Establishing Spatial Resolution Requirements for Digitizing Transmissive Content: A Use Case Approach

Abstract

Guidelines for the digital conversion of transmissive content (negatives and positive slides) in the cultural heritage community have lagged behind those for print content. The primary reasons for this are twofold. Unlike print material, transmissive content is generally an intermediate format (as with negatives) or requires a viewing mechanism (such as a projector). In either case, there is no standard for the viewing of the object. The second challenge for digitization of transmissive content is, in large part, a result of the ambiguity of the visual output for slides or negatives. Typical guidelines for the resolution in digitizing transmissive content have concentrated either on the limits inherent in scanning equipment or attempted to base resolution on the microstructure (e.g., grain) of the negatives or slides, and not on the actual image information content in the film.With special regard to the latter and in response to a project to digitize negatives from the Farm Security Administration (FSA) collection at the Library of Congress, the first part of this study investigated methods for establishing scanning resolution requirements for B&W silver-gelatin film negatives from the early to mid 20th Century based on defined use cases. The use case described by the curators of the FSA collection for resolution was based on the information content of the original photographic image and not on a film's granular microstructure. In other words, actual image information captured and not the inherent grain structure itself. This study describes the methodology used for determining the limiting resolution of film capture, and presents the results of the study for the FSA collection, as well as the application to all similar transmissive content. Examples will be provided to demonstrate the utility of this approach.In addition to silver-gelatin B&W film negatives, the second category of transmissive photographic originals studied was early color positive processes, specifically Autochrome and Dufaycolor. The nature and perception of the image for these processes is very different than the typical image and silver grain structures of typical B&W negatives. In addition to the color information, these photographs represent a very distinct class of original and different use case requirements. Investigation of these materials provided an opportunity to expand the research methodology used for determining appropriate scanning resolution.