Lasers Question

On Wed, 04 Oct 2006 12:53:23 GMT, "Rainbow Warrior" <pizza RemoveThis @spc.com.nz>
wrote:

>Possibly dumb question about lasers assuming they are based on current
>principle metal cutting lasers, if you were to shoot at someone through a
>Glass/Plexiglas window would the glass take damage or reduce damage or be
>counted as normal wall cover?
>
>That is if you were to say put a sheet of glass between an industrial laser
>and some steel would it cut the steel without cutting the glass?
>
>What about invisible PC's?
>

First invisibility should protect a PC 100% if it includes
invisibility to the electromagnetic spectrum. Otherwise its would be
useless for passing though laser fence-alarms and the like.

A laser with a visible wavelength glass provides no protection... see
below. Think of the laser sight on a sniper rifle.

Many types of windows now block IR light, partially at least to reduce
passive solar heating. Many sunglasses block UV. Now if the laser
operated in these lower and higher spectrums... then they might be
partially or totally blocked. Reduce damage by some factor. It may be
ablative DR as the energy has to go somewhere if it is not reflected.
The same would hold true for smoked and frosted glass or those
privacy type glass blocks that let light in but distort things so
badly that no "picture" of what is behind them is possible.

--
Grant
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Rainbow Warrior wrote:
> Possibly dumb question about lasers assuming they are based on current
> principle metal cutting lasers, if you were to shoot at someone through a
> Glass/Plexiglas window would the glass take damage or reduce damage or be
> counted as normal wall cover?
>
> That is if you were to say put a sheet of glass between an industrial laser
> and some steel would it cut the steel without cutting the glass?
>
> What about invisible PC's?
>
>


Standard windowpane glass DOES refract light, including lasers, to some
degree, thus a sniper will still be able to sight, though not as
accurately as truly desired (also remember that distance causes the
"dot" to enlarge- several inches over 300 yds - a good game estimate
would be 1/2"/50yds, 1"/100yds increments.) High power light going
through any medium is just like electricity going through a resister,
some portion of the power is lost, usually as heat, some portion will be
transmitted. What this means is, that getting enough power to burn
something over a distance is extremely unlikely with any portable weapon.

Hollywood frequently shows snipers shooting through glass- in reality, a
VERY risky business, as the bullet, unless armor piercing, will both
deform, and be deflected. Shooting people with armor piercing rounds,
tends to leave small round holes with very little damage -ie the victim
of the gunshot survives, usually capable of some action, which in
situations requiring a sniper is a very bad thing.

Invisibility, if theoretically a physical effect, would bend the
"visible" light around the PC. If Magical/Psi it would be a GM
determination. (My major problem with invisibility for a reality
perspective: the eye sees by capturing the photons reflected from a
body. If invisibility bends the photons around the subject - you see the
objects behind the invisible person- how does the invisible person see?)
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On Thu, 05 Oct 2006 08:44:52 -0400, Roger Connor
<raconnor RemoveThis @email.unc.edu> wrote:

[snip]
> If invisibility bends the photons around the subject - you see the
>objects behind the invisible person- how does the invisible person see?)
>

That is a question no one has ever adequately answered. Even if you
are 100% transparent you can not see either, because light would
passes through your retina. Its been a while but even HG Wells just
did a little hand waving over that fact IIRC. <g>

The techno invisibility of the Predator appeared to work in the human
visible spectrum only while allowing them to see by IR or UV. This is
about the only invisibility that seems to actualy be workable.

--
Grant
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In article <4524fe44$1_3@news.unc.edu>,
Roger Connor <raconnor.TakeThisOut@email.unc.edu> wrote:
>
>Hollywood frequently shows snipers shooting through glass- in reality, a
>VERY risky business, as the bullet, unless armor piercing, will both
>deform, and be deflected. Shooting people with armor piercing rounds,
>tends to leave small round holes with very little damage -ie the victim
>of the gunshot survives, usually capable of some action, which in
>situations requiring a sniper is a very bad thing.

I have seen snipers in movies attach a patch to the (nearby) window
they intend to shoot through, with a wire coming out of it. I have
assumed that this is simply a small explosive charge connected to the
trigger that will break the window after the trigger is pressed but
before the bullet reaches the glass.

