Session 10 – Laser Lenses Part 1

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The Concise RDWorks Learning Lab with Russ Sadler. Session 10: Lenses Part One.

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Well, here we are at session 10. We’ve covered most of the main elements of this machine.

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So you’ve got an idea how this machine is constructed, we’ve done the science behind it to start with.

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And we’ve done the laser tube, the laser beam itself, which comes out of the tube.

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We’ve done the mirrors, we’ve done the motor drive system.

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We’ve done the brain, which is the controller, although we only briefly skipped over that we will come back to that.

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We’ve done the high voltage power supply, which delivers power to the tube, and we looked at mirrors.

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And the last thing in the chain that makes these machines, makes these machines work.

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Are these things, the lenses. Now, we did touch on lenses very, very briefly when we talked about the world that ten point six micron laser light sees.

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Remember, mirrors = metals, these things are a very, very, very small group of materials that will look like windows and past the light through.

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Even though this particular one is completely black to us, we can’t see through it.

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But ten point six micron wavelength light passes right through it.

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As though it was is a piece of glass and I did mention that glass is totally like that to ten point six micron wavelength light, it cannot see through it.

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We can, but to ten point six microns it’s totally black and opaque.

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These things have been around for absolutely ages, maybe not in quite this sophisticated form, but lenses were used by the ancient Egyptians.

Transcript for Laser Lenses – Part 1 (Cont…)

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It sort of went into obscurity with little use until probably about fifteen hundred.

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When Leonardo da Vinci decided he could use mirrors and lenses to focus sunlight and burn up vessels. He was using it as a terror weapon.

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I think it was in the early sixteen hundreds that Galileo decided that he

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could nick an idea from a Dutchman who was using them for improving eyesight,

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basically spectacles. And he could use this idea of lenses to make a telescope.

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That was when really lens theory started to develop. To sort of try and explain some of the strange phenomenon that you could witness.

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The fact that, for example, if you looked through a very, very, very small pinhole, all of a sudden the world turns upside down.

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So the only way that that could happen is if the light from the top is somehow traveling to the bottom and vice versa.

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And that was the beginning of the way in which people started to think about lenses.

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We have still got the same basic principle of what they call ray tracing.

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As I said, this is a very, very well established science. I’ve got no skills in optical engineering.

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All of my engineering career I’ve been involved in all sorts of other things, but not really optics,

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You know, I’ve got involved with optics to a certain extent when I was doing metrology. That’s not understanding optics.

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That’s just playing with something that already exists.

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And yes, look, these things already exist and they’re a well proven technology that works. Like the wheel,

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nobody decides that they’re going to reinvent the wheel. I once thought that square would be an excellent idea.

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Just think about it for a minute. You wouldn’t need handbrake on your car. And who wants to pitch a car with square wheels?

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I’m going to tell you a little bit initially about standard lens technology and theory.

Transcript for Laser Lenses – Part 1 (Cont…)

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I’ve been struggling for about two years now to try and find answers to questions that I can see, but nobody can tell me why I can see them.

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I’ve done all sorts of research and I’ve spoken to all sorts of “learned” people and not come up with a solution to the problem.

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So I’ve had to do my own test work and development work, which, as I mentioned,

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I’ve got another channel called RDWorks Learning Lab, because it’s like watching grass

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grow because you’re looking over my shoulder at me learning how this machine works.

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Now, I still haven’t come to a full conclusion on how lenses actually work,

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but I’ve got an idea now and I’ve got a theory and I’ve got some proof that I do know how they work.

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But it’s a bit early in the day for me to try and pass that information over to you.

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Now, accompanying this video, you’ll find some text, in that text,

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I’ll put some references. Now, I earnestly ask you to go and look at those videos and those PDF’s because you will

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see what the current state of knowledge and technology is associated with lenses. It hasn’t changed in hundreds of years,

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but it doesn’t explain some of the strange things that I have seen while I’ve been doing test work on this machine.

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I didn’t buy these machines to use in any sort of business.

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I bought these machines to keep my two remaining gray cells exercised, trying to understand how it works.

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So there has been, if you like, the final frontier, which I have not fully broken through.

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I’ve stood on the other side of the fence, but I haven’t explored too far yet.

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I’ve got a lot more work to do and I’m not going to share any of that really with you at this moment in time.

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But maybe in a later session when we talk about the laser beam and how the laser beam and lenses react with each other,

Transcript for Laser Lenses – Part 1 (Cont…)

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I will probably start sharing some of that stuff with you, so you can make up your own minds.

