Dark field viewing apparatus

摘要

Dark field viewing apparatus comprises either (a) a source of coherent plane wave radiation and focused coherent receiving means, or (b) a source of focused coherent radiation and a coherent receiver of plane wave radiation, the source and receiver being axially aligned in both cases; and elimination means to eliminate any radiation which in the region of the focus would travel substantially parallel to the axis of the apparatus. The apparatus may be a transmission or a reflection scanning acoustic microscope or a holographic recording system, and the elimination means may be a small acoustically-absorbent stop.