0:10The Destructive Power of Light
0:15Welcome to another Lightblade Learning Lab.
0:19Today I think we’re starting off on a short series of videos about materials,
0:26different materials that we can use and cut on this machine. When I say cut,
0:34engrave, damage; I think damage is a good word because
0:41that’s effectively what we’re doing with this machine we’re damaging material. Now
0:46we’ve talked in the past about various materials and the way in which they have
0:51different… something I call damage thresholds, in other words there is a
0:56certain amount of energy which if you exceed would damage the surface of the
1:01material. Now whether that damage is mechanical
1:04damage or some other form of damage like heat, there will be a threshold for each
1:12material and that’s one of the strange things about this particular technology.
1:19The Laser Technology, because we’ve taken it for granted that it cuts, that
1:28it engraves, that it does strange things to our materials, but have we really got
1:35an understanding of what’s going on? That’s what we’re going to try and
1:40uncover in this session and then we’re going to go on to look at how the
1:46mechanism by which the laser beam works, can damage different materials in
1:51different ways. Now you might imagine that this laser
1:56beam is like a hot knife cutting through butter, wrong!
2:02Neither is it like a hacksaw or any sort of saw, where you cut through your
Transcript for How Do Lasers Work? (Cont…)
2:09material evenly all the way through. These are I suppose, basically what I
2:15would call linear damage mechanisms, they damage the whole of the material at the
2:20same time. The laser beam is not like that at all,
2:27you can watch it cutting through a piece of clear acrylic and you can see a
2:32straight line going through there and you think that it’s a magical hacksaw.
2:36I’m afraid it’s not, the mechanism for cutting with a laser beam is completely
2:43different. Now when we were doing rotary engraving and we use glass, we touched on
2:48the subject of just how it is that glass gets engraved. How you can damage the
2:53surface of the glass and that’s basically where we’re going to pick up
2:58and carry on today, the discussion about the damage mechanism. Glass is just one
3:05damage mechanism, along with stone slate and other mineral materials where you
3:15damage the surface by effectively “stone chipping” but it’s a thermal stone
3:22chipping as opposed to a mechanical stone chipping mechanism. Then we’ve got
3:27wood that looks as though it’s a burning process, I chose my words carefully there
3:35and then we’ve got other things like this, which is a synthetic rubber or a
3:41rubbery type material which to be honest I haven’t got a clue how this operates. I
3:46suspect this is evaporation as well, but it’s one of the products that we’re
3:49going to test in the next few sessions. Now we’ve already spoken of the nature
3:55of our beam of light that we’re producing, our laser beam and the fact
4:00that it has got a brighter intensity in the centre of the beam and it disappears
Transcript for How Do Lasers Work? (Cont…)
4:07away to nothing towards the outside of the beam, but of course we can’t see it
4:10because it’s an invisible beam. So when we look at the beam from the side
4:15here’s what we shall see. We shall see a very dense central part to the beam and the
4:20light will be getting fainter as it gets towards the outside.
4:22Well we’ve also spoken in the past about the energy profile across this beam and
4:27the fact that it is this sort of shape and you can clearly see that from the
4:34way in which this colour density changes. Right, so I’ve got a piece of 3
4:40millimeter plywood which I’ve got sitting in the laser beam well away from
4:45the mirrors and everything else. I’ve got the power turned up towards maximum and
4:49I’m going to hold the pulse button on and we’re going to burn a mark on that
4:55face. Now I’m gonna have to turn the extraction on because we are going to
4:59produce a little bit of smoke.
5:05Now, I’ve stopped on and off, but what I’d like you to see is to watch what happens
5:12right in the centre of that burn, especially when I stop and the flame
5:17disappears. I’ll try and blow the flame away so that you can see what’s going on.
5:25Can you see the intense bright light behind that flame?
6:01The intensity of light is so high there that I’ve actually got those sunspots
Transcript for How Do Lasers Work? (Cont…)
6:07that you see on your retina. Now, the flame has now stopped,
6:17and we’ve just got the glow.
6:21Why is that do you think? Let’s move in and take a closer look, and
6:26there’s the answer to the question, look I’ve burnt a hole in it! Let’s understand what
6:35this wood is, it basically is a cellulose material and it’s an organic material
6:44which burns and it burns with a flame at about 300 ~ 350 degrees C and
6:51once the organic material has burnt away what we’re left with is this black stuff
6:57here. Which basically is pure carbon, this centre part has not burned away it’s
7:04actually evaporated away. You can’t evaporate wood! Well true,
7:11because wood does not have a liquid phase it doesn’t go from solid to liquid
7:17to vapor.
