View Full Version : Which way is your Focal Lens installed

Larry Robinson
08-02-2010, 9:40 AM
Curious as to how everyone has their Focal Lenses installed. I have a drawing below showing two different diagrams. The Mfg of my Laser recommends Diagram 2 convex side down. However, I have notice drawings of other Mfg, showing Diagram 1 convex side up.

I have installed the Lens both ways and notice that the Laser cuts better with the Focal Lens installed as shown in Diagram 1. I did have to change the Focal length down to get the better cut.

Randy Digby
08-02-2010, 9:44 AM
My salesman showed me a little trick to getting mine in correctly. I hold the lens beside the holder and look at it (head about two feet away). If I can see my reflection, it is correct. If not, I have the wrong side up. Don't know if this works for all or not.

Scott Challoner
08-02-2010, 9:59 AM
My salesman showed me a little trick to getting mine in correctly. I hold the lens beside the holder and look at it (head about two feet away). If I can see my reflection, it is correct. If not, I have the wrong side up. Don't know if this works for all or not.
My Laserpro rep told me the same thing, so it must work for all.

Albert Nix
08-02-2010, 10:33 AM
On my MII the lense will not go in the holder up side down???

Dan Hintz
08-02-2010, 10:36 AM

That should work with the typical lens seen with most of your systems (but not all, as some systems do not use the typical Plano-Convex).


Your diagrams show a positive meniscus lens... if that's what you have, Diagram 1 is the correct orientation (the less curved side towards the substrate). If it's a Plano-Convex, flat side towards substrate.

Larry Robinson
08-02-2010, 12:40 PM
Scott, Randy,

If I have to look at myself everytime I change a lens, then I won't be changing lenses very often.:)


Wow, guess I have a long way to go in understanding Optics. "Plano-Convex", had to google that one. :D

I have looked at the lenses again and they seem to be more flat than concave on the one side versus my above drawing. I have dedone the drawing. Perhaps this may help.

So it appears that drawing one is the better way to go. I think.:confused:

Frank Corker
08-02-2010, 5:16 PM
That's what I was told too - flat side down to work piece. Curved up.

James Terry
08-02-2010, 9:05 PM
My college physics expert says that style of lens will focus either direction and that the focal point will simply be at a different location. As such, this might be an interesting way to get a longer focal point for those of us who have rotery jigs where the head can hit.

Perhaps some of you who have reversed the lens before might do some comparative test marking between the two to see what you get as far as focal length.

Dan Hintz
08-02-2010, 10:14 PM
My college physics expert says that style of lens will focus either direction and that the focal point will simply be at a different location. As such, this might be an interesting way to get a longer focal point for those of us who have rotery jigs where the head can hit.
Yes, but the plano-convex suffers from greater spherical aberration when the spherical side is toward the substrate... in other words, you don't get as small of a focal point, which means a loss of power and bigger burn hole.

Jordan Billingsley
02-16-2014, 3:08 PM
I just purchased a laser cutter from China and recently had to replace the lens. I have been digging through this forum and others but have yet to find a definitive answer to focusing lens type and orientation.

My machine uses 150W Reci Laser. According to the manufacturer the machine was equipped with 19.6mm diameter lens with a 101mm focal length. Wavelength 864 Beam Diameter .2mm
The manufacturer also gave me the common tip to look at the lens and if all of my face is visible then "it should down" and if only part of my face is visible "it should up"

However, I bought my replacement lens from Light Object. High Quality 20mm ZnSe F100mm and it is not as obvious which way the lens should be oriented. The replacement lens from Light Object definitely has more of an exaggerated curvature (the original lens seems almost flat in comparison).

So maybe.... the original lens is Plano Convex and the replacement is Positive Meniscus? Or are there other types of lenses commonly used in CO2 laser cutters?

I actually tested the replacement lens using both orientations (I hope I didnt damage the lens this way!) but it was difficult for me to tell which orientation produced a smaller diameter.

I have been reading about "ramp tests" and think that it is possible that the focusing block used with the original lens may not be ideal for the new lens.

Ok I rambled on there for a bit. But basically, is there a way to correctly identify my lens type and proper orientation for my machine?

Also, I am gathering some resources on how to "fine tune" my machine so if you have any tips or tricks I would GREATLY appreciate them!


Dan Hintz
02-16-2014, 4:47 PM
Jordan, PM answered. For everyone else's benefit...

If the lens has a flat side (check with the sharp edge of some black cardstock or paper to prevent scratching), it's a plano-convex lens and the flat side goes towards the table. If both sides are curved (pointing in the same direction), it's a meniscus lens and the inward-facing/concave side should go towards the table.

Dee Gallo
02-16-2014, 6:59 PM
Thanks Dan!

Dave Sheldrake
02-16-2014, 7:48 PM
I posted this on my own forums about a year ago to try and clear up confusion that seemed to be doing the rounds

After quite a lot of misconceptions floating around the web I decided it was time to get the final and conclusive answer on final lens orientation.
My position has always been that the final lens in a CO2 laser optical train should be Convex side towards the beam (or upwards in our case) although of late on a few occasions it has been suggested to be otherwise.

With that in mind I contacted a few people I have met in the optical trade as well as some posted experts from well known optical labs.

Initially I asked Dr Karl Bergman (Of Zeiss) and he responded that the convex side of the lens must ALWAYS face to input beam to reduce spherical abberations.

Armed with this statement I then approached Professor Simon Widlake of Rutherford Appleton Labs who I have known for some 20 years and asked the same. Simon being the kind of *nutty professor* type spent nearly 2 hours turning my brain to jelly with a whole tirade about lens systems and the means of controlling abberation and scatter. Eventually I did manage to get a direct answer and that was that again the Convex side MUST face the incoming beam.

