Abstract: In accordance with an embodiment of the present invention, an apparatus for measuring light scatter loss from a test piece includes a light source, an optical cavity including the test piece and at least one mirror located along a path, and a light detector. The cavity receives an input beam from the light source, circulates a beam within the cavity as a circulating beam, and produces an output beam. Irregularities on the surface of the test piece result in a progressive diffusion of the circulating beam about the path. The light detector provides an output signal based on the intensity of the output beam. The output signal has an initial slope that determines a first saturation value. The output signal has a second saturation value. The difference between the first saturation value and the second saturation value provides a scatter loss measurement.