Abstract: Foreign material in a food substance is detected by placing it in a container filled with water to a level above the top of the food substance and moving the container through an X-raying chamber, the transmitted and detected X-rays being converted to electrical signal form. The signals are thresholded and then differentiated to obtain signals indicative of the edges of the foreign material while minimizing spurious signals due to voids or bubbles in the water or to edges of the container. Preferably the oppositely-directed differentiated pulses produced at the opposite edges of foreign material are both used to indicate presence of the foreign material.