= Speckle contrast of a sinusoidal wavefront error
{title2=$C=(\pi h_0/\lambda)^2$}
For a small sinusoidal <optical path length> error $h_0\cos(2\pi x/\Lambda)$, expand its phase factor to first order. The two exponential harmonics each have field amplitude $i\pi h_0/\lambda$ relative to the unperturbed field, so each speckle has contrast $C=(\pi h_0/\lambda)^2$. Its angular displacement is $\lambda/\Lambda$. Here $h_0$ is optical path error, not physical mirror displacement; a reflecting mirror introduces twice its surface displacement in the near-normal limit.
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