Classical interferometric measurement of mirrors is severely limited in accuracy as well as utility. When considered for use in characterizing dynamic performance of active mirrors, it is only of marginal utility. By a unique utilization of heterodyne interferometry, a method has been developed whose processes are ideally adaptable to the short frame times, high frame rates and high spatial frequencies required in active element measurements. Results showing accurate and high spatial resolution measurements will be presented. Concepts for high frame rates and noise immune systems will be also discussed.