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Now, let’s start off with the basics about lenses. First of all, for our machine here, we normally have got two different sizes of lenses.

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We’ve got lenses that are 20 millimeters diameter and lenses that are 18 millimeters diameter.

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There are other sizes that you can get, but they’re the two most common sized diameter of lenses that we use in these in these machines.

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Now, in addition to that, these are the four sizes that we normally use on this machine. A one point

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five inch focal length lens, thirty eight point five millimeters, a two inch lens,

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which is fifty point eight millimeters focal length, a two and a half inch lens,

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which is sixty three point five millimeters and a four inch ends, which is 101 point six millimeters. Now just in this box here.

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I think you can see quite clearly we’ve got something that’s black, something that’s quite yellowy and something that’s quite honey coloured brown.

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So all these different colours mean something different about the material that the lenses are made from.

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One of them is called gallium arsenide, which I showed you earlier.

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Now, this one, here. And this one here.

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And you can see they’re completely different colors are both the material called zinc selenide.

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This one’s a different grade to this one.

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This one is Chinese because it’s made by a process called PVD – Physical Vapor Deposition, and it produces this brown colour.

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This one is made by a different process called CVD Chemical Vapor Deposition.

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This product is made normally in the USA. Now, the difference really is in performance, not optical performance,

Transcript for Laser Lenses – Part 1 (Cont…)

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although some will claim there is a lot of difference in the optical performance.

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But it’s mainly you’re not advised to use this brown material, this browny material over 80 watts.

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That’s its power threshold. This stuff you can use up to probably 150, 200 depends on the purity of it, they can take this up to kilowatts,

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but the stuff that we use, safely, you can probably go up to about 150 watts.

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This will this will handle several hundred watts. So, yeah, this is basically bulletproof.

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These are cheap. These

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are probably 50 percent more expensive, maybe even 100 percent more expensive.

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Depends where you buy them from. These, well, these will probably be three or four times more expensive than this one.

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Is it worth it? Well, we’ll talk about that in a later session because it really depends what you want to do with your lenses.

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Having said that, there are two basic materials. We only have two commercially available lens shapes.

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And let me explain what I mean by a lens shape. Dodgy diagram time.

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I’ve got a half decent diagram that I’ve prepared for you.

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Which shows you what you would normally expect a lens to look like. This lens here is basically part of a sphere.

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And it’s got a flat on the bottom of it. This is called a plano convex lens.

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That bit’s Plano, this bit is convex, and there are two ways that you can shine light through that lens.

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Light normally coming from a long way off, like the Sun, for example, arrives at this lens with parallel rays of light.

Transcript for Laser Lenses – Part 1 (Cont…)

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And that’s the way in which lenses are designed. They are always assuming parallel rays of light.

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And if the rays of light are parallel and they hit this. What happens is….

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Now, let me just stop you for one second. I’m sure you’ve all seen a stick poked into water and how it appears to bend.

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That’s a physics phenomenon called refraction, where when light travels from one medium air into a denser material,

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which looks like glass but isn’t really glass, the light will change direction, as I’ve shown here.

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And then once it travels out of the dense material back into air again, it will change angle yet again.

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So there are two changes of light happening with the lens this way round.

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And what happens is the light tends to focus down onto a point.

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Now you can see, although we’ve got parallel light approaching this lens,

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what comes out of the lens should technically focus down to a single focal point.

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Look here and you’ll see that it doesn’t actually do what you expect.

Transcript for Laser Lenses – Part 1 (Cont…)

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Those rays that are coming in from the outside are focusing there.

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And those rays that are coming in down the center are focusing here.

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There are several different focal points, not a single focal point with a plano convex lens.

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Now, that’s due to some strange phenomenon called spherical aberration.

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You cannot get the rays to focus down to a perfect focal point.

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You will get a concentration here, which looks like a focal point, but it isn’t the focal point of every single ray.

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We can improve the quality of this focus here, by doing this to the lens.

Transcript for Laser Lenses – Part 1 (Cont…)

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We can put a secondary spherical surface on the lower face of the lens and

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what that will do, will greatly reduce this effect called spherical aberration.

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It still will not make it perfect, but it will greatly reduce the spread of focus.

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You ordered a one and a half inch lens, but when you got your one and a half inch lens, is it a meniscus lens or is that plano convex?

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They probably don’t tell you when you buy them, that you have the choice of either / or you get what they give you.

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Now, there’s a very simple way of telling whether or not you’ve got a meniscus or a plano convex lens.

