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The Concise RDWorks Learning Lab with Russ Sadler.
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Session 9: Mirrors. Welcome to today’s session.
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Now, look, I’m determined to get into the picture somehow, and today’s session is all about these things here mirrors. Now mirrors are a very ignored
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subject. You put them in your machine. They reflect the beam around the machine and you really don’t think about them very much.
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This is not a laboratory machine. We do not need laboratory quality mirrors for this machine, despite what some people are trying to sell you.
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We’re trying to take the mystery out of mirrors today and give you my practical experience where I play, where I have played with lots of mirrors,
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as you will see. I’m not interested in what the science says. I’m interested in what my practical demonstrations of power in and power out.
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I’ve worked out the power losses across each mirror, and they don’t agree with the scientific data that you can see in books.
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But let’s just have a look at some mirrors to start with. And I’ll try and explain some of the differences that you can get.
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Remember I said I was going to repeat things endlessly. Remember those three things in the ten point six universe that laser beams can see?
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One of them was metals, remember? Now, metals equals mirrors.
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Does this look like a mirror? Well, of course it’s not.
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It’s a piece of copper that looks really cruddy. Oh, look, I can see a bit of a vague reflection in there, but trust me, that’s not a mirror.
Transcript for Laser Mirrors (Cont…)
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But then again, let’s move forward to one of the more recent sessions that you looked at,
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where I showed you how reflective this very cruddy surface was.
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It could damage material almost as good as the laser beam itself.
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This is, even in this state, probably something like about 96 or 97 percent reflective.
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It’s nothing to do with the surface finish.
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It’s all to do with the crystal structure in the material itself that’s reflecting the light waves. Equally
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well, we had this pretty cruddy piece of aluminum as well.
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Look, it’s a little bit shinier, but its surface is all scratched.
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And if you look carefully, you’ll see the surface is a grained surface.
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So this is definitely not a mirror, but this performed almost as well as this when we looked at reflected light.
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So metals definitely can be laser mirrors, but it’s nothing to do with the surface finish itself.
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It’s all to do with the crystal structure of the material. Here are five mirrors and in reality, there’s only three different types of mirror there.
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But I’ll explain that in a second.
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I have spent quite a lot of time experimenting with mirrors and making my own mirrors because I wasn’t able to buy them.
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Now I became fascinated by the prospect of copper mirrors because the technical literature indicates that
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copper mirrors might be able to give me something like ninety nine point five or more percent reflection.
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We’ve already seen that copper is extremely good at reflecting even when it’s not shiny. But as I said shinyness
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isn’t the issue. This is one of the mirrors that I did make. It’s quite old now and it hasn’t actually been used.
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This is how it came back from lapping, where I got it lapped extremely flat.
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It performed at around about 95, 96 percent.
Transcript for Laser Mirrors (Cont…)
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And then I finished up polishing it and I got it up to ninety eight, sometimes ninety eight point five percent.
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The scientific promise of this material didn’t materialize. While I was waiting for these to be made.
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I tried to buy some copper mirrors in China and I succeeded in buying some copper mirrors in China, but.
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That copper mirror is actually gold plated and it’s gold plated because everybody said to me, copper is no good, it’ll oxidize very quickly in air.
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Now I have to say that
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the doom merchants that forecast that my copper mirrors would only last a few weeks because oxidization would occur. Were totally wrong,
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because the compound that I used for polishing had some chemicals in it which bonded
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onto the surface and prevented the oxygen getting to the raw copper surface.
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Those mirrors lasted in my machine for over a year before I cleaned them the first time.
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So I was extremely pleased with my copper mirrors. But they’re not the sort of thing that you can buy.
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And they cost me quite a lot of money to get them made. But I had to prove the point and I did.
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Now, I was in contact with a guy in the USA who was not only a laser machine owner, but also was a jeweler.
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And he had the ability to plate copper for me with 24 carat gold, real 24 carat gold.
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Not this stuff here that was on the Chinese copper mirrors. Now, I’ve got no idea what this stuff is.
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It may well be that it was coated with some sort of protective coating because it didn’t perform very well at all.
