Arcane Game Lore

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Building a Laser Rifle

Laser guns are a staple of science fiction books, movies, and games.  The questions that always come to mind (to me at least) when thinking about hand held laser weapons are how realistic are they and how long until I can get one?  On that thought, this video wandered across my RSS feeds the other week and I just had to share and talk about it.

Needless to say, I thought this was pretty cool and I want one.  However, it got me thinking again about what it would take to make a traditional sci-fi laser gun and what components it might have that make it tick.  It gives me ideas of what could go wrong with a weapon and what would need to be fixed if one broke.  So here are my thoughts as a gamer and a physicist.

Energy Delivery

Doing damage with a laser is all about delivering energy from the weapon to the target.  So we need a few definitions. Power (Watts in this case) is defined as energy (Joules) per unit time (seconds). So his 40 Watt laser diode array is providing 40 Joules of energy per second.

Really what you are after is increased energy density on the target.  Or the amount of energy per unit area. Anything that increases the energy density delivered makes the laser weapon more effective.  Things that reduce this make it less so.  Let’s look at how the various aspects of this device contribute to that and how that would be different in a sci-fi weapon.

Light Color

This laser gun is made of blue laser diodes.  Blue is good as shorter wavelengths can be focused to a smaller point.  In fact, blue light, at around 400 nm can be focused to a spot half the diameter of red light at 800 nm since the spot size is directly related to the wavelength.

Which means that given the same amount of power, blue light can theoretically provide four times the energy density (cut the diameter in half and the area goes down by a factor of four). This means you achieve a burn faster with blue light than you would with red or with less power consumed.

Another advantage, albeit a minor one, is that blue light has more energy per photon than red light.  This means you get a bonus from the photoelectric effect (the discovery of which won Einstein his Nobel prize in 1921) as blue light is more likely to ionize some of the target material.  Most of the damage is going to come from pure energy transfer in the form of heat so this doesn’t do much but it is an effect.

Ideally a UV laser would be even better but would not look nearly as stunning as you wouldn’t see the beam.

Continuous vs. Pulsed Beams

In sci-fi most lasers fire a “blast” or “bolt” of very short duration (which in movies always travels way slower than it should  for dramatic effect.  Any true light beam would cover the ranges on set in less than the time between a single frame).  This particular laser is continuous which is why he needs to keep moving it around while he’s talking about it.  If he didn’t, he’d be burning a hole in his wall (maybe, see the Lenses and Mirrors section below).

But this gets back to the energy delivery issue.  A continuous beam delivers its full energy every second, in this case 40 Joules.  However, if this were a pulsed beam, you wouldn’t get nearly as much energy.  Let’s assume a pulse duration of 1/10th of a second.  Since the output of the diodes is 40 Watts (40 Joules/sec) a 1/10th of a second pulse only provides 4 Joules of energy.

In order to get the same effect as one second of a 40 Watt system, a system that used 1/10th second pulses would need 10 times as much power capacity or 400 Watts.  However, you’d still be delivering just 40 Watts of power.  (400 Joules * 1/10 sec = 40 Watts)

You’ll also notice in the video that it typically takes longer than a second for him to ignite or pop his various targets.  So he’s delivering more than 40 Joules to the target.  Which means that given a single 1/10th second pulse, you’d actually need even more power capacity in the system.  To get the same effect from a single 1/10th second pulse would probably require about 1000 Watts of power capacity in the diodes.

So why would you want a pulsed blast instead of a continuous one?  A continuous beam system is definitely easier and cheaper to build as you don’t need as much power capacity.  The main reason is time and energy cost. And that goes back to the energy density issue.

With a pulsed system all the energy is delivered in a very short time span, the smaller the better.  This means the target doesn’t have time to dodge or move out of the way. All the energy is delivered to a single spot, thus boosting the density of the delivered energy.

If you’re using a continuous beam system that requires any significant amount of time to deliver the necessary power and the target moves (spins, rolls, whatever), that power is now being delivered to a series of different locations and delivered energy density greatly decreases reducing effectiveness.  And if you have a continuous beam system that can deliver enough power in a short time to do damage, running it any longer than necessary wastes energy, reducing the number of “shots” you can take before you have to recharge.

Laser being emitted from telescope observatory dome

Everyone knows that the large telescopes of the world are part of a space laser defense system. This is actually a picture of the ARC 3.5m telescope at Apache Point Observatory (I’ve observed on this telescope) using it’s laser system to determine the exact distance to the moon.

There is another reason why you’d prefer pulsed to continuous and that is concealment.  With a continuous beam, there is a beam of light connecting the shooter to the target and anyone that happens to be looking will see exactly where the shooter is.  With a pulsed beam, the length of time that connection is visible is much much shorter and thus makes it harder to see where the shooter is.

Energy Flow and Heat dissipation

This is another advantage a continuous beam system has over a pulsed one.  You would think that since the total energy being delivered (40 Watts in our examples) is the same the amount of heat to be dissipated would be the same.  However, there are some additional complications that relate back to the timing.

