What would cause this Lense issue

My focus lense has a frosted look on the bottom flat surface.
What would cause this. I didn’t notice this when I thought it might be the power supply issues. This may have been my problem all along.

Thanks Rudy

Either you haven’t been cleaning it, or you have been cutting something that has been putting up some flashback. If it comes clean, you’re good, if it’s pitted, time for a new lens.

That looks pretty bad. After seeing yours mine wasn’t bad at all.
I had to work very carefully with Isopropyl alcohol to get the gunk off mine.
I would bet your mirrors are dirty also.

New mirrors tried cleaning the lense no luck I was engraving a lot of tumblers the other day before it just stopped and it wouldn’t even cut wood.
Thats my other post with pictures.
That may be my flash back if that is the cause.


Yeah, if you were working the metal tumblers a bit too strong, or hit a thin spot in the cover material you could easily have gotten some flash back.

Here’s a tip. Next time, set the tumbler just a a few mm off center from the laser beam. You won’t see any real difference, but any flash back will not go straight up into the lens.

Thank you so much for the information.
Location florida

It’s one of those little shared tips. If you are taking the surface off a reflective material, give it a slight angle so the direct reflection isn’t straight back up into the lens. A real critical eye that knows what the original looked like might tell the difference, But other than a few lines being a hair wider than they should you would never be able to tell the difference.

Given how small a sweet spot for a CO2 laser, double the distance from the lens to object and back means there’s going to be very little power left in the beam.

Can you explain how this low-power, unfocused beam is worse for the lens than the full=power beam going through it from the other side?

Are you running air assist? And if so, where does your air come from? That looks like water droplets from an unfiltered compressor, to me.

The air enters the nozzle and comes out the tip.
I do run air assist the nozzle air is filtered once at the compressor and once after the exit of the air valve. I bypassed the valve and let the air blow on a glass plate and found no water so the filters are doing there job plus i drained the compressor.

Thank You.

Yes, you have the sweet spot of the focal point for ALL of the beam, but due to the Gaussian curve, you still have a lot of power going straight down through the center portion of the lens. I have scorched scrap pieces 8" below my focal point with a 2.5" lens on many occasions. If you are engraving with an even shorter focal point lens, that puts that high powered center even closer to the lens in flash back.

Take that fairly large amount of energy doing what ever damage it can, and reflect it back to the lens even occasionally over how many thousands of passes during engraving, and flash back damage is a very real possibility. With the fairly sharp curve of a tumbler, shifting a few mm off center is a very easy thing to do to protect your lens.

set the laser slightly off centre on rotary engraving … of course. that’s a really simple and effective solution. thank you for sharing it.

it’s so obvious now you’ve said it !

How do you think I guessed correctly that you were using a compressor and not an air pump? Did you think it was a guess?

You are heading down a rabbit hole of bad information.

Just take a moment to understand what happens when the beam exits your lens, travels to the object, and is reflected back.

Add to that the diffusion of the formerly-coherent beam and the obvious picture of a series of splatters of water vapour on a hot lens and that’s what you have.

But you carry on with the ‘flash’ theory. It’ll keep you busy for hours.

I have tried it before, almost the same image of a lens. In my case, it was definitely water in the compressed air, even though I have a filter and water separator installed.

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And in my experience of servicing customers machines, too.

It looks like water vapour/splatter.

Thats what I thought I put a new lens in and still I have very low power will not even cut cardboard. The splatter will not wipe off.

Rudy, it does not sound so good, but should not be related to the original “water / reflection” problem. You probably have to start by troubleshooting, hopefully it’s just a bad focus of a mirror or pollution. Good luck

Waiting on a new power supply before I start a warranty claim on my tube.

Did you use air, at a low psi, like 5 - 10 you can keep that from happening. Plus get in the habit to clean your lens after a job or a couple hours

I somehow used the wrong kind of cleaner or had a contaminant on the cloth I used. It destroyed my lens. It looked a bit different than that but it was ruined all the same.