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And let me show you. If you wear glasses, you’re going have to take them off.

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Now, normally, you wouldn’t be handling these lenses because, hey, this yellow material is actually toxic.

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But this lens is covered with an anti reflective coating, and that coating protects me from the toxic material that’s underneath.

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If ever you break a lens, be very careful. Don’t handle it.

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I’ve got very acidic sweat on my hands. And you wouldn’t normally see me handing lenses quite like this.

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I’m likely to do more damage to this lens than it’s going to do to me.

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The curved side should be away from your eye and you’re going to hold it up to

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your eye and twist it away so that you can see a reflection of the background.

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Now, make sure that the background is something like a window or something that’s well lit.

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If you can see everything in that surface crystal clear, like a mirror, then that must be a flat surface and it’s a plano convex lens.

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Now, if you do the same thing and everything looks fuzzy.

Transcript for Laser Lenses – Part 1 (Cont…)

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Then it’s a meniscus lens and then the other thing that you can do is just turn it very slightly to look at the hair,

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maybe on the side of head and you’ll see that that is actually in focus, crystal clear.

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You absolutely know that then what you’re holding is a meniscus lens.

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If I pick these two lenses up, there’s no way that you can easily tell what the focal length of those lenses is.

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So I’ve got a little clamp there that holds that lens in front of a little viewing window.

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And I’ve made myself a fixture here, which has got a target on it.

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And that target moves backwards and forwards relative to the lens. And on the side here, I’ve got some marks. Now,

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I only have to put four sets of marks on there because as I said,

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we’re only going to be using four types of lens. This enables me to look very carefully through here.

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And I can bring this into focus and say, yeah, the focus is about there somewhere.

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And then I could look on the edge to see roughly which band of marks

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I’m in. And in this instance, it’s the one and a half inch band.

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Now, it will not be absolutely perfectly one and a half inch, even though this is a one and a half inch lens,

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because white light refracts differently to ten point six micron wavelength light.

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So the chances are that this lens, which is designed as a one and a half inch or thirty eight point one millimeter lens,

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will come up short when we check it in white light. It won’t be thirty eight point one millimeters.

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It’s more likely to be something like about 35, 36 millimeters.

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So the lenses have to be designed for the wavelength of light that they were using.

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Although the meniscus plano convex trick still works for these gallium arsenide lenses. Because you can’t see through them,

Transcript for Laser Lenses – Part 1 (Cont…)

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we can’t tell exactly what focal length they are.

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|The only way that we can tell what focal length they are, is to put them in the machine and try and test them.

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Let me show you what I mean. I’m just gonna to remove the lens and lens tube. We’re just going to fire the laser beam at this piece of wood.

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And theren we go, look, we can clearly see the size of the beam.

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The damage that it does to a piece of card. Trust me, the laser beam is actually bigger than that,

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but that’s the amount of damage that I could do in the very short time that I allowed the beam to burn onto that paper.

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And if I was to leave it on there long enough, I would probably find that that diameter would grow to about 10 millimeters.

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I’ve got a plano convex lens in this lens holder here, and it’s sitting here and I think it’s a two and a half inch.

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I’m going to pulse it.

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And, you’ll notice immediately there’s a difference in the diameter, because the laser beam is being focused down to a smaller diameter.

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Now it’s getting even smaller and even smaller, now it’s getting bigger.

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There we go. So basically what I’ve done, is Iheld my paper there and we got a large burn. Then as I moved the paper up,

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we got closer and closer to this point here, which is the point through which most of the rays are passing.

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And so we produced a small burn here. And then I went above the focal point and we started to get big again.

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So the only way that we should be able to determine what the focal length of this gallium arsenide lens is, is to do something

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like this very quick test and then measure the distance where we get the smallest burn in relation to the lens.

Transcript for Laser Lenses – Part 1 (Cont…)

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But! When the lens is this way round, it’s going to have a much larger focal spot than when it’s this way round.

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So when you turn the lens over, you’ll find that you’ll be able to get different size burn marks,

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depending on which way you have your lens. No, I’m not going to get into a very old myth,

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an argument about whether there is such a thing as a right way and a wrong way to use your lens in this machine.

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That’s a whole different subject, which I’ve done a lot of study and work on.

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But very crudely, in general terms, people will tell you the right way to have your lens is flat side down.

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I’m not going to argue with that because in general terms, that’s a good way to use your lens.

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So when you turn the lens over, I think it’s pretty obvious there’s a big difference in the performance of the lens.

