Help picking a lens

[Walt Langhans]

Hi all,

I need some help figuring out what size lens I need. I understand how a lens works and what happens when you increase or decrease the focal length, but I don't have any experience actually seeing the results.

I currently have an 18mm diameter 40mm focal length lens. The distance from the tip of my nozzle to my material needs to be 7mm to be in focus. I'm working with mostly 1/8" (3mm) MDF and when things are in focus I'm happy with the results. But the boards are imperfect and there are spots where the focus distance can very 1-2mm because of warpage, which then cause bad results, and some nasty flair ups.

Here's a picture of what I'm doing:

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What I would like to do in increase my focus distance as much as possible to help adjust for warpage, but loose as little detail as possible. But as you can see, my detail is simple it's not like I'm tying to laser peoples portraits or anything. Basically I'm looking for 'my perfect' lens.

And now for the questions.

1/ What lens should I be looking at?

2/ How do I put in a different diameter lens? The nozzle I have doesn't look like it will hold any other size than 18mm. Do I need to get a new nozzle, or a new tube that comes down from the mirror with a new nozzle, or an entire new mirror, tube nozzle set up?

3/ What makes one lens 'better' than that other? I see the ZnSe and the GaAs lens (I understand the different elements) but what the effect? Also looking at the lenses over at lightobjects and they have 'Improved' & 'High Quality' but are both ZnSe type lenses, any idea what the differences is?

4/ I've also noticed that 2 lenses can have the same diameter but then have different focal lengths listed, how's that work and is there any advantages / disadvantages with the way they change the focal length but keep the lens diameter the same?

Thanks!

[Rodne Gold]

The further you are away from your materials the better, less chance of smoke damage and so on. Try a 50mm and a 63.5mm , see what works for you. DOF will be more with either and you wont have as many issues with warped material. Try focus a little into the material , like a 1/3rd of the thickness in.
You can make a holder of sorts to house a smaller lens , or you can order a bigger nozzle/head (they are the princely sum of $12 or a bit more) You cant mess with the lens itself.
If your lens is Yellow , its ZnSe , the grind and the coatings as well as most likely the purity of lens make a difference , buy the cheapys if you run a high risk of damaging em. The top quality (circa $150-200) lenses do do a better job , but not by a huge margin.
Bigger diameter, whatever the Focal lenght, is better as centering the beam on it is less critical and generally the grind is less critical as well , the sweetest spot of any lens is it's centre , you get all sorts of optical problems near the edge , so you want to avoid the incoming beam being near the edges before being focused. The smaller the diameter , the more chance you have of edge diffraction etc.

[Walt Langhans]

All of that makes sense. How will the longer focal lengths effect the engraving / rastering?

Also, I understand what you are saying about diameter of the lens, but I'm not sure I'm understanding how the effects the end result of the product being cut/engraved. How do I know if I'm getting edge diffraction, are there any tell tail signs? If I have the laser aligned well is edge diffraction still an issue, and / or does the large lens diameter help pick up the slop from things being a bit out of alignment?

[Richard Rumancik]

. . . What I would like to do in increase my focus distance as much as possible to help adjust for warpage, but loose as little detail as possible. But as you can see, my detail is simple it's not like I'm tying to laser peoples portraits or anything. Basically I'm looking for 'my perfect' lens. . . .

Looking for the holy grail, I see. Unfortunately, as you probably already know, if you increase the focal length with a different lens, you will increase depth of field (good for warped substrates) but will also increase spot size (increase kerf) and reduce detail. There's no right answer other than to say you need to try a few lenses and see what works. "Detail" is subjective and only you (and your customer) can say what is good enough. It's all a matter of tradeoffs.

I can't answer all your questions but this link might help answer some of your questions about tradeoffs in selecting a lens.

http://www.parallax-tech.com/faq.htm

As far as mounting I don't know exactly what the hardware looks like but Rodne indicates holders are cheap so it would be best to give yourself a variety of lens assemblies to work with for different kinds of projects.

[Rich Harman]

I have tried 40mm, 50mm, 63.5mm and 100mm lenses. Almost everything I do is cutting, very little engraving. The 63.5mm lens has good depth of field and small spot size. It is what I use 99% of the time.

