Velocity interferometer system for any reflector is a time-resolved velocity measurement system that uses laser interferometry to measure the surface velocity of solids moving at high speeds. For solids experiencing high velocity impact or explosive conditions, VISAR plots the free-surface velocity against time to show the shock wave profile of a material. VISAR is a useful tool in determining the pressure-density relationship of a material known as the Rankine-Hugoniot conditions or simply the "Hugoniot". In recent years another time-resolved velocity measurement tool called laser Doppler velocimetry has achieved popularity in the shock physics community as an adjunct or replacement for VISAR. This device is essentially a displacement interferometer of the normal Michelson variety. As such it requires extremely fast data acquisition devices and is limited in the range of velocities it can cover. As the surface moves, the reflected light interferes with itself and sinusoidal 'fringes' in light intensity are produced and recorded. A cycle of light intensity or fringe count indicates a displacement of the surface corresponding to one wavelength of the light. The rate at which these fringes occur is thus proportional to the velocity of the surface. To derive a velocity history the fringe data must be differentiated with respect to time, usually by Fourier analysis. This differentiation or FA step inevitably reduces the time resolution and accuracy of the velocity history. The VISAR on the other hand is configured to 'optically differentiate' so that the light intensity variation due to interference varies sinusoidally with the velocity of the surface not the displacement. Also called a 'Delay-Leg Interferometer', it remains the best and most accurate method for recording the velocity history of fast moving surfaces. The original VISARs were built at the National Laboratories and had free-space beams on optical tables with discrete optical components such as beam-splitting pellicles, mirrors, quarter wave delay plates, glass etalons, high voltage photo-multiplier tubes, argon ion lasers and so on. Modern versions such as the Mark IV-3000 from Martin, Froeschner & Associates implement the same optical arrangement entirely in single-mode optical fibre with all solid state telecomm components such as InGaAs photodiodes, Er doped fibre amplifiers and extremely high purity lasers. Velocity resolution down to 0.01 m/s has been demonstrated with time resolution <1ns.
