Strehl, RMS and Peak-Valley Calculator

Enter either a Strehl, RMS or Peak-Valley. Press the corresponding button to calculate the other values.

Strehl =
Peak-Valley =


The conversions assume a smooth optical figure.

A Strehl of 1.0 means that all the light is focused to a point. Because of diffraction caused by the finite aperture, at least 16% of the light will be thrown outside the point in the form of surrounding rings of diminishing brightness, and the point will become a disk containing at most 84% of the light. The linear size of this Airy Disk depends on the focal ratio.

A quarter-wave optic has a Strehl of 0.80, an RMS of 1/14 wave and a Peak-Valley of 1/4 wave. The Airy Disk will contain 64% of the light and the surrounding diffractions rings 36% of the light.

See my Rating Mirrors webpage for more.

Mel Bartels