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Crystran’s new ‘Low Coherence Interferometer’

01st Jan 2020

Read our January 2020 newsletter. 


Surface Flatness Measurement - Made Easy! 
Quick and accurate (Lambda/20) analysis of surface flatness can now be made on parallel transmissive parts without painting the rear face!!

Most optical manufacturing companies use a Fizeau interferometer to measure surface flatness. This is perfectly fine for materials that don’t transmit in the visible, or have a wedge angle larger than roughly 1 arc minute. But the long coherence length of their HeNe laser proves problematic when looking at more parallel parts (etalons etc) as the interferogram is mixed with a fringe pattern formed from the part’s rear face. For materials with a refractive index of around 1.5, painting the rear surface can suppress these ‘static fringes’, but this is both messy and time-consuming. However, for higher index IR materials, such as ZnSe / ZnS / KRS5, this doesn’t work – the index match between the part and paint just isn’t close enough to adequately reduce the Fresnel reflection.

Based around a Twyman-Green design, the OptoFlat is able to use an LED as its reference source. With an emission linewidth of roughly 0.5 nm, its coherence length is long enough to easily see fringes across the complete surface, yet small enough to ensure there’s no phase relationship (and therefore interference) with the back reflection. Additionally, with some extremely clever triangulation, it takes seconds to align the part both in tilt and z-position.

This means that, for high index parallel flats where the specification just states a surface figure, it’s a great relief to no longer have to estimate what the transmitted wavefront error ‘could be’, and explain our need for concession.

Similarly, as we polish many optical flats using a double-sided technique, where parallelism between front and rear surfaces is a given, our polishing and inspection technicians are using our new interferometer all the time. No painting, no confusion - just fast, accurate, repeatable and fully-traceable surface flatness measurements that we’re sure all our customers will love.