The minimum distance separating two parallel planes between which the surface can be contained.
Theory and Application
Scale of Flatness
Although in theory there is no minimum or maximum, in engineering practice we take as minimum size the field of view of a microscope, even an STM type, and as a maximum the size of large telescope mirrors or very large surface plates. This means that sizes where flatness is relevant can range between a few nm2 and a few m2.
A flatness deviation is a geometrical error, where this error is determined, implicitly or explicitly, for surface wavelengths from a minimum value, while all longer wavelengths that define the geometrical deviation are not filtered. This implies that the bandwidth can be rather small when flatness is assessed over a course grid. For example, for a square grid, this implies that surface wavelengths smaller than twice the pitch are neglected and are...
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