Session 09 – Laser Mirrors

The Concise RDWorks Learning Lab Series

Welcome to the new Concise RDWorks Learning Lab Series with Russ Sadler. In this session, Russ discusses his 6 years of practical experience with laser mirrors as well as his numerous experiments with differing laser mirror types. He also advises on the best value for money laser mirror for both performance and durability.

Over the last 6 years, Russ has built up a formidable YouTube following for his RDWorks Learning Lab series which currently has over 200 videos.

The original RDWorks Learning Lab series on his “Sarbar Multimedia” YouTube Channel, follows Russ as he tries to make sense of his new Chinese laser machine and to sort out the truths, half truths and outright misleading information that is available on the web.

Six years later with over 4 million YouTube Views under his belt, Russ has become the go to resource for everything related to the Chinese CO2 laser machine user or wannabe user.

A Variety of Laser Mirrors
A Variety of Laser Mirrors

In this new series, Russ has condensed his knowledge and experience of the last 6 years to provide valuable information and insights into the purchasing, understanding, use, repair and maintenance of the Chinese CO2 laser machines and their key component parts.

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A Damaged Gold Plated Laser Mirror
A Damaged Gold Plated Laser Mirror

Transcript for Laser Mirrors

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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 silicon 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.

165
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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.

166
00:19:25,130 –> 00:19:30,230
And in the bottom of, in that fringe, it indicates that the depth,

167
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the bottom of that green depression there. Is approximately because it’s one fringe, 34 nanometers.

168
00:19:42,990 –> 00:19:49,240
Let’s just extract the basic facts from that simple little picture there.

169
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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…)

170
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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.

171
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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.

172
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What we’re going to be able to do is to calculate what the size of that angle is.

173
00:20:16,730 –> 00:20:24,470
Point zero zero one seven one nine degrees.

174
00:20:24,470 –> 00:20:26,720
I didn’t just calculate that in my head.

175
00:20:26,720 –> 00:20:34,610
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,

176
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it hits a small imperfection on the surface of the mirror and it veers off at this angle here.

177
00:20:41,690 –> 00:20:47,690
So that angle there is the same as that angle there.

178
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But now we’re going to assume what happens when it veers off at one meter.

179
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How far is that out of position?

180
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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.

181
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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.

182
00:21:20,690 –> 00:21:25,550
Is it going to have an effect? I’ll leave you to laugh.

183
00:21:25,550 –> 00:21:35,710
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

184
00:21:35,710 –> 00:21:41,970
I’ve convinced you that you’re not operating a piece of really laboratory scientific equipment.

185
00:21:41,970 –> 00:21:47,790
You don’t need super duper mirrors. You’ve seen the clouds.

186
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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.

187
00:21:56,580 –> 00:22:02,640
And I hope you understand why now. So there’s your friend for life.

Transcript for Laser Mirrors (Cont…)

188
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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.

189
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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

190
00:22:20,260 –> 00:22:26,950
all the way through mirror three, mirror two and bouncing off mirror one.

191
00:22:26,950 –> 00:22:37,390
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.

192
00:22:37,390 –> 00:22:48,981
Does the circular outlet from the tube look around? That’s the real question.

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Last updated August 26, 2021

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