Laser lens 'dull spot'

[Bob Crimmins]

I've been paying more attention to the lens of my 80w Co2 since I changed in a new lens. Very soon into the new lens, I noticed that about a 2.5mm dull spot developed in the center of the lens. I assume this is where the laser contacts the lens. This raises a few questions.

1) Is this normal or is this an indication of some contamination on the lens that caused that bit to over heat and distort?

2) If it's likely contamination that caused it, is it more likely to be smoke/fumes from material or moisture from the air?

3) How much power is likely lost with this condition?

4) Is this a "replace the lens immediately" situation or is it just a "yeah, you're gonna a lose a little power until you replace it" situation?

5) What's the cheapest source for quality replacement lenses?

Thanks for all the terrific contributors on this forum. I'm on the lookout for opportunities to add value but y'all are still so far ahead of me. Someday though....

Bob

[Bill George]

What are you cutting or engraving? My machine is 9 months old and on the same Lens that came with it, been cleaned once.

[Dan Hintz]

I've been paying more attention to the lens of my 80w Co2 since I changed in a new lens. Very soon into the new lens, I noticed that about a 2.5mm dull spot developed in the center of the lens. I assume this is where the laser contacts the lens. This raises a few questions.

1) Is this normal or is this an indication of some contamination on the lens that caused that bit to over heat and distort?

2) If it's likely contamination that caused it, is it more likely to be smoke/fumes from material or moisture from the air?

3) How much power is likely lost with this condition?

4) Is this a "replace the lens immediately" situation or is it just a "yeah, you're gonna a lose a little power until you replace it" situation?

5) What's the cheapest source for quality replacement lenses?

Thanks for all the terrific contributors on this forum. I'm on the lookout for opportunities to add value but y'all are still so far ahead of me. Someday though....

Bob

1) Contamination. If you clean the lens and you can still see a spot, either you didn't clean it well enough yet or the lens/coating is getting hosed.

2) Either. Smoke/fumes are a constant hazard, but all it takes is a drop of water spatter from a dirty air line to start the avalanche.

3) Unknown. Could be 1/2W right up to enough power to eventually break the lens.

4) It's a "Buy another lens because this one is going to break on you at the worst possible moment" situation. But if you do not notice a lack of power or image crispness, I'd keep using it until it goes south.

5) You'll have to define cheap... quality, price, or both.

[Dave Sheldrake]

Bob,

Remember to take the lens out when you clean it :) the topside often gets forgotten and is where most of the detritus gathers leading to over heated lens's / cracked lens's. Dan covered everything else :)

[Bob Crimmins]

Thanks guys! As always, good stuff. Clearly I need a small stash of lenses.

RE quality vs. cost, I'm a balance kind of guy. Since this is not yet my business I don't feel like the best available quality would be warranted. That said, I realize that there are almost always performance trade offs with cheap stuff so I don't necessarily want the cheapest available lenses. Thoughts?

RE detritus gathering on the top side of the lens, could that be greatly reduced be placing some kind of barrier at the entrance to the laser head assembly? Of course, you'd need to have a clear patch for the laser, but that's only a mm or two worth of clearance. I could imagine just putting a piece of wide double sided foam tape across the aperture and then fire the laser to create the clearance hole. Is there a better way?

Thanks!

Bob

[Dave Sheldrake]

Yup, split off a line from the air assist and pipe it across the front of the hole :)

You can buy ZnSe windows for the same purpose but air works just as well (and is free)

[Bob Crimmins]

Ah... I hadn't heard of those ZnSe windows. Dang... pricey. I like the air idea. I'll do that.

I looked on eBay for lenses. Looks like they range from about $25 to about $40. Is there a less expensive source? Can you buy them directly from China?

[Dave Sheldrake]

Ah... I hadn't heard of those ZnSe windows. Dang... pricey. I like the air idea. I'll do that.

I looked on eBay for lenses. Looks like they range from about $25 to about $40. Is there a less expensive source? Can you buy them directly from China?

Err $40 is a cheap lens if you want something that will last more than a couple of weeks Bob :) Mid quality from II-VI run about $150 - $200, RMI can go to $450 easily

An ultra cheap lens with an 80 watt tube isn't a good combination :(

[Bob Crimmins]

Good to know. What are the indicators of quality in a ZnSe lens? How can I know whether I'm buying something that will perform well? I see lots of charts like this:




Specifications

Standards


Focal Length Tolerance

±1%



Diameter Tolerance

+0 / -0.1mm



Centre Thickness Tolerance

±0.2mm



Clear Aperture

>= 90% Diameter



Edge Thickness Tolerance

±0.15mm



Scratch/Dig

60/40



Surface Accuracy

lamda/2 @632.8nm



Centration

< 3’



Bevelling

< 0.2 x 45°



Coating

AR Both Sides/ custom design

[Rich Harman]

I'm still using the original cheap lenses that I bought with the laser. Since I moved the air assist to outside the nozzle I rarely need to clean the lens - like maybe once a month. One time a bit of debris made it's way to the top side of the lens and caused a spot but I was able to clean it off.

[Dave Sheldrake]

Only two that actually mean anything of worth are Centration and Surface Accuracy, both of which if it's a Chinese lens are probably made up.

Think of the lens as the final part of a long optical train, you could have great mirrors and optics inside the tube, good quality / flatness on the directional mirrors all being used to focus a beam through a lens that should be on a kids toy camera.

The effective differences between a cheap lens and a decent quality lens become apparent when you swap them out to compare them. II-VI in the US do some very good mid range lens's at the $100 mark, not the best in the business but still very good. Their high grade stuff is amazing and provides beam modes of superior quality but expect to pay $150 for a good one with a useful life of some 3 years if taken care of.