Inspection of text on the inner surface of plastic bottle caps

Raised alphanumeric characters embossed on the inner side of a green cap are inspected using a camera and blue coaxial lighting. OPX coaxial lighting can be used to capture an image in which the flat surface of the cap appears white and the raised alphanumeric characters appear black. In addition, since plastic bottle caps are thin and the external logo or design is visible through the material, a lighting color that minimizes transmission is selected.