Hardness evaluation with an optical ring light

Evaluation of hardness by interrogating the residual indentation in a static test using a microscope can be hampered by poor image quality. Image digitization with processing may overcome this but is limited by cost, convenience, and speed. It is shown that the indented surface on the sample in a Brinell test acts effectively as a spherical mirror. Thus, placement of an optical ring light coaxial with the microscope objective and at varying distances from the sample surface produces a distinct right light image that can be correspondingly altered in diameter. The Brinell hardness can be computed by determining the ring light to sample distance after adjusting the ring light image diameter to equal the indentation diameter. This approach is verified experimentally and found to be accurate, cost effective, and expeditious.