Beam expanders/collimators

[Steve Clarkson]

The Feb issue of Engravers Journal says that if you are doing photos on marble you should be sure to get a laser with a Beam expanders/collimator.......anyone know if I have that?

[Margaret Turco]

Hi Steve, I found this online so it looks like you are covered!
http://www.epilogfiles.com/brochures/radiance.pdf

[Steve Clarkson]

Well......I guess I can't use THAT excuse anymore.........

[Mike Mackenzie]

I am not so sure I agree with this assumption, Beam expanders / Collimator's are used to shape the beam and keep it more consistent over larger distances.

There are thousands of laser systems that are doing photographs without the expanders/collimator's. I don't see how this could effect the quality of a photo unless you are doing 24" x 36" photos and then all they would do is to keep the beam consistent across the entire field which would control any fading possibilities from one edge to another.

I would like to read this article if anyone can post a link to it I would appreciate it.

[Rodne Gold]

Expanders and collimators are useless unless your alignment is spot on across the table , and alignment is the reason that power varies across the table rather than beam divergence over the small distances involved.
I havent read the article , but have fitted collimators to 3 of my machines with NO improvement of power variance , but have had an improvement of beam quality to a small extent.
Using a short focal length lens is about 10x more effective in terms of spot size and power density then using a collimator and or expander for photos , the smaller the spot size , the more effective resolution you can get as you can increase the half tone "matrix" to represent more shades of grey.
This is wrong in concept - but to illustrate the point , if you have a 0.003" spot size , you can print 300 discrete dots per inch , if you have a 0.006 , you can only print 150 discrete dots.
Correspondingly , if you have a misaligned 0.003" spot size , with poor beam quality , it becomes ovaloid and this your 300 dots start overlapping in one direction or the other , effectively lowering the resolution.
Remember spacing between dots defines the illusion of shades of grey , black would be no spacing in terms of lets say a 3x3 matrix of dots , all the dots would be filled in , the lightest shade of grey would be 1 dot filled in in that same matrix, if you can get a smaller spot size , you can increase that matrix to lets say a 4x4 matrix in the same space you would fit a 3x3 matrix and thus reperesent more shades of grey.

[John W. Love]

Well, when I read the title of this thread a shiver went down my spine as the term "collimator" reminded me of the last "full checkup" at the doctor's office. However, after reading what Dr. Rodne, our resident Physics Professor, had to say, I can now say that I not only learned a little more, but also reinforced what I had already known and I am no longer afraid of the word Collimator.

[Jon Colley]

I would agree with most of the points above. The use of a collimator is not going to improve the variation of real power over the engraving area but it can affect the spot size of your laser.

The collimator typically used to enlarge the diameter of the laser beam and at the same time reduce the beam divergence. The larger the laser beam is when it hits the lens, the smaller the spot the lens can focus down to. This small spot size gives engraving results that look like there is more power but its actually the same amount of power concentrated on a smaller spot creating higher power density.

The reduction in beam divergence is also important because the spot size can change slightly over the size of the table due to the beam diameter being larger as the head moves further away from the laser source.

Despite all of this technical info, it is worth noting that the lower power lasers on the market typically have minimal beam divergence so the small difference in beam diameter over the size of a standard table really doesn't show up in the engraving quality. You are more likely to see issues caused by the motion system or power fluctuation on some systems than you are due to beam divergence. On systems with more power or larger tables, the beam divergence becomes more of an issue, so a collimator should be used. We use them on all systems with 60W or higher and on our large Speedy 500 and Professional series machines.

I hope this helps!

[Mike Mackenzie]

The shape of the beam also has a lot to do with the MFG of the laser tubes. Most of the MFG's beam shapes are oval which causes the difference in X and Y quality. That is the number one reason that system mfg's use collimator's on there systems. The beam spot size is still controlled through the Len's although through the collimator if it is properly adjusted will improve the spot density. If the collimator is not properly adjusted it does more harm than good.

[Dan Hintz]

Mike,

If the beam is elliptical, wouldn't that require asymmetric lenses to rectify the issue? If some manufacturers had such a poor less-than-Gaussian profile to their beam, then the collimator lenses would need to be mounted in a specific orientation (axially). All collimators I've ever seen use off-the-shelf symmetric lenses... if the beam is elliptical, those standard lenses will do nothing to change the elliptical shape of the final spot.

[Jon Colley]

Hi Dan,

You are correct. There is a special beam shaping optic that takes the elliptical beam and makes it round. It looks like a lens but it is shaped differently. A collimator does not shape the beam.

All of our systems that have collimators use Synrad tubes that have round output beams. The collimator is used for expanding the beam diameter and reducing beam divergence only.

[Mike Mackenzie]

The beams are elliptical on most laser tubes Look at the spec sheet that is on synrad's site http://www.synrad.com/48series/48_25.htm. So mfg's are putting in shaping optics if they are stating that they are getting a round beam.

I meant to put this into this sentence "That is the number one reason that system mfg's use collimator's / shaping optics on there systems.

Synrad's are pretty good but others are really bad. Remember when creating a laser beam you are bouncing the beam between optics so it will naturally be an elliptical shape.