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It performed at about 96 percent. Some of the best companies in the world,
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and here we’ve got one from Thor Labs, which have got a graph which tells us what happens to the coatings that they put on mirrors.
Transcript for Laser Mirrors (Cont…)
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They put coatings on these gold mirrors to try and enhance the performance of gold.
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Why? Gold is supposed to be ninety nine point five percent anyway.
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And in fact, when we look here, we find that there is one particular treatment, which at a specific frequency of around about ten point six microns,
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takes us up to probably something like ninety nine point eight or ninety nine point nine percent.
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But it’s a very, very, very special coating. And any other coating that they use comes nowhere near that.
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We’re around 97, 98 percent. And that very much accords with the performance data that I was getting from gold plated mirrors.
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Now, the 24 karat gold plated actually performed worse than the protected gold plating.
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This was about 94 percent. This was about 96 percent.
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I got these up to ninety eight point five percent. And then we’ve got these mirrors here now, these two on the end.
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This one is gold plated. It might not look it.
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It’s got an atomically thin layer of gold plated on top of silicone.
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But silicon is not a reflective material. It’s only the gold that is reflective on the surface.
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I think that mirror is no good. My acidic fingerprint has eaten through the coating and damaged something on the surface of that mirror.
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So that shows you how sensitive these mirrors are.
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You wouldn’t normally put a fingerprint on a mirror like that, but even rubbing that with isopropyl alcohol and a cotton bud,
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there’s a very distinct possibility that you will eat through various parts of the gold, the atomically thin gold coating that’s on there.
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And if you do that, what’s underneath is something that does not reflect.
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It will take in the energy and heat up, so when you work your way through the gold after several cleans,
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you may well find a bit surprisingly that your mirror doesn’t work.
Transcript for Laser Mirrors (Cont…)
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Now,
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this particular type of mirror, silicon, is very expensive and it must be the base material because the base material is inherently flat to start with.
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Whereas these. This one. This one. And this one. And this one, which I’ll talk about in a second, all have to be mechanically lapped
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flat. Molybdenum, which is this on the end here is a very heavy element.
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It’s a metal which is very hard, a bit like tungsten carbide and it’s extremely heavy.
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When you pick the silicon up, you will immediately say, oh, that’s very, very light.
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And when you pick this one up, it looks like the size of a coin, but it’s probably three times the weight of any coin you’ve ever felt.
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And you will automatically say this is heavy. That’s a very quick way of deciding whether or not your mirror is molybdenum.
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And apart from anything else, it’s the only one that’s silver.
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All these others are gold and gold is a good warning that, hey, this is not solid gold.
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This is gold plated. Now, I do not like plated lenses of any sort,
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for the reason I just mentioned. You can wear through the atomically thin layer of gold when you clean it.
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Now, if you’ve got a copper mirror, it doesn’t really matter.
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The gold is there to supposedly stop it oxidizing.
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And if you wear through the gold, well, the copper underneath is just as reflective as the gold itself.
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So it doesn’t really matter if you wear away the gold on a copper mirror. But copper mirrors are expensive and they’re really a waste of time.
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Here are the various materials that we’ve been talking about and here are some of the mechanical properties of these materials that are used
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as sometimes positive and sometimes negative advantages as to why you should buy one type of mirror as opposed to another type of mirror.
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We’ll talk about those in a minute. Here we’ve got the reflectance of this material at ten point six microns.
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The black numbers down here represent the text book
Transcript for Laser Mirrors (Cont…)
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umbers, that are promised reflectance values for each of these materials.
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And in red down the side here are the actual physical test values that I’ve been able to achieve for these mirrors.
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Now, I’ve got no idea how they achieve these numbers here, but I do know how I’ve been able to achieve my numbers.
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My numbers are real world numbers. So I use a power meter to measure the watts
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entering a mirror, and then I use a power meter to pick up the reflected watt’s coming out of the mirror.
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And the difference between the two gives me this percentage. You can’t actually go out and buy such a thing as an aluminum mirror.
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But I made one. It was made from an old hard drive disk, which is basically aluminum that has been plated with various materials that looks silvery.
