. If the light is amplified enough and focused into a small enough spot, it can heat the matter it is incident upon until it burns, evaporates, or even blasts into star-hot plasma. As laser technology advances, lasers become more destructive, starting with continuous infrared heat rays of late TL 8 that burn, melt, and boil their targets through the exteremely short pulse trains of TL 11 that blast deep holes.

A laser emits light of exactly one color. If this light is visible to the human eye (such visible colors are red, orange, yellow, green, blue, and violet), the beam can be seen as a faint trace in clean air in full daylight, or a bright ray indoors, in dusty air, or at night. The spot the beam illuminates scatters some of that light, enough to produce a dazzling flash of the beam color from the target. Ultraviolet beams will either be invisible or produce a blue-violet trace, depending on the type of laser emitting them (heat rays are invisible, pulse lasers fluoresce blue-violet due to two-photon absorption, and ultraviolet blasters can't get through air at all). If the laser beam is in the infrared, you cannot see the beam or the flash without special sensors (or character advantages such as Multispectral Vision), but effects of the beam on the target (such as the flare of white hot plasma, splattering material, and flying sparks) will be obvious. Weapons grade near IR beams are not visible to infravision (which detects light in the far IR part of the spectrum, not the near IR). A laser beam acts like a tracer round for those who can see it. Any beam will be invisible in vacuum - there is no air or dust to scatter from.

Because lasers propagate differently through the air than bullets, there are differences when using the advanced rules given in GURPS Tactical Shooting.

Visible and Near-Visible Lasers

Visible and near visible (near infrared and near ultraviolet) light can shine through the air with little interaction, allowing most of the beam to reach its target. Since light from the target to the laser follows exactly the same path as the light in the beam from the laser to the target, lasers in these frequencies automatically hit where it looks like they are aimed, regardless of any bending or distortion of the beam. The typical aiming aid is a single lens reflex (SLR) scope, which uses the laser's own focusing optics as the scope to show where the beam will land, eliminating any parallax due to offset sights. When the laser is discharged, a mirror flips to allow the full powered beam to be reflected down the scope's axis in the same manner as a single lens reflex camera. SLR scopes are standard on all visible and near-visible laser weapons with Acc of 5 or more. SLR scopes should not be confused with the similarly named reflex sights from GURPS High-Tech pg. 156. If set to magnification of ×1 the SLR sights can give a similar bonus as a reflex sight (+1 to Beam Weapons skill with sighted or aimed shooting, not cumulative with targeting lasers or HUD bonus), otherwise they act as a normal scope at higher magnifications.

Lasers can project a low powered beam for targeting. This will be the same color as the high power beam, so most characters will be able to see the targeting spot of visible light lasers but without special optics will not see those of infrared or ultraviolet lasers. This acts as a targeting laser from GURPS High-Tech. The bonus to hit is not cumulative with that of a SLR scope or of any HUD. Typically, if the weapon is set to use its targeting laser the beam is activated when the gunner's finger touches the trigger; the full powered beam is emitted when the trigger is depressed.

The air a laser beam must travel through has density fluctations that will act to slightly defocus a laser beam, keeping the pulses from reaching destructive intensities at their target. To correct for this, lasers reflect their beam off a mirror with a deformable surface before sending it through the final focusing optics. Subtle warping of this mirror can "pre-focus" the beam to exactly cancel the defocusing of the atmosphere. This deformable mirror also allows some other tricks. It allows the direction of the beam to be shifted slightly from the main axis of the laser weapon, so that the beam to track the slight movement of the target over the duration of the pulse so the beam illuminates just one spot rather than a track across the target (for heat rays, with continuous beams, this ability is necessarily limited to relatively short time periods). It also means that electronic image stabilization routines can be implemented that automatically correct for any jitter of the weapon, such as that caused by breathing, heartbeat, pulse, or muscle tremmors. Laser beams can thus be held at rock steady aim, far superior to what can be acheived by mere human dexterity. This, and the very sophisticated imaging optics that are necessary for the laser to operate in the first place, the lack of recoil, the lack of drop due to gravity, the lack of need to lead the target with a light speed beam, the lack of drift due to wind, lack of parallax, and the fact that the laser beam follows exactly the same path to the target that the light from the target took to the laser (automatically compensating for large scale lensing effects of the air, shooting from air into water, and so on) make visible and near-visible lasers incredibly accurate.

