The Destructive Power of Light
Welcome to another Lightblade Learning Lab.
Today I think we're starting off on a short series of videos about materials,
different materials that we can use and cut on this machine. When I say cut,
engrave, damage; I think damage is a good word because
that's effectively what we're doing with this machine we're damaging material. Now
we've talked in the past about various materials and the way in which they have
different... something I call damage thresholds, in other words there is a
certain amount of energy which if you exceed would damage the surface of the
material. Now whether that damage is mechanical
damage or some other form of damage like heat, there will be a threshold for each
material and that's one of the strange things about this particular technology.
The Laser Technology, because we've taken it for granted that it cuts, that
it engraves, that it does strange things to our materials, but have we really got
an understanding of what's going on? That's what we're going to try and
uncover in this session and then we're going to go on to look at how the
mechanism by which the laser beam works, can damage different materials in
different ways. Now you might imagine that this laser
beam is like a hot knife cutting through butter, wrong!
Neither is it like a hacksaw or any sort of saw, where you cut through your
material evenly all the way through. These are I suppose, basically what I
would call linear damage mechanisms, they damage the whole of the material at the
same time. The laser beam is not like that at all,
you can watch it cutting through a piece of clear acrylic and you can see a
straight line going through there and you think that it's a magical hacksaw.
I'm afraid it's not, the mechanism for cutting with a laser beam is completely
different. Now when we were doing rotary engraving and we use glass, we touched on
the subject of just how it is that glass gets engraved. How you can damage the
surface of the glass and that's basically where we're going to pick up
and carry on today, the discussion about the damage mechanism. Glass is just one
damage mechanism, along with stone slate and other mineral materials where you
damage the surface by effectively "stone chipping" but it's a thermal stone
chipping as opposed to a mechanical stone chipping mechanism. Then we've got
wood that looks as though it's a burning process, I chose my words carefully there
and then we've got other things like this, which is a synthetic rubber or a
rubbery type material which to be honest I haven't got a clue how this operates. I
suspect this is evaporation as well, but it's one of the products that we're
going to test in the next few sessions. Now we've already spoken of the nature
of our beam of light that we're producing, our laser beam and the fact
that it has got a brighter intensity in the center of the beam and it disappears
away to nothing towards the outside of the beam, but of course we can't see it
because it's an invisible beam. So when we look at the beam from the side
here's what we shall see. We shall see a very dense central part to the beam and the
light will be getting fainter as it gets towards the outside.
Well we've also spoken in the past about the energy profile across this beam and
the fact that it is this sort of shape and you can clearly see that from the
way in which this colour density changes. Right, so I've got a piece of 3
millimeter plywood which I've got sitting in the laser beam well away from
the mirrors and everything else. I've got the power turned up towards maximum and
I'm going to hold the pulse button on and we're going to burn a mark on that
face. Now I'm gonna have to turn the extraction on because we are going to
produce a little bit of smoke.
Now, I've stopped on and off, but what I'd like you to see is to watch what happens
right in the centre of that burn, especially when I stop and the flame
disappears. I'll try and blow the flame away so that you can see what's going on.
Can you see the intense bright light behind that flame?
The intensity of light is so high there that I've actually got those sunspots
that you see on your retina. Now, the flame has now stopped,
and we've just got the glow.
Why is that do you think? Let's move in and take a closer look, and
there's the answer to the question, look I've burnt a hole in it! Let's understand what
this wood is, it basically is a cellulose material and it's an organic material
which burns and it burns with a flame at about 300 ~ 350 degrees C and
once the organic material has burnt away what we're left with is this black stuff
here. Which basically is pure carbon, this centre part has not burned away it's
actually evaporated away. You can't evaporate wood! Well true,
because wood does not have a liquid phase it doesn't go from solid to liquid
to vapor.
But carbon is a rather special material, carbon exists as a solid material and
at about 3,000 degrees C it changes directly from a solid material to a gas
and just disappears.
Hence the reason we have a hole in there. We've turned the wood to carbon and then
we've supplied so much heat to the carbon at the center of our beam here,
we've evaporated the carbon away and the other thing I want you to note about
this, although the flame was blowing across this way and we got a little bit
of a distortion or a halo over here. What we have got is from the centre, we've got
a gradual burning process which is from a very high temperature to a much lower
temperature out here, where it was scorching and as you can see, look it's gone away to a
very faint Brown right at the edge. Where we're just having enough heat to damage
the surface of the material. We can almost measure the diameter of our laser
beam by looking at the size of this. Although the power is in the middle,
there is energy that goes out to maybe 10 or 12 millimetres diameter.