Or it could just be my overactive imagination

Cheers
Bent D
--
Bent Dalager - bcd.TakeThisOut@pvv.org - http://www.pvv.org/~bcd
powered by emacs
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Roger Connor <raconnor.DeleteThis@email.unc.edu> writes:

> Invisibility, if theoretically a physical effect, would bend the
> "visible" light around the PC. If Magical/Psi it would be a GM
> determination. (My major problem with invisibility for a reality
> perspective: the eye sees by capturing the photons reflected from a
> body. If invisibility bends the photons around the subject - you see
> the objects behind the invisible person- how does the invisible
> person see?)

Some options:

- The invisibility tech captures *all* photons it receives and
processes them. It presents photons to the user's eye that would
have reached the eye. It also regenerates all photons on the far
side of the user's body at the same trajectory, as if none were
intercepted.

This means the invisibility tech needs to be very fast, but doesn't
require it to fudge the image.

- The invisibility tech allows photons to be captured by the user's
eye, and bends the rest around the user, in such a way that the
missing ones aren't necessary; the eye is a very small surface, so
it's virtually impossible to tell.

This requires the *processing* power of the suit to be fast, but the
resulting far-side image is still made up of the original photons
still travelling at light speed.

--
\ "I have one rule to live by: Don't make it worse." -- Hazel |
`\ Woodcock |
_o__) |
Ben Finney
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On Thu, 05 Oct 2006 07:08:15 -0600, All hail Discordia
<wyrm_ksc RemoveThis @hotmail.com> wrote:

>That is a question no one has ever adequately answered. Even if you
>are 100% transparent you can not see either, because light would
>passes through your retina. Its been a while but even HG Wells just
>did a little hand waving over that fact IIRC. <g>

Well, IF -I- remember correctly, Wells stated that just a small
portion of the retinae were still visible. ^_____^

I remember that, in the book, the first experiment was a cat, and
there was a quite upsetting scene of a pair of light-reflecting spots
moving fast at a few centimeters from floor... O_______O


Korin Duval

--

"Truth requires a great amount of courage;
Fiction requires a great amount of maturity."
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Rainbow Warrior wrote:
> Possibly dumb question about lasers assuming they are based on current
> principle metal cutting lasers, if you were to shoot at someone through a
> Glass/Plexiglas window would the glass take damage or reduce damage or be
> counted as normal wall cover?

A bit about the theory and operation of laser weapons.

The light in the weapon itself needs to be low enough intensity that
the weapon is not harmed. You don't want the mirrors and windows and
lenses inside your laser gun to warp or distort or melt. However, the
intensity needs to be high enough to rapidly vaporize the target, even
if the target is made out of hard to vaporize stuff like steel or
graphite. How do you accomplish both of these goals? Simple, you make
it so the light intensity is higher at the target than inside the
weapon. You do you do that? You focus the light down to a small point
at the target using lenses or mirrors.

Very intense light interacts differently with matter than light of
lower intensity. Linear optical properties such as transparency and
reflectivity no longer matter, as the light couples to the surface
either through a plasma layer or even, in extreme cases, through simply
ripping electrons off. The laser's intensity at the target will be so
high that neither mirrors nor windows will work, both will absorb the
beam and take damage. Before reaching the target, however, the laser
beam will not have been focused down to highly damaging levels
(although it would probably cause painful but superficial burns to
unprotected skin) and will pass through windows and bounce off mirrors.

So, if the glass or plexi window is very close to the target, it counts
as a normal barrier. Farther away, it lets the beam through (assuming
the laser operates at a wavelength at which the window is transparent -
I can go into more detail on this if anyone is interested. It is an
interesting but complex subject).

For very intense beams at tight focus, you can get interesting self
focusing behavior, where the laser beam creates its own plasma-filled
optical waveguide through which it passes without losing focus
(assuming the beam is in air - this trick does not work in vacuum).
The light intensity in these self focused fillaments is so high that
matter cannot withstand it, even glass or mirrors, but for practical
purposes you probably don't want the beam to do this except very close
to your target.

> That is if you were to say put a sheet of glass between an industrial laser
> and some steel would it cut the steel without cutting the glass?