7:20But carbon is a rather special material, carbon exists as a solid material and
7:27at about 3,000 degrees C it changes directly from a solid material to a gas
7:37and just disappears.
7:41Hence the reason we have a hole in there. We’ve turned the wood to carbon and then
7:48we’ve supplied so much heat to the carbon at the center of our beam here,
7:52we’ve evaporated the carbon away and the other thing I want you to note about
7:56this, although the flame was blowing across this way and we got a little bit
8:01of a distortion or a halo over here. What we have got is from the centre, we’ve got
Transcript for How Do Lasers Work? (Cont…)
8:06a gradual burning process which is from a very high temperature to a much lower
8:11temperature out here, where it was scorching and as you can see, look it’s gone away to a
8:15very faint Brown right at the edge. Where we’re just having enough heat to damage
8:21the surface of the material. We can almost measure the diameter of our laser
8:25beam by looking at the size of this. Although the power is in the middle,
8:32there is energy that goes out to maybe 10 or 12 millimetres diameter.
8:37Now we’ve also witnessed this energy profile in the beam by firing the beam
8:42at a block of acrylic and here we can see the energy density at the centre of
8:48the beam is much higher, because it’s penetrated deeper into the acrylic. We
8:53just witnessed exactly the same thing happening with the wood, this very high
8:58intensity part of the centre of the beam has been able to generate huge
9:03temperatures and that’s before it passes through the lens. Anybody that’s
9:08messed around with lighting or photography will understand this color
9:12temperature chart and it was a scientist called Kelvin who basically realized
9:17that the filament in a light bulb, glowed at different colours and those
9:24colours were representative of different temperatures. Now, here we’re talking
9:29about temperatures that are going up into the thousands of degrees Kelvin or
9:34thousands of degrees C. They’re more or less the same thing with an offset, let
9:41me explain. This same guy, was also responsible for determining something
9:50called absolute zero. Now all molecules sit
9:58there, at room temperature doing this. They’ve got vibration and basically the
Transcript for How Do Lasers Work? (Cont…)
10:09amount of vibration is actually an indication of their temperature. The
10:17faster they vibrate the higher the temperature. Now that sounds like a
10:22strange concept, but that is actually what temperature is and here it’s
10:28defined; “how fast its atoms and molecules oscillate” that’s what temperature of an
10:33object is. He took that concept to the opposite extreme and said; well, look if
10:40they’re doing this at room temperature there must be a point at which, when I
10:45lower the temperature all motion will stop and that is what absolute zero was
10:55defined as. Now in modern day terms, with quantum physics and all the rest of it,
11:00they have established that at absolute zero there is still a small amount of motion
11:05that goes on, but to all intents and purposes absolute zero means the atoms
11:12are not vibrating and as you raise the temperature, the atoms vibrate faster and
11:18faster and faster. Now that concept of temperature and
11:22atoms moving fast, was also discussed when we talked about the nature of the
11:29laser tube and the mechanism that takes place within the tube itself and how the
11:34nitrogen gets very very excited by being threatened with 25,000 volts, and if you
11:43allow more current to pass through the nitrogen, it gets more and more and more
11:48excited and it can do damage to the co2 molecules because it’s so excited. It’s
11:56all related to this same concept of atomic motion, so if you can carry
12:02forward this idea of vibrating molecules, vibrating faster and faster, getting
Transcript for How Do Lasers Work? (Cont…)
12:08hotter and hotter, that is basically the mechanism by which
12:11the laser beam works. Back to this little picture here. Now Mr. Kelvin, working the
12:18other way, away from absolute zero determined that the colour of a filament
12:24in a bulb was able to determine its temperature.
12:28Now absolute zero was determined as minus 273 degrees C and so that is where
12:36the kelvin scale starts, at minus 273 degrees C. So these values here are
12:44degrees K Kelvin and they start at minus 273, I mean they are degrees C but
12:55they’ve got this offset on here so to get them into numbers that we understand
13:00we had to have to add about another 300 say another 300 degrees C onto these.
