Image chopping techniques for fast measurement of irradiance distribution parameters

A set of parameters employed to evaluate the irradiance distribution of an image formed by an active optics system is described. Relative to an arbitrary frame of coordinates, these parameters are the variance in the x and y directions, the covariance, the angle of the principal axes, and the eccentricity. The summary parameters are defined in terms of the moments of the distribution. It is shown how the first and second moments can be measured in a fast analog way by using a technique based on image chopping in three directions with a knife edge or a slit and collection of the transmitted energy on a detector.