Since the pulsed system delivers the energy in 1/10th the time, the instantaneous heat generated at the time of lasing is going to be 10 times higher (at least).  Thus whatever we are working with to dissipate that heat has to be able to handle that heat impulse.  This means that there may need to be some different materials use for construction.

Additionally, the shorter time scale mean that we are going to have a higher current flowing through our circuits during the lasing interval to deliver the energy from our power source to the laser itself.  It turns out that this source of heating in the electronics, called Joule heating is proportional for the current squared.  So if we increase the current by a factor of 10 (necessary to get the same energy delivered in 1/10th the time), the amount of heating goes up by a factor of 100.  Now it only lasts 1/10 as long but that still gives us 10 times as much heating as the continuous case.  All that heat has to be dealt with.

For our sci-fi laser weapons, however, there is a help and that comes from assuming we are using some sort of superconductor in our electronics.  The other thing the Joule heating is proportional to is the resistance in the wires.  If we have a superconductor, which has a resistance of zero, then we get no Joule heating.  So if we just have an exotic material with very low resistance, even if it is non-zero, that greatly improves things.

Lenses and Mirrors

I thought one of the best things about the video was his off-hand comment about putting the short range lens on for the demos he was doing.  Since he had 8 diodes, in order for them to work together, they had to be focused to a single point in order to concentrate the power and increase the energy density on target.

The same will be true in a sci-fi laser gun.  If you’ve got multiple laser emitting elements (which makes the power distribution and cooling issues a little easier) or a single element, you’re going to need to focus the beam.  You need mirrors and lenses in your system to control the way the light is emitted.

In the case of multiple emitting elements, you’re going to need to focus the various beams at exactly the right range to hit your target.  This will probably take the form of some sort of deformable lens system that will allow you to change the focus of the lens to adjust the range.  Of course you need to get that range and get it exactly as being off by even a little bit means your laser energy is spread out and you don’t do as much damage because the energy density isn’t high enough.  You could take this as the explanation of why you roll dice for damage, the variation represents how accurate you got the range to concentrate the laser fire. (This is also why he may not have burned through the wall if it wasn’t moving the laser while he was talking.  If the beam was dispersed enough, it’s no different than a really bright light.  But better safe than a house on fire.)

The range finding would need to be mostly automated as the operator probably won’t be able to estimate the distance accurately enough.  However a small ladar system (laser distance and ranging) built in to the weapon would do it.  It could send out a low power IR laser beam, get the return signal and use that to estimate the distance just before firing the main blast.  Maybe the operate has to pick a general range to get the right deformable lens selected (i.e. short, medium, long, etc.) and the weapon does the rest.

For a single beam you still have a focusing effect.  The beam has a “waist” where it is the narrowest and thus has the highest energy density.  You’ll want to adjust the focus of your lens so that the waist is on the target for maximum effect.  The same focusing system described earlier would work here as well.

Backscatter

Finally, you have the issue of backscatter of the light beam.  This is caused by photons in the laser beam reflecting off of surfaces or air molecules along their flight path.  It’s why when you see pictures of people working with lasers, they are wearing dark glasses.  It helps to protect the eyes of the operators from backscatter.  This is also why you can even see the laser beam at all.  If there were no backscatter and all the photons were going forward, you wouldn’t see the beam.  It’s only visible to you because some of the photons are bouncing off air molecules toward your eyes.

In the video, he definitely needed them as he had that big lens sitting right out in front of the beam.  Typical glass reflects about 4% of the light that hits it unless it has had special coatings applied to reduce this value (This is why you can see yourself in a window).  That means that of his 40 Watts of power going out, 1.6 Watts was being reflected back at him from the lens. It was diffuse and not focused, but that’s still a lot of laser power going back.  Just look at his shadow from the laser light.  Upping the power just makes it worse.

In a weapon system, your lenses will be coated to make sure as much light passes through as possible since any reflected light is going back into the belly of the weapon were you don’t want that energy bouncing around.

For a system that is completely enclosed until the beam is emitted, the backscatter to the operator wouldn’t be as bad but there would still be an intense flash of light as the beam left the barrel and some of that light was backscattered.  It wouldn’t be a problem in space but anywhere there is air you’d have this effect.  And more power means brighter light.  Maybe firing a laser weapon requires special eyewear connected to the weapon to prevent spot blindness from the beam or there is a targeting penalty after the first shot as you now are seeing spots.

Final Thoughts

Have you ever thought about how exactly laser guns work in your game?  Is there anything I missed?  Hopefully this gives you some ideas of ways to add a little flavor or color to your games when your characters pull out their blasters and take a couple shots.  There is a lot of engineering that goes into a laser gun.  Do they cost more in your game than more “traditional” weapons?  Should they?  Let me know your thoughts in the comments below.


Categorised as: General


One Comment

  1. [...] spread out.  They don’t stay a tight beam of light forever.  I touched on this a bit in the laser rifle post a couple of weeks ago when I talked about focusing the laser beam at the proper range for the [...]

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