# Optical Forums > Optical and Ophthalmic Equipment >  How do optical, non mechanical, tracers work with cyl?

## mmarin

Hi,
I can't get my head around how do optical tracers (like Essilor Delta 2, Briot Perception...) work, if you have to edge a lens with cylinders? 
You put the demo lense in the tracer, scan it, but then how does it know what is the right angle of the demo lense that it just scanned? Because you need that so that you can adjust the prescription lens accordingly. 
Regards, Marin

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