A simple change to the CLAVIS optical meter was a redesign of the sensor head for the U.K. Royal Mail. With over 70 sorting offices throughout the country, millions of letters are sorted each day by sophisticated letter sorting machines which are driven by synchronous drive belts. Correct belt tension is essential for these machines to run with the minimum of down time. The orientation of the belts on this equipment required the sensor to be 'side looking' rather than the standard 'end looking'.