Now we've also witnessed this energy profile in the beam by firing the beam
at a block of acrylic and here we can see the energy density at the centre of
the beam is much higher, because it's penetrated deeper into the acrylic. We
just witnessed exactly the same thing happening with the wood, this very high
intensity part of the centre of the beam has been able to generate huge
temperatures and that's before it passes through the lens. Anybody that's
messed around with lighting or photography will understand this color
temperature chart and it was a scientist called Kelvin who basically realized
that the filament in a light bulb, glowed at different colours and those
colours were representative of different temperatures. Now, here we're talking
about temperatures that are going up into the thousands of degrees Kelvin or
thousands of degrees C. They're more or less the same thing with an offset, let
me explain. This same guy, was also responsible for determining something
called absolute zero. Now all molecules sit
there, at room temperature doing this. They've got vibration and basically the
amount of vibration is actually an indication of their temperature. The
faster they vibrate the higher the temperature. Now that sounds like a
strange concept, but that is actually what temperature is and here it's
defined; "how fast its atoms and molecules oscillate" that's what temperature of an
object is. He took that concept to the opposite extreme and said; well, look if
they're doing this at room temperature there must be a point at which, when I
lower the temperature all motion will stop and that is what absolute zero was
defined as. Now in modern day terms, with quantum physics and all the rest of it,
they have established that at absolute zero there is still a small amount of motion
that goes on, but to all intents and purposes absolute zero means the atoms
are not vibrating and as you raise the temperature, the atoms vibrate faster and
faster and faster. Now that concept of temperature and
atoms moving fast, was also discussed when we talked about the nature of the
laser tube and the mechanism that takes place within the tube itself and how the
nitrogen gets very very excited by being threatened with 25,000 volts, and if you
allow more current to pass through the nitrogen, it gets more and more and more
excited and it can do damage to the co2 molecules because it's so excited. It's
all related to this same concept of atomic motion, so if you can carry
forward this idea of vibrating molecules, vibrating faster and faster, getting
hotter and hotter, that is basically the mechanism by which
the laser beam works. Back to this little picture here. Now Mr. Kelvin, working the
other way, away from absolute zero determined that the colour of a filament
in a bulb was able to determine its temperature.
Now absolute zero was determined as minus 273 degrees C and so that is where
the kelvin scale starts, at minus 273 degrees C. So these values here are
degrees K Kelvin and they start at minus 273, I mean they are degrees C but
they've got this offset on here so to get them into numbers that we understand
we had to have to add about another 300 say another 300 degrees C onto these.
Well look at these numbers these are thousands of degrees C so another 300
degrees C added onto these numbers it's not actually going to give us a great
deal of change, but the point that I want you to understand is that we've been
looking at colors for our glowing wood up here in this bright yellow to white
region and that is somewhere in the region of anything between four and five
thousand degrees C. At 3,000 degrees C carbon will sublimate, it will go
directly from a solid to a gas and so here we've got confirming information
that what we're looking at in the center of that beam. I've got to be very careful
what I say here, because the temperature is not in the beam itself, but the beam
can, depending on the material that we're firing at and we fired it at wood. In
that particular instance it was able to generate temperatures up in the four to
five thousand degrees C, because it was hot enough to sublimate carbon. Now that
does not mean to say that the beam itself is hot,
that is a misconception. The beam is a beam of light and it has no temperature,
so how do we get the damage? How do we burn things if there's no
temperature in this light? I'll bring you back to this little statement here, where
it says temperature is actually defined by how fast the atoms and molecules of a
material vibrate or oscillate. When this light hits the surface of that material,
it could do one of two things. If this is a metallic surface, it will certainly
reflect, but of course like all light the surface has to be flat to act like a
mirror, if it's in any way distorted the light will disappear off at different
angles. Things like gold, silver, copper and aluminium, particularly those four
metals will reflect 99% or better. So they are literally mirrors, lower grade
materials like mild steel for example or iron will be somewhere in the region
about 60 or 65 percent reflective. The other thing that could happen is the
light can be absorbed into the surface. Oooh, I've got to be careful about this
word absorption, because it gives you the impression the material is like a sponge,
wrong! What actually happens is, just at the surface of the material, there are
atoms which will be stimulated just like when you put things in your microwave
and it heats up, that's what would happen here. These light particles are
stimulating the atoms on the surface of the material. It's light, it can't
penetrate the material itself, it can only interact with a surface and that's
the important thing to remember throughout this. The solid starts to
vibrate because it's stimulated by the light and as we've just discussed,
vibration, extra stimulation, heat. Because these materials that we're
going to be firing this at are non metallic materials, they have got very
poor conduction properties, they do not transfer heat easily and this heat
builds up on the surface and because the heat can't disappear, it stays on the
surface and it gets hotter and hotter and hotter. As we've seen in the Wood
experiment, it starts to glow white-hot and it glows white-hot in the centre here.
