My review of buying a Triumph laser.

[Rodne Gold]

Most of it can be summed up with "does my machine, Chinese or Western do what I want it to do"

If yes, the physics rarely matter
If no, then it gets complicated.

cheers

Dave

+100000
These things are just tools ..either they do what you want or not.
The only difference really is affordability ... some folk can afford a $5k entry fee and live with compromises , if you cant afford $20k for a "better" machine any compromises are academic.

[Dan Hintz]

]
When I first saw that with my beam diameter and focal length the best spot size that I could get was 0.08mm, and I had an actual spot size of about 0.2mm, and I knew that I was only achieving 1/6 of the potential energy density - it seemed like I should upgrade the lens.

But then I read on, and this FAQ says that 0.2mm is an "excellent" spot size and 0.1mm is "amazingly good."

"[/COLOR]Therefore, if you get a CO₂ laser and an ordinary (affordable) ZnSe lens, you should be happy to get a spot size diameter anywhere from 250 to 500 micron = 0.25 to 0.5 mm diameter. From your spot diameter you can calculate the spot area and from there you can calculate your beam intensity at any power setting."

It says I should be happy, but I need to find out what I can expect for a spot size reduction with a ZeSe meniscus lens.

Don't get too worked up about the Parallax site... their overall description is correct, but they also give rules of thumb rather than hardcore numbers. Western lasers are typically at 5 mil spot size for 2" lenses, 3 mil for 1.5", and approaching 1 mil for the setups which include beam collimators (like ULS's HPDFO).



Dave, you'll have to verify me on this one as it's been a long time since I ran the numbers... didn't the meniscus lens have a bigger issue with aberration near the lens edge compared to a plano-convex (given the same basic production quality), which meant you needed a larger diameter meniscus to avoid the edges? Someone do a search here, I vaguely recall writing up a minor thesis on the subject a few years back.

[Dave Sheldrake]

Dave, you'll have to verify me on this one as it's been a long time since I ran the numbers... didn't the meniscus lens have a bigger issue with aberration near the lens edge compared to a plano-convex (given the same basic production quality), which meant you needed a larger diameter meniscus to avoid the edges? Someone do a search here, I vaguely recall writing up a minor thesis on the subject a few years back.

Spot on Dan Plano suffer from the Zernike aberrations more so than Meniscus but to avoid spherical abbs the lens needs to be bigger

cheers

Dave

[Robert Silvers]

welcome to the exponential function. Power density is a measurement of power / area in 2 axis. Laser cutting is a product of power/area/depth (3 axis)

cheers

Dave

Still the relationship between the two lenses would be the same with various depth materials. So for a 3mm material, the 2.25x difference in power density would still be 2.25x for a 9mm thick material.

Has anyone bought one of those $30-$50 Chinese Meniscus lenses that say they are made in the USA? For this wavelength, it would seem easier to make a perfect shaped lens than visible light since you have 10x looser tolerance for perfection.

[Dave Sheldrake]

So for a 3mm material, the 2.25x difference in power density would still be 2.25x for a 9mm thick material.

Power density is an area function, cutting power is a volume function.

it would seem easier to make a perfect shaped lens than visible light since you have 10x looser tolerance for perfection.

?? Visible light runs 400 or so nm to 700 or so nm,

lenses that say they are made in the USA

Made in the USA means nothing, it depends on the manufacturer. II-VI, Zeiss, RMI are all great "Bobs backroom lens shop LLC" doesn't confide anything.

cheers

Dave

[Robert Silvers]

Power density is an area function, cutting power is a volume function.



?? Visible light runs 400 or so nm to 700 or so nm,



Made in the USA means nothing, it depends on the manufacturer. II-VI, Zeiss, RMI are all great "Bobs backroom lens shop LLC" doesn't confide anything.

cheers

Dave

Going from a spot size X to a spot-size X/1.5 is the same as going from a 100 watt to a 225 watt laser. Do you disagree?

Ok, so going from an 10600 to 700 is a 15x difference, so an IR lens is 15x less critical for lens imperfections because the wavelength is 15x longer.

BTW, the took apart my lens unit. It is 20mm in diameter, and was in upside down. The convex shape was pointing down. I changed it to point up.

[Scott Shepherd]

Going from a spot size X to a spot-size X/1.5 is the same as going from a 100 watt to a 225 watt laser. Do you disagree?

I'm not a physics student, but from know what I know about lasers, I'd say that's a very false calculation there. If you put a 2.0" lens in and cut 1/2" thick acrylic with 100P and 2S, putting a 1.5" lens in, it's NOT going to cut at 100P and 4S, or 50P and 2S. It doesn't work that way at all with the laser I have been exposed to.