The diameter of the lens is irrelevant - unless you are using a beam expander. If you want the beam to come out of the lens perfectly perpendicular, you need to hit the center of the lens (on axis).

[Richard Rumancik]

Rich says that the diameter of the lens is irrelevant - I think he is saying that you won't gain by increasing the diameter from what your machine was designed for. Or at least, that is what I will say. Larger diameter lenses are used on metal cutting lasers for a few reasons. You want to keep metal debris farther away from the optics. You can only put so much power in a small lens without causing heat damage so with kilowatt+ lasers a larger lens is easier to keep cool. So you need a larger diameter input beam to match the larger lens (small beam on large lens is not good, as hot center and cool outside can cause thermal gradients and cause cracking.) And the lens equations will tell you that if you have a large input beam diameter you need to use a longer focal length to avoid the spherical aberation that Rodne was talking about - problems with the edges of the lens. The edges won't "point" the light quite correctly and will focus at a somewhat different point than the intended focal length. The end result is that there isn't one real focal point to the lens. The bad aspects of this is that it has the effect of making the spot side seem larger and the focal point becomes less defined.

For a given lens OD, they can make a lot of different focal lengths by changing curvature. If you try using a smaller lens than the laser was designed for you will run the risk of hitting it off-axis as Rich and Rodne alluded to. And it will be harder to mount and keep perpendicular. Bottom line, probably best stay with your existing OD and find a suitable focal length lens for the job.

[Joe Hillmann]

Since your problem is with warpage I would say to work on that rather than investing money in lenses that will only make the problem less of a problem. Depending on how large the pieces are that you are cutting I would suggest building a vacuum table to hold the mdf flat. Although the smaller the pieces you are working with the stronger your vacuum needs to be so for small models you are making it may not work.

[Walt Langhans]

If you want the beam to come out of the lens perfectly perpendicular, you need to hit the center of the lens (on axis).

How do I make sure that's happening?

So if I get a 50mm lens, and mine is a 40mm with a focus distance of 7mm from the material to the end of the nozzle, then the new focus distance should be 17mm from the end of the nozzle to the material correct?


Since your problem is with warpage I would say to work on that rather than investing money in lenses that will only make the problem less of a problem. Depending on how large the pieces are that you are cutting I would suggest building a vacuum table to hold the mdf flat. Although the smaller the pieces you are working with the stronger your vacuum needs to be so for small models you are making it may not work.

Rodney had also suggest a vacuum table but I'm not sure it would work and I don't know much about them. The MDF is 12" by 24" by 1/8" so I'm not sure how strong the it would have to be, plus with MDF sitting directly on a table like that wont the underside end up getting scorched pretty bad?

[Joe Hillmann]

Walt,

From pictures you posted last year I thought you were making much much smaller parts than that. At the size you are working with even the weakest vacuum will pull out any bow in 1/8 inch mdf.

My vacuum table is just a box with 1x4 sides and a plywood bottom with pieces of 1x4 placed randomly inside to prevent the vacuum from pulling a large bow into the center of the work area. To prevent the underside from getting burned I put a florescent light defuser grid on top of the box. It still burns a bit where the laser passes directly over the grid but since the wood is now flat you can use less power so the burning on the back is even less.

Here is a link to the grid I use so you know what I am talking about.
http://www.menards.com/main/paint/drop-ceilings/ceiling-light-panels/plaskolite-egg-crate-white-louver-light-panel/p-1474053-c-7945.htm

Your vacuum need to be exhausted outside or through a filter because it will suck up a lot of the smoke from the wood being cut.

[Rodne Gold]

Walt , To ensure the beam is centered on the lens , you need to do a beam alignment.
Do not trust a mathematical calculation to determine the exact focal distance , you need to test it.

[Walt Langhans]

@ Joe, I'm pretty sure I'm understand what you are saying but could you post some pics of your set up when you get a chance.


To ensure the beam is centered on the lens , you need to do a beam alignment.
That was the first thing I did. I'm dead center on the mirror closest to the laser tube, and then no more than .5 - 1mm from the dots lining up with each other when I test it by marking the tape at x-0 / y-0 and then x-0 / y-12, and then the x-0/ y-0, x-24 / y-0. I was understanding that that was good enough or do I need to try to get it all dead on?