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So I couldn’t tell you what the material was that it was coated with, but it gave a result of about ninety six per cent.
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So it was, you know, if you really get stuck, you can always make a mirror out of an old aluminum hard disk. Silicon?
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Well, I was a bit disappointed with the silicon mirror because again that was around about 96.
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Hmm. I did get some 97 percent results from it, but there was nothing staggering.
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This one, this one, this, this one was not gold plated, but it was plated with something else.
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And the copper ones that were gold plated all performed at about ninety six to ninety four per cent in some instances,
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very poor performance overall when gold promises to be so good.
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And when I compare those results with molybdenum at 97 percent, they were, molybdenum was just about as good or better than most of the other results.
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The only thing that I found that was beating molybdenum were my pure copper, raw copper mirrors that were unplated.
Transcript for Laser Mirrors (Cont…)
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You get a lot of people talking about these numbers. And look, at the end of the day, you’ve got 100 watt tube.
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So what’s the difference between 98 percent and 96 percent?
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Two watts. What can you do with two watts? Is it going to allow you to cut faster?
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Is it going to allow you to engrave deeper? In real terms,
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the answer is no. You are not going to notice the difference. So you can spend a lot of money on a silicon mirror,
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you can you can spend a stupid amount of money trying to find a copper mirror.
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Don’t bother. The most cost effective mirrors out of the whole of this combination are the molybdenum mirrors.
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Now, molybdenum, as I said, is very hard,
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you will not be able to scrape the surface off them at any stage because they are solid molybdenum all the way through.
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So they’re virtually bulletproof and mirrors for life.
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When I took my original molybdenum mirrors out of the machine and replaced them with copper mirrors. They weren’t quite as bad as this.
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Do they look like mirrors to you? No. They’re very badly oxidized on the surface.
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Now, this molybdenum over a period of time will oxidize a bit like that.
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So you need to keep these every six months, at least probably, serviced. Here, we’ve got a little teeny weeny bit of acetone.
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Is that having any effect on the surface? No.
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So acetone is fine for taking off a film that will build up over maybe a few days if you really need to clean your mirrors.
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But most of the time I don’t touch my mirrors. I keep an eye on them, but I don’t touch them.
Transcript for Laser Mirrors (Cont…)
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Now, what are we going to do now is completely restore that mirror to its brand new condition for nothing.
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Silver Polish. There’s the tarnish.
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So you can now see that I brought that mirror back to life again. That’s what you can do with molybdenum.
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As I said, it is a mirror for life. But on the other hand, it’s the cheapest almost of all the mirrors in this list.
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You can get glass based mirrors which are coated with an atomically thin layer of gold.
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They have got zero reflectance, the glass underneath, and it’s only the gold surface on the top that is reflective, but it’s atomically thin.
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And after a few cleans,
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you would definitely go through that and you will then find you’re putting all your energy into heating up the glass and it will crack.
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Now heating mirrors up. Or not, heating mirrors up is a benefit that manufacturers will claim for their mirrors.
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But is it really a problem when we mount our mirrors?
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They’re mounted in blocks of aluminum. Aluminum is a very good transmitter of heat,
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and it will spread the heat out very quickly over the whole of these areas.
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So you’ve got a huge radiator there. I mean, look what we’re talking about.
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We’ve got a mirror that is 98 percent or 97 percent efficient.
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So that means three watts are continuously heating up your mirror,
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potentially. That mirror will never heat up.
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It might get slightly warm. Provided you’ve got this large heat sink around the mirror and the mirror itself has got good conductive properties.
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Then the heat’s going to dissipate away from the mirror. Silicon mirrors are probably four or five times the price of a molybdenum mirror.
Transcript for Laser Mirrors (Cont…)
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What do you get as an advantage? Ah! The claimed advantage is flatness. Flatness, you’ve got to be a long way out to produce distortion.
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And the flatness of this surface, which is what we’re really talking about,
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the light rays are going to bounce off atvery slightly different angles because of minute surface imperfections.
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And that’s why this doesn’t look shiny.
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The reflectivity of it hasn’t changed, only the way in which you perceive the image of the light that’s gone on to it.
