A new derivative of Arizona sand, ISO Medium Test Dust, is coming into use for calibrating liquid optical particle counters. ISO Medium Test Dust and optical particle counters ave used to test filtering efficiency. Particle shape plays a role in both filtering and optical detection. To establish a new Standard Reference Material for the fluid power industry, the National Institute of Standards and Technology (NIST) performed scanning electron microscopy (SEM) on large numbers of medium test dust particles over the size range of 0.5-50 mum. Aspect ratio, longest chord to projected area diameter ratio, and circularity were derived from the boundaries of the SEM images.