Commonly referred to as the Bessell UBVRI filter set, these filters are primarily intended for photometric measurements of stellar objects using a standard color system. They are 5mm thick, 50mm in diameter and have hard AR broad band coatings on both sides. These filters are also available in 1-inch diameter and mounted for insertion into the popular CFW filter wheel from SBIG. A clear filter to maintain parfocality is also available in both sizes. Gerald Persha, founder of Optec, has written a white paper discussing the subject of filter standards for CCD photometry. In 1990, Dr. M. S. Bessell published a detail description of these filters as used in CCD photometry. Click here to retrieve the original paper.