In many problems in nondestructive testing, imaging is not a practical method for detecting flaws. An example of this is the nondestructive testing of airplane canopies in which the absorbing anisotropic material is degraded through prolonged exposure to ultraviolet radiation. In cases such as this, the use of target signatures (if they exist!) may present an alternative to imaging. In this talk we consider the situation when small changes in the (possibly complex valued) index of refraction are to be determined from changes in the measured far field (or near field) data.