Thread: Strehl ratio
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Old 03-09-2019, 05:00 AM
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Slawomir (Suavi)
Going visual

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Quote:
Originally Posted by Wavytone View Post
UPDATE: updated graph - see below. I've coaxed Excel into doing a quadratic least-squares fit instead of linear, and zoomed in on the region of most interest. FWIW the curves are automatic in Excel, not of my doing.

I was interested because previously from the Russian examples I had guessed a few rough approximations, as follows:

PV 0.30 (⅓ wavelength) ~ Strehl 0.90
PV 0.25 (ľ wavelength) ~ Strehl 0.94
PV 0.16 (1/6 wavelength) ~ Strehl 0.95
PV 0.12 (1/8 wavelength) ~ Strehl 0.98

Though it's only a rough correlation as pointed out before, eg a turned edge vs a central hump.
Nick, try using the commonly used approximation formula for Strehl, instead of doing linear fits or quadratic least-squares :

Strehl = 1 / (e^(2pi rms)^2

I donít think there is a simple equation for calculating Strehl from PV, but it will be similar to the one above when PV approaches RMS wavefront error.

Again, PV IMO should not be used in evaluating the quality of optics.

Example: two optical surfaces, one with PV 1/7 lambda, one with PV 1/10 lambda. Many would jump into a conclusion that the one with PV 1/10 must be better.

Your graph suggests that optical surface with PV 1/7 can have Strehl of 0.97, and perhaps even higher if the surface was perfectly smooth.

If the supposedly superior optical surface with PV 1/10 had a rougher polish and RMS wavefront = PV, then it could at the most reach Strehl of 0.67, if we utilise the commonly used formula above.

It does make perfect sense to me, since rougher polish will scatter light more (plus will cause more small scale interference and diffraction patterns).

To me it is clear, RMS wavefront gives a much more accurate indication of the quality of the optical surface, while PV alone can be misused by manufacturers who might be trying to hide rough polish.
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