Mechanical properties of metals depend strongly on the grain size. The yield strength, for example, is generally inversely proportional to the square root of the grain diameter (Hall-Petch equation) [1]. For this reason, steel and other industries spend a great deal of effort in measuring and ensuring the desired grain size. In a common practical technique, samples are cut from the actual product and the number of grains per unit area are measured by optical methods. This method is time consuming and a hundred-percent inspection is impossible because the delay would affect the product’s net output. Thus, there is an interest to develop a nondestructive, on-line technique capable of rapid measurement of grain size.