Unsteady compression waves in interferometer windows

Optical windows for the velocity interferometer system for any reflector perturb the measurement of specimen/window interface velocity during compression wave experiments. The perturbation is caused by the variation of window index of refraction with density. The correction for shocks and steady waves is the ratio of apparent to actual interface velocity and is called 1+Δν/ν0 in the literature. We find 1+Δν/ν0 for unsteady waves also depends only upon interface velocity and not on details of the unsteady compression wave as previously reported. The correction 1+Δν/ν0 for unsteady waves is the same as for steady waves and shocks only if the index of refraction is linear in density in which case 1+Δν/ν0 is constant equaling the zero-density intercept of the index of refraction function. Differences in LiF window performance for shocks and for unsteady waves are shown to be small compared with other experimental uncertainties.