Abstract: A video camera records a series of interferometric images of a test object, while the object is subjected to a repeated and varying level of stress. At least one pair of interferometric images, taken during the same video frame, and corresponding to different levels of stress, are added together, in an analog manner, to form a composite image which is then stored in digital form. During the next video frame, the phase of the light used to generate the interferometric image is altered, and the process is repeated. The digitally-stored composite images obtained during two successive video frames are subtracted from each other to produce a second-order composite image which reveals information about the condition of the object. The second-order composite image includes fringes of high contrast, and is not adversely affected by most ambient noise.