An improved Asay window technique is employed to experimentally investigate the micro-spalling fragments of a melted tin subjected to high explosive loading. Compared to the traditional Asay window, details of the new design are illustrated, through the use of photonic Doppler velocimetry to record high-quality micro-spall signals. The analytical method is established to convert the measured data into the spatial volume density distribution, being in quantitative agreement with that obtained from the x-ray radiograph. This improved non-radiographic technique greatly promotes a wide application in diagnosing the micro-spall.