Industrial lasers are usually either CO_2 lasers or Nd:YAG lasers.
CO_2 lasers emit beams at 10.6 microns wavelength, where glass and
plexi are both opaque. CO_2 lasers will etch, drill, or cut glass just
as well as they do any other material (with the exception of the very
few materials that are transparent to this radiation - table salt is
one of these).

The 1.05 micron near infrared light of Nd:YAG lasers is almost visible.
For all practical purposes, anything which is transparent to visible
light is also transparent to Nd:YAG laser light. A continuous beam
will pass through glass without significant damage unless it dwells on
the glass for a long time (which will cause some heating and distortion
or local melting of the glass) or is very focused. Pulsed beams, often
as short as 1 nanosecond, have power levels so high that glass ceases
to be transparent, at least near the point of focus - the surface of
the glass (or any other material) absorbs all of the light, vaporizes,
and then explodes.

I understand people have been looking into Ti:Sapphire lasers for
industrial purposes. These operate at a range of near infrared
wavelengths. The main attraction of Ti:Sapphire lasers is that they
can be used for chirped pulse amplification, compressing pulses down to
picoseconds or hundereds of femptoseconds with power levels so high
that, at tight focus, no matter can withstand the light intensity.
Again, at tight focus, a Ti:sapphire pulse will damage glass, blasting
out craters, just as it will to any other materials. Interestingly,
the temperature of matter heated by these ultrashort pulses often
reaches temperatures higher than that in the core of a detonating
nuclear weapon or the heart of the sun. That's some significant energy
density we're talking about!

> What about invisible PC's?

It depends on the method of invisibility. If it is a mental suggestion
of "I'm not here. Ignore me" then invisibility will not do any good if
a stray beam hapens to intersect you.

My favorite justification of invisibility (which might actually be
accomplished with nanoscale optical phased arrays) has the
"invisibility suit" recording all information about the incoming light
wavefronts (the intensity, phase, and polarization of the light as a
function of frequency) and then recreating that wavefront on the other
side as if it had passed through empty space (and also recreating it at
the wearer's eyes, so he can see). Such a suit would conceivably have
an upper limit to the intensity it can handle, meaning a tight focus
laser beam would blast through. On the other hand, a magic spell or
ultra tech "field" might not (or might, for all we know) have such a
limit. One big challenge, however, will be finding the proper distance
to focus the beam - if low power laser ranging pulses are not returned
and if passive optical focusing techniques will not work, how do you
know where to focus the laser beam to zap the invisible guy?

Luke
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"Ben Finney" <bignose+hates-spam@benfinney.id.au> wrote in message
news:874puie883.fsf_-_@benfinney.id.au...
> Roger Connor <raconnor.DeleteThis@email.unc.edu> writes:
>
>> Invisibility, if theoretically a physical effect, would bend the
>> "visible" light around the PC. If Magical/Psi it would be a GM
>> determination. (My major problem with invisibility for a reality
>> perspective: the eye sees by capturing the photons reflected from a
>> body. If invisibility bends the photons around the subject - you see
>> the objects behind the invisible person- how does the invisible
>> person see?)
>
> Some options:
>
> - The invisibility tech captures *all* photons it receives and
> processes them. It presents photons to the user's eye that would
> have reached the eye. It also regenerates all photons on the far
> side of the user's body at the same trajectory, as if none were
> intercepted.
>
> This means the invisibility tech needs to be very fast, but doesn't
> require it to fudge the image.
>
> - The invisibility tech allows photons to be captured by the user's
> eye, and bends the rest around the user, in such a way that the
> missing ones aren't necessary; the eye is a very small surface, so
> it's virtually impossible to tell.
>
> This requires the *processing* power of the suit to be fast, but the
> resulting far-side image is still made up of the original photons
> still travelling at light speed.

Another possibility would be that they aren't invisible at all, but are
casting a "spell" on the people around them (or have he technology...) so
that the other people's *perception* of them is nearly 0%. (Sorta like
hypnotism). The effect is dependant on the number of people that need to be
hypnotised, the sort of place they are in (harder to hypnotise people who
expect to see something, so standing on a stage in front of an audience and
then going *poof* invisible is harder than just not being noticed in a crowd
of similar numbers) and of course, what it is the character is actually
doing (people are likely to notice someone rigging up a device that looks
like a huge bomb in the middle of a public area than they are going to
notice someone just casuall;y strolling along like everyone else).

