A coating film is formed on a surface of rubber by applying a coating material containing a solvent which swells a rubber and a resin on the surface of the rubber material. A cutting element for the roof-bolt drill bit may include a cutting face and a peripheral surface extending around an outer periphery of the cutting face. An interconnect pattern is patterned and etched into said insulating layer. Each upper surface continuously and smoothly curves in the same direction when viewed along a plane through a diameter of the substrate. A slant reflector is built on the path of the back light to reflect the transmitted light to the reflective portion so that the back light and ambient light follow similar paths.