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What is an optic surface figure?

The surface figure, or surface quality orsurface cosmetics all refer to the deviation between an actual optic andits ideal surface. There are basically two set of information that are commonlygiven by manufacturers: the surface flatness and what is referred to as"scratch-dig".

Why is it important?

Well, an optic with a bad rating on surfaceflatness will introduce some wavefront distortions, which are responsible for aberrations and badquality focus. Aberrated wavefront leads to poor ratio (ratio of theobserved intensity at the image plane compared to the theoretical maximumintensity of a perfect optical system), so poor optics makes one loosevaluable optical power at focus. Plus scratches or digs on an opticcreate diffraction and stray light, which no one wants either.

Surface flatness




less than λ/2

very low

non critical divergent applications only



Often best standard for cube beam splitter.  Not suitable for high power applications or when wavefront control is  important



General standard for quality manufacturer.  Suitable for most laser and scientific application.


very good

Manufacturers who specify surface flatness  in advise this flatness for critical wavefront control applications such as  interferometry or intense femto-second lasers. But honestly, if this is your  case you wouldn't want to leave room for imprecision, and you would choose a  manufacturer able to specify the RMS flatness.

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