Radius gauges require a bright light behind the object to be measured. The gauge is placed against the edge to be checked and any light leakage between the Plates and edge indicates a mismatch that requires correction.
It is a kind of observational measurement because of its visual test way.
R 7.5-15mm includes 15pcs at each end, radius as 7.5mm, 8mm, 8.5mm, 9mm, 9.5mm, 10mm, 10.5mm, 11mm, 11.5mm, 12mm, 12.5mm, 13mm, 13.5mm, 14mm, 14.5mm, 15mm.