Non-contact length measurement

Anyone out there have experience with non-contact length measurement? If so, please share your successes and lessons learned. I am looking for a way to measure the length of material that is paid off from rolls onto an extruded substance. I cannot use an encoder with a measuring wheel and the variety of materials and their thickness variances will not allow calculation based on roll revolutions or roll diameter. Ideally, our Mfg Engrs want the data to go directly to a database, but it would be best to buffer the data through a PLC to know when the machine is running and allow direct association with the products we run. We use A-B Compact/ControlLogix so an E/IP interface would be the best form of comms for us. Any experience (good or bad) you have would be greatly appreciated. I have searched the web and found a few options, now I need to know what works and what does not before I talk to someone in sales.
Thanks for your time!

what measurements?
Length, width, thickness?

what is the min and max distance you are wanting to measure?

james

I've used the Beta Laser Mike device for length measurement on clear plastic film at speeds up to 1200m/min. Interfaced to a S7-300 plc over Ethernet using UDP

Two things I recall dealing with laser doppler velocimiters:
1) Very expensive, although the prices may have come down in the last 15 years since I last worked with them
2) You need to control the distance from the laser sensor to the product. the unit I dealt with had a decent tolerance, but there was a distinct limit

I worked with a unit made by Canon, and I had a colleague who used hardware from Laser Beta Mike. I had no complaints about the accuracy or reliability.

I haven't done a non contact web length before. Would you do length correction based on tension, Young's modulus of the material and cross sectional area?

Proton InteliSENS is another option for a laser doppler sensor, I had a similar experience as OkiePC, expensive but works well.

As the other guys say, these are not cheap....

some of our other factories use this :

Limab UK
GA5103-0112
LaserSpeed Pro LS8500-403

And we are in the process of looking at getting one to measure the length of board as it goes along the outfeed conveyor from our press.