Session 10 – Laser Lenses Part 1

The Concise RDWorks Learning Lab Series

Welcome to the new Concise RDWorks Learning Lab Series with Russ Sadler. In this session, Russ takes a real life look at the workings of laser lenses used in laser cutting and engraving machines, and explains how lens theory does not fully account for how laser lenses actually work in practice.

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 Lenses
A Variety of Laser Lenses

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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Transcript for 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.

167
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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.

172
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But not many people would agree with me, because it looks as though you get better performance this way round.

173
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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.

175
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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

180
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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,

181
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not that you’re going to use the lens this way round. Now to cut thick materials,

182
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you don’t necessarily need a high wattage tube.

183
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You need time and exposure to cut.

184
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But there comes a point in time where you cannot go slow enough for the power

185
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of your tube to cut through the material thickness that you are dreaming of.

186
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So typically you might say, well, we can cut six millimeter material with a 50 watt tube.

187
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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.

189
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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.

192
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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.

193
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But this is a piece of 26 millimeter thick hardwood.

194
00:22:33,180 –> 00:22:39,630
Look what I’ve been able to do. How is it possible to get a 26 millimeter deep cut like that,

195
00:22:39,630 –> 00:22:50,520
which is parallel and narrow from a beam that’s doing this. It just doesn’t make logical sense.

196
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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,

197
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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.

198
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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.

199
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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.

200
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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.

201
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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.

203
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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,

204
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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.

206
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I’ve managed to burn off the anti reflective coating. Now I did that in a completely different way,

207
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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.

208
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And there’s another one. Look, you can see the film that I’ve put on there.

209
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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?

210
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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.

211
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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.

212
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In some future session. I will probably start sharing some information about my theory and approach to lenses.

213
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But for the time being, you can be as happy as I’ve been for the past five years,

214
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just using lenses and playing with lenses without actually ever having to understand how they work.

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Last updated August 26, 2021

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