Method and apparatus for detection of an edge of a printing plate mounted on a drum imaging system

The invention is in the field of imaging systems for use in the printing industry. More particularly, the invention relates generally to the field of edge detection of imageable printing plates mounted in or on an internal or external drum platesetter used for imaging printing plates. The invention is also suitable for use in an imagesetter used for imaging a film media, which is thereafter used in the process to image a printing plate. A method and apparatus is disclosed to precisely locate an edge of an imageable plate mounted on a drum or other support surface. A light source and light sensor are used to measure the difference in reflectivity between the plate and the support surface. The drum or support surface contains at least one groove to increase the difference in reflectivity between the plate and the support surface. The groove may also contain an anti-reflecting layer to further increase the difference in reflectivity. The groove may also have a geometric shape that causes incident light to be directed away from the light sensor.