This test slide is made in the UK and is under license from the National Physical Laboratory. It is an epoxy replica of a master slide produced and certified by that laboratory. The replicas are mounted on microscope slides of 1.2mm thickness with cover glass of 0.17mm thickness. This information is transferred to the accompanying table, which indicates the maximum phase change passing through test objects to test slide. A satisfactory system will detect block 5. Full details are supplied with the slide.