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.
R15.5-25mm includes 15pcs at each end, radius as 15.5mm, 16mm, 16.5mm, 17mm, 17.5mm, 18mm, 18.5mm, 19mm, 19.5mm, 20mm, 21mm, 22mm, 23mm, 24mm, 25mm.