I've used a 2.0 lens and a 4.0 lens over the years and the 4.0 lens doesn't produce 50% of the power of the 2.0 by any means.

[Dave Sheldrake]

Do you disagree?

Yes, you are basing the calculation on power density in relation to better cutting ability,

You have a finite power in the incident beam, at the infinite level of the material surface you have X watts per SqMM, that is an area function, once the beam transgresses the material surface you are working in 3 dimensions not 2.Not only is the radiation being absorbed by the flat 2 dimensional surface it is also expanding due to the curve in the 3rd dimension as well hence being lost in progressively higher rates as the beam diverges.

o an IR lens is 15x less critical for lens imperfections because the wavelength is 15x longer.

Not quite sure how you got that idea?

doesn't work that way at all with the laser I have been exposed to.

Quite right Scott, Rob is getting confused between an area factor and a volume factor. If the beam at focus was a cylindrical shape into infinity then it may have some merit but it's a (hopefully) Gaussian curve with a finite limit on the amount of energy being focussed.

Rob, Lasers follow the inverse square law and the exponential function when cutting, unless of course you don't factor in the finite power levels being emitted. If they didn't you could build a star wars death star for 50 bucks out of a laser pointer with a G2 focus lens attached to it.

cheers

Dave

[Scott Shepherd]

Quite right Scott, Rob is getting confused between an area factor and a volume factor. If the beam at focus was a cylindrical shape into infinity then it may have some merit but it's a (hopefully) Gaussian curve with a finite limit on the amount of energy being focussed.

I just know it doesn't work

[Robert Silvers]

I removed the lens and cleaned it, and flipped it over so that the convex part now points up.

I have auto focus working, but I am perturbed that I need to make new height gauges. I think in the future I will be able to take this apart and put it back where I don't lose focus, but flipping the lens over may have made that impossible this one time.

While it was apart, I replaced the 3mm Phillips screws with 3mm socket head cap screws.

My beam is almost 0.002" smaller now.

I believe this upside down lens was increasing spherical aberration and making my 100 watt laser behave like a 65 watter. This may explain why my cutting speed was slower than the charts.

[Robert Silvers]

I want to take this step by step, because there may be some steps that you agree with.

Step A:

Reference http://www.iiviinfrared.com/resources/spherical_aberration.html

Spot size due to spherical aberration: = (k * (D * D * D)) / (F * F)

F = 50.8mm
D = 8mm
K = 0.0187 for Meniscus and 0.0286 for Plano-Convex.

So approximate possible spot size is 0.094mm for Meniscus and 0.144mm for Plano-convex.

Agree or disagree?

Step B:

If you divide these, then we can see that a Meniscus lens would be expected to reduce the spot size by a factor of 1.528.

Agree or disagree?

Step C:

http://www.buildlog.net/cnc_laser/laser_calcs.htm

Power density of 100 watt laser with perfect beam and 0.094mm spot size is ~14348 watts/mm^2.
Power density of 100 watt laser with perfect beam and 0.144mm spot size is ~6140 watts/mm^2.

Agree or disagree?

Step D:

If you have a 0.144mm spot size, and you want a power density of about 14348 watts/mm^2, you need a laser with about 234 watts.

Agree or disagree?

Step E:

Therefore, a 100 watt C02 laser with a Meniscus lens can perform the same as a 234 watt laser with a Plano-convex lens.

[Robert Silvers]

I've used a 2.0 lens and a 4.0 lens over the years and the 4.0 lens doesn't produce 50% of the power of the 2.0 by any means.

The comparison has to be done on lenses of the same focal length. The 4 inch has more depth of field, so the spot does not get larger as quickly throughout the depth of the material, so the effects cancel each other out.

[Scott Shepherd]

The comparison has to be done on lenses of the same focal length. The 4 inch has more depth of field, so the spot does not get larger as quickly throughout the depth of the material, so the effects cancel each other out.

Where do you suggest getting lens with the same focal length and different spot sizes from? On all the mainstream lasers, the spot size is relative to the focal length, so there's no way to separate the two.

[Robert Silvers]

Where do you suggest getting lens with the same focal length and different spot sizes from? On all the mainstream lasers, the spot size is relative to the focal length, so there's no way to separate the two.

By one lens having more spherical aberration than the other.

[Rodne Gold]

Surely if it was so easy to get "extra" 2.5x the power with merely a lens shape change , all laser mnfgrs would supply their lasers with such a lens.