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OK, dodgy diagram warning. I very much respect science, but I’m not a slave to it.
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Facts and figures, as I’ve demonstrated to you with this session, can be a bit different to reality.
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And one of the things that I will agree with is that a silicon mirror is about as good a mirror as you can buy at a reasonable price.
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But the question is, do we need it or are you being sold something that is totally unnecessary for this type of machine?
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Now, here we are in my yard. I’ve got a mirror. You can see the edge of the lens cover there,
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and beyond that, look, we’re looking at the sky and those clouds are several thousand feet in the air.
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Do they look fuzzy? I know they’re clouds. Of course they’re fuzzy. But is the picture fuzzy?
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The answer is no, because that mirror is doing exactly what it’s supposed to do.
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It’s reflecting an image and it’s not distorted.
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Well, I don’t think there can be a much clearer demonstration than that one.
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As to how a mirror works and why it’s not that important to have a super duper, incredibly flat surface.
Transcript for Laser Mirrors (Cont…)
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We’re not trying to fill in a crossword puzzle on the moon. All we’re trying to do is to send a laser beam bouncing off of three mirrors, over distance of
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maybe at the maximum, two meters. So I think you can begin to understand how I’m really almost ridiculing the technobabble that will be thrown at
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you by manufacturers, telling you you’ve got have absolutely superb quality mirrors to get the best beam possible
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arriving at your lens. Now, just to rub salt into the wound, here’s some very, very basic mathematics. Here is my mirror,
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which happens to be 25 millimeters and the beam 10 millimeters diameter.
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OK, now on the surface of this blue mirror, we have got some imperfections and let’s just say those imperfections look like this.
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So if you go on to Google and look at optical flatness measurement, you see all about the way you use light fridges and a special type of light
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and an optical flat to determine the flatness of a surface that’s almost flat.
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I’m not going to go into too much detail, but very crudely, let’s just say the width of a fridge is approximately two millimeters.
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And in the bottom of, in that fringe, it indicates that the depth,
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the bottom of that green depression there. Is approximately because it’s one fringe, 34 nanometers.
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Let’s just extract the basic facts from that simple little picture there.
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There’s half of our two millimetre pit and we’re looking at half way to get the depth of the pit.
Transcript for Laser Mirrors (Cont…)
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And we said that that depth of pit was roughly zero point zero three microns, which is equivalent to 30 nanometers.
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So we’re not going to get too fussy about this because we know this dimension and we know the depth of the pit.
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What we’re going to be able to do is to calculate what the size of that angle is.
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Point zero zero one seven one nine degrees.
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I didn’t just calculate that in my head.
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Now, if we assume that a light beam is coming along here and it would normally hit a flat upright surface because it would be a mirror,
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it hits a small imperfection on the surface of the mirror and it veers off at this angle here.
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So that angle there is the same as that angle there.
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But now we’re going to assume what happens when it veers off at one meter.
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How far is that out of position?
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One meter away from the mirror from where it should be? 25 microns, with the hairs off of my head, what, 25 microns looks like.
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So maybe we’re talking about two hairs at the full extremity, two meters of the beam length. Our beam is 10 millimeters diameter.
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Is it going to have an effect? I’ll leave you to laugh.
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I’m not going to laugh out loud because that would be rude. OK, I think that’s probably about the end of it. Now I hope
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I’ve convinced you that you’re not operating a piece of really laboratory scientific equipment.
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You don’t need super duper mirrors. You’ve seen the clouds.
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What more can I say? You don’t need to be told by a salesman that you’ve got to have the best mirrors in the universe.
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And I hope you understand why now. So there’s your friend for life.
Transcript for Laser Mirrors (Cont…)
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You can be rude to it from time to time. And like any good friend, it won’t take any offense and it will keep loving you.
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We’re going to do one final check. I’m going to put the camera down here and we’re going to look up the lens tube
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all the way through mirror three, mirror two and bouncing off mirror one.
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We’re going to take a look at this round circle here, which is the outlet to the laser tube. Although the mirrors are oval, because they’re tipped at forty five degrees.
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Does the circular outlet from the tube look around? That’s the real question.