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The light comes in and because it hits the surface at 90 degrees, it doesn’t get refracted.

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It carries on straight on through the lens like this until it hits a surface at an angle.

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And again, this spherical surface refracts the light, but it refracts in a completely different pattern to when it was this way round.

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And look, we’ve got a much larger separation of the focus points from various diameters across the lens.

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So used this way round the lens is going to have a completely different performance characteristic to when it’s used this way round.

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These are the sort of characteristics that most people do not want.

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When you use a lens this way round, whether it be a plano convex or a meniscus, the focus is reasonably tight or very good.

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You get reasonable performance, particularly for engraving.

Transcript for Laser Lenses – Part 1 (Cont…)

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This way round has potentially got better cutting ability.

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But not many people would agree with me, because it looks as though you get better performance this way round.

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So we’re going to leave that for the moment. As I said, at a future date, we’ll come back and discuss the performance of lenses.

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But to keep it simple, stick with flat side or meniscus side downwards and you can’t go far wrong.

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Now, the next question is what sort of lens should I use for what application?

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We’ve got all these lenses, four inch, two and a half , two, one and a half and even a one inch lens is available.

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Well. Basically, the shorter, the focal length, at typically one and a half inch,

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the smaller this neck will be just here. Which means you’ll get the smallest or narrower width of cut.

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The longer the focal length, the slower the cut will have to go and the wider

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will be the cut that you produce. I’m using this just to show that it’s going to be a wider cut,

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not that you’re going to use the lens this way round. Now to cut thick materials,

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you don’t necessarily need a high wattage tube.

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You need time and exposure to cut.

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But there comes a point in time where you cannot go slow enough for the power

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of your tube to cut through the material thickness that you are dreaming of.

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So typically you might say, well, we can cut six millimeter material with a 50 watt tube.

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So I would generally recommend that a two inch lens is the best General All-Purpose lens.

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If you want to do some fine engraving work, then maybe a one and a half inch lens.

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Certainly a meniscus lens for engraving would be very beneficial for you.

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But for cutting, I would still stick with a plain convex lens. Just to get your appetite with the sort of things that have puzzled me.

Transcript for Laser Lenses – Part 1 (Cont…)

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You can see how a lens works with these rays. OK, and although these pictures are exaggerated, look at that cut.

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Now, that cut was done with a two and a half inch lens like the one I’ve got in there at the moment.

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But this is a piece of 26 millimeter thick hardwood.

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Look what I’ve been able to do. How is it possible to get a 26 millimeter deep cut like that,

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which is parallel and narrow from a beam that’s doing this. It just doesn’t make logical sense.

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Now, I’m just showing you that, to, if you like illustrate the sort of thing I’m going to have to be talking to you about in the future,

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about my theory of lenses as to how that is possible, as opposed to the conventional theory of lenses, which says that’s not possible.

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I’ve done some silly experiments with lenses, and I don’t mean silly things like poking my nose and things like that.

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No, I mean, I’ve done things like this. I’ve drilled a hole right through the middle of the lens.

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And then I’ve also machined away. The outside part of a lens and all I’ve done, I’ve left a very small three millimeter lens in the middle.

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The results are interesting. We’re going to leave lenses there. That’s enough for you to understand that a lens is a very vital part of your machine.

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You need to look after your lens carefully, do not clean them with anything other than isopropyl alcohol and a small Q-tip.

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Check them from time to time to make sure that you’ve got no fumes on the face of the lens,

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because if you put fumes on the face of the lens, the fumes will heat up and they will damage the face of the lens.

Transcript for Laser Lenses – Part 1 (Cont…)

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Now, look, when I hold this lens up right in the center there.

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I’ve managed to burn off the anti reflective coating. Now I did that in a completely different way,

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but that’s the sort of thing that you could do damage to your lens with by allowing it to smoke up.

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And there’s another one. Look, you can see the film that I’ve put on there.

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OK, now that’s a meniscus lens. I think you can see how it’s not flat, but can you also see what I’ve done to it?

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Look, it’s cracked. So, yeah, I’ve got lots of experience destroying lenses, doing silly things with lenses and getting good performance from lenses.

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That is the whole point of my RDWorks Learning Lab. It’s me learning to use specific aspects of this machine and understand how they work.

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In some future session. I will probably start sharing some information about my theory and approach to lenses.

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But for the time being, you can be as happy as I’ve been for the past five years,

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just using lenses and playing with lenses without actually ever having to understand how they work.