13:05Well look at these numbers these are thousands of degrees C so another 300
13:09degrees C added onto these numbers it’s not actually going to give us a great
13:13deal of change, but the point that I want you to understand is that we’ve been
13:19looking at colors for our glowing wood up here in this bright yellow to white
13:27region and that is somewhere in the region of anything between four and five
13:33thousand degrees C. At 3,000 degrees C carbon will sublimate, it will go
13:41directly from a solid to a gas and so here we’ve got confirming information
13:47that what we’re looking at in the center of that beam. I’ve got to be very careful
13:52what I say here, because the temperature is not in the beam itself, but the beam
13:58can, depending on the material that we’re firing at and we fired it at wood. In
Transcript for How Do Lasers Work? (Cont…)
14:02that particular instance it was able to generate temperatures up in the four to
14:07five thousand degrees C, because it was hot enough to sublimate carbon. Now that
14:14does not mean to say that the beam itself is hot,
14:19that is a misconception. The beam is a beam of light and it has no temperature,
14:27so how do we get the damage? How do we burn things if there’s no
14:32temperature in this light? I’ll bring you back to this little statement here, where
14:37it says temperature is actually defined by how fast the atoms and molecules of a
14:43material vibrate or oscillate. When this light hits the surface of that material,
14:47it could do one of two things. If this is a metallic surface, it will certainly
14:55reflect, but of course like all light the surface has to be flat to act like a
15:02mirror, if it’s in any way distorted the light will disappear off at different
15:08angles. Things like gold, silver, copper and aluminium, particularly those four
15:14metals will reflect 99% or better. So they are literally mirrors, lower grade
15:22materials like mild steel for example or iron will be somewhere in the region
15:27about 60 or 65 percent reflective. The other thing that could happen is the
15:33light can be absorbed into the surface. Oooh, I’ve got to be careful about this
15:39word absorption, because it gives you the impression the material is like a sponge,
15:43wrong! What actually happens is, just at the surface of the material, there are
15:50atoms which will be stimulated just like when you put things in your microwave
15:58and it heats up, that’s what would happen here. These light particles are
Transcript for How Do Lasers Work? (Cont…)
16:05stimulating the atoms on the surface of the material. It’s light, it can’t
16:11penetrate the material itself, it can only interact with a surface and that’s
16:17the important thing to remember throughout this. The solid starts to
16:22vibrate because it’s stimulated by the light and as we’ve just discussed,
16:29vibration, extra stimulation, heat. Because these materials that we’re
16:34going to be firing this at are non metallic materials, they have got very
16:40poor conduction properties, they do not transfer heat easily and this heat
16:47builds up on the surface and because the heat can’t disappear, it stays on the
16:53surface and it gets hotter and hotter and hotter. As we’ve seen in the Wood
16:59experiment, it starts to glow white-hot and it glows white-hot in the centre here.
17:06Basically the wood has burnt away, but then after that we saw this white glow
17:11in the center here and the white glow was the carbon, it wasn’t burning it was
17:17heating up and it heated up to a white-hot temperature where it couldn’t
17:21resist anymore and it turned into a gas and so gradually what was happening is
17:26here, we get a little bit of erosion in the surface as the carbon
17:32evaporates and it leaves clean carbon behind and so the light beam stimulates
17:37this new carbon that’s behind here and gradually what we’ve got is a process of
17:45erosion and it eventually worked its way through the three millimeter thick
17:54material. The important thing I need to stress here is this is not like the
18:01hacksaw, this is not like the knife, this this is not a continuous process this is
Transcript for How Do Lasers Work? (Cont…)
18:06almost like a wood pecking action, it’s a gradual erosion process, even with the
18:11very high energy levels that we had here it took time to burn through the wood.
18:16But what we can categorically say is, that when we fire this beam at wood it’s
18:21capable, because of the carbon content of generating at least 3,000 degrees C. Now
18:29the beam itself is not 3,000 degrees C, it’s just a beam of light now if this
18:35was just a piece of acrylic,
18:40this damage here took place at a much lower temperature than 3000 degrees C,
18:47because the threshold, the damage threshold on this material is different
18:52to the damage threshold of wood. So every material will react to this light
18:57stimulation in a different way, but it is the light stimulation that heats up the
19:04material, it’s not the light itself that is hot. That’s an important concept to
19:13remember, we are now going to take that beam and we’re going to magnify it.