Cats are almost completely immune to this effect, of course.

Yowie
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On Fri, 06 Oct 2006 07:57:48 +1000, Ben Finney
<bignose+hates-spam@benfinney.id.au> wrote:

>Roger Connor <raconnor.TakeThisOut@email.unc.edu> writes:
>
>> Invisibility, if theoretically a physical effect, would bend the
>> "visible" light around the PC. If Magical/Psi it would be a GM
>> determination. (My major problem with invisibility for a reality
>> perspective: the eye sees by capturing the photons reflected from a
>> body. If invisibility bends the photons around the subject - you see
>> the objects behind the invisible person- how does the invisible
>> person see?)
>
>Some options:
>
>- The invisibility tech captures *all* photons it receives and
> processes them. It presents photons to the user's eye that would
> have reached the eye. It also regenerates all photons on the far
> side of the user's body at the same trajectory, as if none were
> intercepted.
>
> This means the invisibility tech needs to be very fast, but doesn't
> require it to fudge the image.
>
>- The invisibility tech allows photons to be captured by the user's
> eye, and bends the rest around the user, in such a way that the
> missing ones aren't necessary; the eye is a very small surface, so
> it's virtually impossible to tell.
>
> This requires the *processing* power of the suit to be fast, but the
> resulting far-side image is still made up of the original photons
> still travelling at light speed.

The Man From Uncle handled the physical invisibility field just by
combining it with infra-red goggles.
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"Yowie" <yowie9644.DIESPAMDIE.TakeThisOut@yahoo.com.au> writes:

> Another possibility [for the technology of invisibility] would be
> that they aren't invisible at all, but are casting a "spell" on the
> people around them (or have he technology...) so that the other
> people's *perception* of them is nearly 0%. (Sorta like
> hypnotism).

Note that this would not work on anything immune to hypnotism, nor on
anything that the technology couldn't affect for some other reason,
and would require more power depending on how many beings were being
hypnotised.

The technologies involving shuffling photons around will work on
anything that sees photons.

Whether the GM likes one result or the other will determine which one
costs less, of course

--
\ "A 'No' uttered from deepest conviction is better and greater |
`\ than a 'Yes' merely uttered to please, or what is worse, to |
_o__) avoid trouble." -- Mahatma Gandhi |
Ben Finney
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Yowie wrote:

> Another possibility would be that they aren't invisible at all, but are
> casting a "spell" on the people around them (or have he technology...) so
> that the other people's *perception* of them is nearly 0%.

Alternately, a simple SEP [1] field and a coat of pink paint would do
the trick.

cheers

Sara

[1] Somebody Eles's Problem
http://en.wikipedia.org/wiki/Somebody_Else's_Problem_field
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lady_sara wrote:
> Yowie wrote:
>
>> Another possibility would be that they aren't invisible at all, but are
>> casting a "spell" on the people around them (or have he technology...) so
>> that the other people's *perception* of them is nearly 0%.
>
> Alternately, a simple SEP [1] field and a coat of pink paint would do
> the trick.
>
> cheers
>
> Sara
>
> [1] Somebody Eles's Problem
> http://en.wikipedia.org/wiki/Somebody_Else's_Problem_field
>

Here's a scary piece of recent news.
http://news.search.yahoo.com/search/news?p=Cloak&fr=yfp-t-501&toggle=1...p=mss&e

Yes someone has invented a cloaking device!
Pagrin
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Wow, good posts everyone so far.

My response is further down.

> One big challenge, however, will be finding the proper distance to focus
the beam - if low power laser ranging pulses are not returned and if passive
optical focusing techniques will not work, how do you know where to focus
the laser beam to zap the invisible guy?

> Luke

Simple enough; do without the rangefinder. If the laser can adjust it's
range on the fly by moving mirrors & such, have the focus move quickly from
just in front of your weapon to it's theoretical max-range and back again,
therefore slicing neatly through everything between here and there.
Invisible or not, you'll still hurt it if it can be hit.

--U
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