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An interferogram analysis of the quality of an imaging optical system is always supposed to be presented as wavefront error unless otherwise specified . The setting in the analysis software of the 'waves per fringe spacing' determines that the results are correctly presented and quantified .
In the case of this RC opticsal auto-collimation tested system the spacing would have been 0.5 waves per fringe. If you ordered a flat you would expect the results to be expressed as a surface error. It is possible for an optician to fiddle this aspect by setting a less sensitive wave per fringe factor than is technically correct for the testing set up , thus making the optics appear to be better than it really is . It happens.
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