19:18We’re going to concentrate it down and we’re going to concentrate it down from
19:22six millimeters diameter to 0.1 millimeters diameter. Basically we’re
19:28going to amplify the light at this point, the energy density, to some phenomenal
19:34value and in fact if we do a very simple bit of maths you’ll find that that, is
19:40about three thousand five hundred times smaller than that. So we’ve amplified
19:50potentially, the ability to do three thousand degrees C’s worth of damage,
19:57there is a huge amplification of the energy density.
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20:03Watch very carefully here, I’m just going to do a very simple, very very quick
20:10pulse. You may or may not have seen what happened there, but let’s first of all
20:17check. How long was that pulse? Maybe a tenth of a second? The pulse has burnt
20:24right the way through and here’s what the underneath looks like, it’s burnt
20:29right through it in less than say a tenth of a second or maybe even less
20:34than that. We’ll do it one more time, now if you watch very carefully you’ll see
20:39two things happening, you’ll see a little bit of a flame coming up where the wood
20:44is burning away, but then you’ll see this white flash as the carbon evaporates.
20:56We’ve still got the same energy profile, it’s just been magnified up to some
21:06phenomenal intensity as it’s been decreased to 0.1 of a millimeter
21:13diameter. The same damage principle is existing after the lens as the one that
21:22I demonstrated to you before the lens and whereas with the unfocused beam it
21:29was taking it’s possibly a minute to burn through this piece of 3 millimeter
21:33material. When we amplified the beam it was taking less than 0.1 of a second!
21:42This piece of wood is the same piece of wood that took over a minute to pierce
21:47through with low energy density, once we amplify the energy density up, we
21:55can do the same amount of work in a tenth of a second. Time is something
22:00that’s going to come into our cutting and our engraving discussions as we push
Transcript for How Do Lasers Work? (Cont…)
22:06on with this subject. Now, we saw that in a tenth of a second we could burn a very
22:12compact little hole through. Let’s see what happens if I leave the beam on for
22:18about a second. One second. We’ve got our scorch Corona around the
22:27outside, the same as we did here. We’ve started to generate additional damage
22:35around the outside of the hole, whereas here,
22:40we had it on for such a short period of time that all we did was to pierce a
22:44hole, instantly through there, right with the centre of the energy beam and we
22:50didn’t get a chance for the lower energy levels around the outside to have any
22:55burning effect on the wood. So that’s another important lesson I’m trying to
23:00get over to you about time, you need just enough time to do the damage that you
23:06want to do and not so much time that you actually cause collateral damage as well.
23:14Now, these are very important concepts when it comes to understanding the
23:18cutting process, because this is effectively the process that causes
23:24charring on the edge of your cut. The idea is to cut as quickly as possible
23:32with as high an energy level as you can. The way in which the laser beam
23:37damages material is a difficult concept to try and describe to you, but I
23:44hope that I’ve broken it down into small enough chunks, that like a jigsaw puzzle
23:49you’ll be able to put the pieces together for yourself. The fact that
23:53we’ve got energy density which causes gradual erosion and not an
23:58instantaneous cut, even that instantaneous tenth of a second cut that
Transcript for How Do Lasers Work? (Cont…)
24:04took place as we burn that hole through, was not an instantaneous pierce. It was
24:10still done by exactly the same pecking mechanism, it’s just that it happened in
24:15such a short period of time that it looked as though it was an instant cut,
24:21like a hacksaw or a knife, but it’s not, it’s a woodpecker.
24:27A very fast-acting woodpecker and that process applies to every material that
24:35we’re going to fire this beam at, whether we’re doing engraving or whether we’re
24:39doing cutting, we’re basically either going, we’re going to be damaging the
24:44material surface with this high-energy beam of light and it’s the interaction
24:50of the light with the surface that causes the damage. It causes heating and
24:56that heating causes different types of damage in different types of material.
25:01Well, I think that’s enough mental gymnastics for today we’ve basically
25:06talked about the damage process for wood and organic materials in this session
25:12and we’ve used that as a demonstration of how the laser beam damages material,
25:18because it’s one of the easier concepts to understand. Now there are other damage
25:24concepts which we’ll move on to in future sessions, but they’re all based on
25:28this same principle. So the hard work is done, if you can understand
25:34what’s been going on today. So thank you very much for your time and I’ll catch
25:39up with you in the next session
