When equipped with a back-scattered electron detector, an electron microscope has the capability to produce images in which the contrast is controlled by differences in atomic number across the specimen. Three reference samples are now available for testing the differences of atomic number . Each of the reference samples consists of two high purity elements that have an atomic number difference of 1. They are in form of a wire of the low Z element embedded in a matrix of the high Z element. The samples are mounted onto 5 mm diameter blocks.