Measuring ejecta particle size distributions using dark-field imaging versus light-field imaging

The characterization of ejecta particle size distributions from shock-loaded metals has significant interest to National Laboratories. Traditional measurement techniques have employed holography, Mie scattering, and low-energy X-ray imaging. Recent advancements in optical diagnostics have demonstrated that dark-field (Schlieren) imaging enhances the contrast of ejecta images by emphasizing high-frequency spatial information, thereby improving particle detection and size distribution measurements. To achieve simultaneous light-field and dark-field imaging, an optical system has been designed to split the ejecta motions onto two independent cameras. This configuration enables the concurrent recording of both conventional light-field images and dark-field images, facilitating a comprehensive analysis of ejecta dynamics. High-speed imaging sequences will be utilized to capture the temporal evolution of ejecta dispersal, providing critical insight into the underlying physical mechanisms. Dark-field imaging is particularly advantageous in that it suppresses coherent laser artifacts, such as mode structures and speckle, which can obscure fine-scale features in recorded images. The imaging system is configured such that the ejecta volume is illuminated by a high-intensity laser source. A Fourier optical relay system introduces a well-defined spatial stop, where incident laser light is focused onto an absorbing element, preventing direct illumination from reaching the imaging sensor. As a result, the camera records only scattered light, enhancing contrast and enabling improved particle size discrimination. A 50/50 beam splitter directs a portion of the light to a secondary camera, allowing for the simultaneous acquisition of light-field images. An additional benefit to using dark field imaging occurs when optimizing the alignment of complex optical imaging systems. Alignment set up time is greatly reduced. However, this approach necessitates direct access to the Fourier plane stop position. Analyzing the astigmatism of an optical imaging system is much easier in dark field imaging mode.