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 well.
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.