Doubt about lens

Community Laser Talk
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I have seen several topics here about lenses. I know they are co2 machines, and eventually galvos and fibers.
I only have a laser diode and I have never needed or seen the lens size changed because it is a fixed distance (20mm) and because it is protected by glass (I have already changed this).

As I don’t want to interrupt a topic to clarify my ignorance, I prefer to do it here where it won’t bother anyone (I hope) :innocent:

The doubt:
The size of the lenses, for example “200mm lenses or 100mm lenses”, what does the value refer to? Diameter, I don’t believe it.
Focal length, will it be? That’s what makes the most sense to me but since I’m not sure…

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On C02 lasers the lens length, i.e. 1.5", 2", 4", etc. refers to the focal distance. The different focal distances also have different depth of focus, or depth of cut that is considered in focus. This page does a good job describing it.

I don’t have a Galvo, but if I understand correctly the lens size refers to the working area. A 200 mm lens has a 200x200 usable size. I also know the longer the lens, the higher the focal distance, but I don’t know if the lens size equates to actual distance or just the working area. Someone who knows Galvo’s can correct my statements.

2 Likes
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This make sense too!!!
Thanks!. :wink: :+1:

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All lenses are like this, in general have this type of layout. If I speak about a short or long lens, I’m speaking about it’s focal length. With a gantry coverage usually isn’t focal length related.

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In simple terms, the longer the lens (focal length) the larger the spot and larger depth of focus (dof).

In general you pick a lens for a particular purpose, if you want detail you go shorter, thicker material, probably longer.

Most led type diodes don’t have any kind of a standard in how the lens mounts. The manufacturer picks the what they think is the best lens length for a customers needs.

Galvo type heads are generally mounted to some fixed part of the machine. They are developing what’s called flying galvo machines that incorporate a galvo mounted to a gantry machine.

Lenses for fiber machine, IMHO, be specified in focal length. Lens types for galvo machines are designate F for flat focus. Focal length effects coverage, but they are not the same. Note an F210mm lens could cover 150x150mm or 160x160mm depending on the scan angle. I think this graphic was from Cloudray.

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Drawback with gantry is they are size limited. As the focal length increases, the spot size increases.

Reducing the spot size by 1/2 increases the observed power by 4 times.

Russ Sadler has some very good Youtube videos on lenses.. I like they way he presents the ideas …

:smiley_cat:

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The Depth of Focus (DoF) increases proportionally with lens focal length.

BUT, there are two other critical parts.

The Focal Spot Dia (FSD) also increases proportionally. It doesn’t actually focus to a “point”, but a circle of finite diameter.

Double the lens’s focal length, double the FSD. But the area of a circle is the PI*r^2/. Double the dia, the area is quadrupled. The energy density drops by a factor of FOUR.

The last part is nuanced. A CO2 beam cutting thick material actually does NOT follow the profile of the point-up cone the beam expands on below the focal point. It cuts a straight walled channel. This is because a lot of materials are reflective to CO2’s 10.6um beam at low angles. The way cutting works, the beam naturally makes a “funnel” on top that tends to direct the light at low angles and direct it into the cut, “beam capture” and it “channels”. Once it’s in the channel, it will continue bouncing off opposing walls at low angles with surprisingly little loss until it reaches the bottom. It will NOT have a low angle when it contacts there, it delivers its energy and this ablates material on the bottom, deepening the cut and making more straight-wall channel.

On CO2, the 2" lens is best for this on most materials. Assuming the beam is 6mm dia (it varies, this is a number for some 130W glass tubes) at the lens, the cone of energy has a half-angle of 3.4 deg from vertical so it is easy to direct the energy into a channel.

A 1" engraving lens has a half angle of 6.8 deg, and it will NOT channel because that initial angle is too high.

You’d think a 4" lens would channel even better, but reducing the initial energy density by a factor of 4x tends to make it much worse. The only case I know of is 2" of microcellular PE foam. 4" lens will cut 1.5" depth with very clean vertical, parallel sides. The std 2" will cut about 1" before the channel craps out. I can get to 1.5" but it gets really slow and the channel isn’t very parallel anymore.

Beams don’t always channel. Like I say, a 1" engraving lens won’t. The capacity to channel does depend on the material and laser wavelength to. CO2’s 10.6um is great for this on many materials. Diode will be different. I do have small Atomstack laser diode machine too. I don’t know how it behaves with different focal lengths, I have never tried to change it

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Can you actually get different focal length lenses for your Atomstack?

:smiley_cat:

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I don’t know, never looked. The market certainly has lenses of different focal lengths for that diode wavelength, in different dia. Whether one matches the mount, and if there’s any point to it, I don’t know

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I wanted to ask a question to clear up my curiosity and I get a free lesson!!! :laughing:
Thanks for all the information! :ok_hand:

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When I decide to buy a CO2 machine I have to come back here to remember this lesson!!! :laughing: :wink:
Thank you for all the clarification!!!