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 Plate and edge indicates a mismatch that requires correction.
It is a kind of observational measurement because of its visual test way.
R4-24mm includes 15pcs at each end, radius as 4mm, 5mm, 4.5mm, 6mm, 7mm, 8mm, 9mm, 10mm, 11mm, 12mm, 14mm, 6mm, 18mm, 19mm, 20mm, 22mm, 24mm.