Looking around I found that the supplier I often use for lens's in the US (worldwide actually) is II-VI infrared so I dropped them a mail and got a very nice reply from one of their engineers Mr Andrew Waterhouse who again stated that the Convex side of both meniscus AND Plano Convex lens should always face the incoming beam. Mr Waterhouse went to far as to provide some simple drawings to illustrate the placement to avoid and eliminate any confusion. (see below)

Finally I contacted Knight Optical here in the UK and again was provided with drawings illustrating the orientation of CO2 Laser Lens systems (see below)

For reference you may find the following interesting:


II-VI Incorporated, a global leader in engineered materials and optoelectronic components, is a vertically-integrated manufacturing company that creates and markets products for diversified markets including industrial manufacturing, military and aerospace, high-power electronics and telecommunications, and thermoelectronics applications. Headquartered in Saxonburg, Pennsylvania, with manufacturing, sales, and distribution facilities worldwide, the Company produces numerous crystalline compounds including zinc selenide for infrared laser optics, silicon carbide for high-power electronic and microwave applications, and bismuth telluride for thermoelectric coolers.

In the Company's infrared optics business, II-VI Infrared manufactures optical and opto-electronic components for industrial laser and thermal imaging systems and HIGHYAG Lasertechnologie GmbH (HIGHYAG) manufactures fiber-delivered beam delivery systems and processing tools for industrial lasers. In the Company's near-infrared optics business, VLOC manufactures near-infrared and visible light products for industrial, scientific, military and medical instruments and laser gain materials and products for solid-state YAG and YLF lasers. Photop Technologies, Inc. (Photop) manufactures crystal materials, optics, microchip lasers and opto-electronic modules for use in optical communication networks and other diverse consumer and commercial applications. In the Company's military & materials business, Exotic Electro-Optics (EEO) manufactures infrared products for military applications and Pacific Rare Specialty Metals & Chemicals (PRM) produces and refines selenium and tellurium materials. In the Company's Compound Semiconductor Group, the Wide Bandgap Materials (WBG) group manufactures and markets single crystal silicon carbide substrates for use in the solid-state lighting, wireless infrastructure, RF electronics and power switching industries; Marlow Industries, Inc. (Marlow) designs and manufactures thermoelectric cooling and power generation solutions for use in defense, space, photonics, telecommunications, medical, consumer and industrial markets; and the Worldwide Materials Group (WMG) provides expertise in materials development, process development and manufacturing scale up.

II-VI is also well known as a supplier to both Lockheed Skunkworks as well as various departments at both N.A.S.A and the DOD.

Based on that I think it can be assumed that with the 4 biggest producers of laser lens systems in the world all concurring that it is wise to fit your final lens with the convex side upwards / towards the input beam.

Best wishes

ATTACHMENTShttp://oplaser.co.uk/opsupport/download/file.php?id=126&t=1 (http://oplaser.co.uk/opsupport/download/file.php?id=126&mode=view)http://oplaser.co.uk/opsupport/download/file.php?id=125&t=1 (http://oplaser.co.uk/opsupport/download/file.php?id=125&mode=view)http://oplaser.co.uk/opsupport/download/file.php?id=124

Matt Turner (physics)
02-17-2014, 12:22 PM
Sounds like it's been answered, but as the resident physicist I thought I'd just add one more voice to confirm the convex side up. Here's a simple explanation why: Light is only bent (deflected, focused) by the surfaces of the lens, and if it hits a surface straight on (perpendicularly) it isn't bent at all. If the flat side is up the collimated beam isn't bent or focused at all by the flat surface, and all the focusing is done by the convex surface. If the beam enters the lens through the convex surface, it is partially focused by that surface, and then the flat surface can do the rest of the focusing.

Mark Ross
02-17-2014, 7:53 PM
On epilogs, you can only install all lenses 1 way.

Kev Williams
02-18-2014, 1:52 AM
I've experimented with the lens in my Triumph both ways. While it does work better overall (like it should) rightside-up, I've found that I can cut 1/16" and 1/8" thick Rowmark using less power with the lens upside down, and the cut edge is cleaner than I've ever gotten from my LS900. Why, no clue...?

Chris J Anderson
02-18-2014, 6:19 AM
OK, To add to the question, on an Epilog Helix, does the o'ring go above or below the lens ?

This will affect the focal length.

(Mine is below)


Dan Hintz
02-18-2014, 7:09 AM
If the O-ring holds the lens in, then whatever seems most appropriate to do that job...

Amos De Pasquale
02-18-2014, 8:26 AM
Oh!! Wow!! I have always thought it was the other way around? out to the laser I will go and have a fiddle!! I will let you know(eventually) which way I find is best, the proof of the pudding is in the eating! Amos:) Thanks for all the input to this thread,

Dave Sheldrake
02-18-2014, 9:58 AM
This will affect the focal length.

The focal length is defined by the curve on the lens, no matter where the lens is fitted the focal length will be the same BUT may move in relation to the material surface (depending how far away from the material the lens is at any given time)

When switching lens's round to the right way up on a Chinese machine please be aware that China tends to machine a shoulder for the flat face on the lens to fit in, if you swap the lens round you will be tightening it up onto a sharp corner and risk cracking it. Fitting the lens convex down makes the beam more scattered, this can give the impression it is cutting better when in reality it is just a fatter spot with a deeper (effective) depth of field due to the scattering. Power density will drop as well.



Sergio Sarmiento
06-20-2017, 12:38 AM
I have an epilog mini 24 35W. The original lens are plano-convex 19mm FL 2". And now I have to replace the lens. Can anyone guide me if a meniscus lens produces the same, better worse results? Thank you.

Dave Sheldrake
06-20-2017, 9:18 PM
better results usually