Do not trust a mathematical calculation to determine the exact focal distance , you need to test it.
And the best way to do that is?

Also since I'm moving father from the material do I need to adjust the angle of my air assist so it's pointing more down toward the material, (right now it's more at an angle that is more parallel to that material) ?

[Richard Rumancik]

Walt, to find the real focal length of a lens, most people use something like black anodized aluminum to do test plots as it is a high resolution material (i.e. you can see fine detail). It is an iterative process - you make your best guess and raster some text and/or vector some lines. Record the height setting (how you do this and where you measure from depends on your laser but you need a reference if the laser manufacturer has not defined one.) Sounds like you want to use the tip of the nozzle as a reference - I guess that works.

Then increase the focus distance from the first trial and observe the results. Is the graphic crisper or worse? Experiment until it is optimized. You may need to try a smaller distance. An optical loupe would be handy.

The process can be made faster by inclining the material on one end by say .25" or 6mm and plotting the graphic repeatedly up the incline, so you can see where it looks best. Then measure the approx. distance from your reference point to the best graphic. You can fine-tune from there. There are other threads on this subject; maybe you can hunt them down.

This distance that you measure won't necessarily be the focal length of the lens - since your reference point on the carriage probably won't coincide with the lens itself. I would make an accurate spacer or probe for setups and mark it as being the setup tool for a specific FL lens.

[Walt Langhans]

Thanks Richard, where can I get some black anodized aluminum from?

Here's how my air assist is currently set up.

254179254180

[Rich Harman]

You don't need anodized aluminum to do the tilted material focus test. I've tried both anodized aluminum and crappy plywood. They both do the job.

It looks like you air assist is blowing across the nozzle instead of being directed at the cutting point.

To make sure the beam is perfectly perpendicular it needs to enter the last mirror dead center, but first use a square and verify that the nozzle assembly is perpendicular to the table both left/right and fore/aft. Judging by the above photo it needs some adjustment.

Then, you need to align the beam so that it is centered in the nozzle. Once that is done burn a mark on a piece of material, lower the table a couple inches and burn again. If the marks are not centered on each other then further adjustment is necessary. It may be that beam is going perfectly perpendicular to the lens but still off axis due to the mirror being screwed in, or out too far. Or it could be that the beam needs to be steered towards the center. Or both.

[Richard Rumancik]

You can probably get black anodized from your engraving supplier, but if you don't have scraps around to use then it will end up being too expensive as you just discard it after. Maybe you could get some painted or powder coated aluminum flashing or something similar from a hardware store. A company that installs evestroughs might have some scraps of coated aluminum. Sure, other materials will also give you an idea of your focal length, but wood probably isn't the best option.

[Rich Harman]

Sure, other materials will also give you an idea of your focal length, but wood probably isn't the best option.

You only need to discern where along the line the engraving is at it's finest. The best option is to use what you have on hand that will do the job.

I suppose if you were burning the line at too slow of speed it could make it hard to read using wood. I use something like 600mm/sec at 70% power and it makes a nice, easily readable line.

I prefer to use a long piece, about a foot long or so, and prop up the end so that one side is a bout two inches below the nozzle and the other just clear of the nozzle. There will be certain distance along the engraved line where it all looks the same. That distance represents the depth of field, the longer the focal length of lens - the longer this distance. I then make pencil marks at the extents of this region and then take the middle as the focal distance.

[Dan Hintz]

Go to Home DEpot and pick up some gutter material... it's aluminum flashing with a polyester-based paint (black, white, and brown are common colors). Take a section (doesn't matter how long, but 10" is a good round number that makes the math easy) and put a 1" high block of wood under one end so the section rises from one end to the other. Draw a series of straight lines 0.5" apart, separated so each new line is drawn at a different height along the section.

Once engraved, one of those lines will be sharper than the others. That's the perfect focal height. Since you have a 10" run over a 1" rise with a line every 0.5", every line represents a 0.05" increase in focal height off of the table. Line #4 from the bottom would mean your focal point is now 0.2" above the current height of the table. Move your table up 0.2" and you are perfectly focused on the table. You can then move the table down, as necessary, to account for the thickness of the substrate you're going to engrave.