Basically the wood has burnt away, but then after that we saw this white glow
in the center here and the white glow was the carbon, it wasn't burning it was
heating up and it heated up to a white-hot temperature where it couldn't
resist anymore and it turned into a gas and so gradually what was happening is
here, we get a little bit of erosion in the surface as the carbon
evaporates and it leaves clean carbon behind and so the light beam stimulates
this new carbon that's behind here and gradually what we've got is a process of
erosion and it eventually worked its way through the three millimeter thick
material. The important thing I need to stress here is this is not like the
hacksaw, this is not like the knife, this this is not a continuous process this is
almost like a woodpecking action, it's a gradual erosion process, even with the
very high energy levels that we had here it took time to burn through the wood.
But what we can categorically say is, that when we fire this beam at wood it's
capable, because of the carbon content of generating at least 3,000 degrees C. Now
the beam itself is not 3,000 degrees C, it's just a beam of light now if this
was just a piece of acrylic,
this damage here took place at a much lower temperature than 3000 degrees C,
because the threshold, the damage threshold on this material is different
to the damage threshold of wood. So every material will react to this light
stimulation in a different way, but it is the light stimulation that heats up the
material, it's not the light itself that is hot. That's an important concept to
remember, we are now going to take that beam and we're going to magnify it.
We're going to concentrate it down and we're going to concentrate it down from
six millimeters diameter to 0.1 millimeters diameter. Basically we're
going to amplify the light at this point, the energy density, to some phenomenal
value and in fact if we do a very simple bit of maths you'll find that that, is
about three thousand five hundred times smaller than that. So we've amplified
potentially, the ability to do three thousand degrees C's worth of damage,
there is a huge amplification of the energy density.
Watch very carefully here, I'm just going to do a very simple, very very quick
pulse. You may or may not have seen what happened there, but let's first of all
check. How long was that pulse? Maybe a tenth of a second? The pulse has burnt
right the way through and here's what the underneath looks like, it's burnt
right through it in less than say a tenth of a second or maybe even less
than that. We'll do it one more time, now if you watch very carefully you'll see
two things happening, you'll see a little bit of a flame coming up where the wood
is burning away, but then you'll see this white flash as the carbon evaporates.
We've still got the same energy profile, it's just been magnified up to some
phenomenal intensity as it's been decreased to 0.1 of a millimeter
diameter. The same damage principle is existing after the lens as the one that
I demonstrated to you before the lens and whereas with the unfocused beam it
was taking it's possibly a minute to burn through this piece of 3 millimeter
material. When we amplified the beam it was taking less than 0.1 of a second!
This piece of wood is the same piece of wood that took over a minute to pierce
through with low energy density, once we amplify the energy density up, we
can do the same amount of work in a tenth of a second. Time is something
that's going to come into our cutting and our engraving discussions as we push
on with this subject. Now, we saw that in a tenth of a second we could burn a very
compact little hole through. Let's see what happens if I leave the beam on for
about a second. One second. We've got our scorch Corona around the
outside, the same as we did here. We've started to generate additional damage
around the outside of the hole, whereas here,
we had it on for such a short period of time that all we did was to pierce a
hole, instantly through there, right with the centre of the energy beam and we
didn't get a chance for the lower energy levels around the outside to have any
burning effect on the wood. So that's another important lesson I'm trying to
get over to you about time, you need just enough time to do the damage that you
want to do and not so much time that you actually cause collateral damage as well.
Now, these are very important concepts when it comes to understanding the
cutting process, because this is effectively the process that causes
charring on the edge of your cut. The idea is to cut as quickly as possible
with as high an energy level as you can. The way in which the laser beam
damages material is a difficult concept to try and describe to you, but I
hope that I've broken it down into small enough chunks, that like a jigsaw puzzle
you'll be able to put the pieces together for yourself. The fact that
we've got energy density which causes gradual erosion and not an
instantaneous cut, even that instantaneous tenth of a second cut that
took place as we burn that hole through, was not an instantaneous pierce. It was
still done by exactly the same pecking mechanism, it's just that it happened in
such a short period of time that it looked as though it was an instant cut,
like a hacksaw or a knife, but it's not, it's a woodpecker.
A very fast-acting woodpecker and that process applies to every material that
we're going to fire this beam at, whether we're doing engraving or whether we're
doing cutting, we're basically either going, we're going to be damaging the
material surface with this high-energy beam of light and it's the interaction
of the light with the surface that causes the damage. It causes heating and
that heating causes different types of damage in different types of material.
Well, I think that's enough mental gymnastics for today we've basically
talked about the damage process for wood and organic materials in this session
and we've used that as a demonstration of how the laser beam damages material,
because it's one of the easier concepts to understand. Now there are other damage
concepts which we'll move on to in future sessions, but they're all based on
this same principle. So the hard work is done, if you can understand
what's been going on today. So thank you very much for your time and I'll catch
up with you in the next session
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