Peak-to-Valley vs. RMS
While P-V measurements have been the most common method of specifying optical
system accuracy, RMS is a much better method for quantitatively measuring
the quality of an optic. P-V only gives a measure of the difference between
the highest point and the lowest point. For example, suppose an otherwise
good mirror has a bump in the center occupying only 10% of the mirror
surface, but it is one wave high. The P-V reading will be one wave, yet
the RMS will show that the optic is still of good quality. Obviously the
tall peak will result in a certain loss in performance, but since only
a tiny portion of the light is affected the optic will still perform very
For this reason, we recommend that
the RMS measurement be used to decide optical quality, rather than P-V.
RMS readings are intrinsically lower than P-V readings. Typically a wavefront
better than 0.075 wave is considered to be diffraction limited.