In the operation of a projection printer (11) photomasks (22), the patterns (21) of which are protected by coverplates (39), are used interchangeably with other photomasks, the patterns of which are located at an open surface thereof. The photomasks (22) feature shims (46) which space the patterns (21)...http://www.google.com/patents/US4537498?utm_source=gb-gplus-sharePatent US4537498 - Focal plane adjusted photomask and methods of projecting images onto photosensitized workpiece surfaces
