In 1971, the Smithsonian Institution Office of Printing and Photographic Services (OPPS) was formed (later known as the Office of Imaging, Printing and Photographic Services, OIPP and Smithsonian Photo Services, SPS) to provide original and copy photography service to all interested bureaus of the Smithsonian Institution, as well as to provide permanent storage for film-based still photography. Still picture film formats deposited with OPPS were assigned Smithsonian Institution (SI) negative numbers and retained in the OPPS cold storage vault. In 1999 the calendar-based numbering format altered slightly to accommodate the century change and OPPS (then OIPP) abandoned the bulk of their film-based operations and switched to digital imagery. Between 1971 and 1999 SI negative numbers were assigned to a large number of images from the collections of the National Air and Space Museum (NASM). The actual number of NASM copy photographs is unknown, but may be in the range of 50,000 to 60,000 images. Photographs from a variety of sources were routinely copied by NASM-branch OPPS photographers at the request of NASM staff members. Until circa 1985 it was standard practice for NASM to keep large three-ring binders of copy photographs (sorted by subject) on hand for the reference convenience of staff and patrons. Additional copy photographs were kept in the Technical Reference Files. This practice became less prevalent in the 1990s, and ceased for all intents and purposes in 2000 with the switch to the use of digital image systems for reference and reproduction.