Laser light acts like any other light - it will pass through transparent materials like windows and air, and reflect off of mirrors. This allows a gunner to aim at the image of his target in the mirror, and the beam will reflect off the mirror in such a way as to hit the target. Typically, the high intensity of a focused beam is enough to degrade mirrored surfaces so that the reflectivity of the target does not matter, but surfaces encountered before the beam reaches its tight focus at the target can reflect the beam. Normal glass is opaque to much of the ultraviolet - UV-B and UV-C beams will not pass through glass unhindered (although they can burn or blast the glass out of the way like any other barrier). In settings with superscience force screens, if the screens are transparent they will offer no protection against lasers emitting in bands (colors) to which the screen is transparent. Usually, this is the entire visible spectrum and is likely to extend into the near-visible parts of the spectrum as well. In some settings, you can have force screens that allow light through only one way, such as the Opaque option (pg. UT192). In this case, if people inside the screen can see out, light from outside can get to those people and the laser can go through freely to hit the people inside (although the gunner will not be able to see them, making aiming more difficult).

Laser beams are attenuated by poor atmospheric conditions. Mist, smoke, fog, and dust can all scatter laser light from the beam so that it has less energy with which to damage its target. Halve damage for every -4 penalty to vision from smoke, mist, fog, and haze. An extra -2 vision penalty reduces damage by an additional 2/3.

Different beam colors will change the beam performance. Shorter wavelengths allow longer ranges, but attenuate more rapidly in air.
ColorRange Water
UV-C×6 10 m 2 km
UV-B×5 20 m 4 km
UV-A×4 30 m 10 km
violet×4 40 m 15 km
blue×3 50 m 30 km
green×3 50 m 50 km
yellow×2.5 20 m 80 km
orange×2.5 10 m 100 km
red×2 5 m 120 km
1 μm IR×1.5 0 m 650 km
1.5 μm IR×1 0 m 3400 km

Wavelengths shorter than 1.5 microns (such as 1 μm, red, green, UV, etc.) are much more dangerous to unprotected eyes - LC becomes 1 if it was not already lower. Flash protection goggles will provide complete protection from accidental blinding from lasers (but not from direct hits to the eye). Anyone who is not wearing adequate eye protection and who can see the point of incidence (where the beam hits) of a non-eye-safe laser beam must roll vs. HT at a bonus equal to minus the Speed/Range penalty for the range to the point of incidence and a penalty equal to the damage on the Speed/Range table. If you are actually hit by the laser, the Speed/Range bonus is equal to your SM; if hit in the face, SM-5. Failure causes blindness for one minute for every point by which the roll failed. Failure by 5 or more causes lasting blindness. Treat this as a crippling injury, with effective injury for recovery purpose equal to the margin of failure (but no actual wound is taken). A hit to the eye which penetrates armor always causes permanent blindness.

The Water Attenuation column gives the distance through clear water at which damage is halved. The Air Attenuation is, likewise, the distance through perfectly clear air at which the damage will be halved due to scattering. Half this distance reduce damage by a factor of 2/3. This is cumulative (for example, going through 2.5 times the Underwater Attenuation distance halves damage twice - a total of 1/4 damage - and then reduces damage again by 2/3 for a total of 1/24 damage). This is for clear air or water - water with a large suspended sediment load or lots of algae follows the rules for vision penalties, above, as will air with lots of smoke, dust, fog, mist, or smog if those are worse than the attenuation penalties. If the beam cannot propagate through water (i.e. the Water Attenuation is zero), treat each meter of range through the water as DR 40 that is fully hardened against the laser's armor divisor (if any). Ultraviolet tends to be scattered more by haze than visible or infrared - double the effective vision penalty for the purpose of determining damage reduction. The UV-C band is rapidly absorbed by ozone - the GM may drastically reduce the attenuation length of UV-C beams in air with lots of ozone. Smog (which contains ozone) is especially effective at blocking UV-C, quadruple the effective vision penalty for the purpose of decreasing damage of UV-C beams in smoggy environments.

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