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Topic: Job Shop forum / Focal length

Page: 1

Posted: 30-01-2007 11:01 by Lexpert
Yes - i are a boffin - and i hav the knolidge;

You can set up a lens holder mount attached to a vertical vernier or height gauge ... then find the focal point for light from a strip light or any sort of lamp - measure the distance from the focal point to somewhere on your lens holder mount.

This 'focal length' will not be the same as for the laser light - at this visible wavelength you will get a shorter focal length - and anyway you have chosen an arbitrary measurement point on your lens holder mount- however you are not interested in measuring the actual focal length - you are interested in VARIATIONS in focal length between lenses -

Do the above experiment for a lens you know is correct and use that as your datum measurement (ie the 'focal length measurement' for a 190mm focal length lens might be 179 mm from the focal spot to your arbitrary point on the lens holder mount.) if the next lens you try has a focal point which is a couple of mm away from the datum measurement you have identified the problem. If you try 5 lenses you will quickly be able to identify which ones pass the variability test and which ones are too far out. If you were feeling particularly brainy you could calculate the exact focal length of the lens at 10.6 microns from your 'visible light' measurement but there really isnt any need.
John Powell
Posted: 30-01-2007 20:40 by Cirrus
We have purchased Trumpf focussing lenses for the past 5 years since on our small usage they are cheaper than buying direct from II-VI. Not only do we use them on our Trumpf machine but also on our older non-Trumpf machines. The sales pitch from Trumpf is that II-VI check the focal length and only ship to Trumpf those lenses that are within a small error of focal length. We have recently installed a new 3030 and have had issues with focus position due to (unusual) variations in the Trumpf / II-VI lenses focal length.
Does anyone know of a simple piece of equipment that can check focal length, would we need to check at 10.6 microns wavelength?
A quick check on the web shows that an auto-collimator method through a pinhole and optical bench might work but a bit complicated for a job shop or a Gevena mechanical gauge might be sufficiently accurate and easy to use.
Any boffins out there with knowledge?
Posted: 19-03-2007 10:38 by
There is a newly developed tool that may be able to measure the curvature of either side of a lens and there by determine the focal length.
I will be able to follow up on this, what would job shop owners pay for such a device?

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