Table of Contents

1.1. Context
1.2. Scope

1.1. Context

This is the third version of the California Digital Library Guidelines for Digital Images (CDL GDI). This version is based upon earlier versions of the CDL Digital Image Format Standards (September 1999 and July 2001) and companion CDL Best Practices for Image Capture (February 2001).

These guidelines were prepared by the CDL Digital Object Working Group during the winter of 2004. This version of the CDL DIG draws substantially on the U.S. National Archives and Records Administration's (NARA) Technical Guidelines for Digitizing Archival Materials for Electronic Access: Creation of Production Master Files -- Raster Images. Because the communities represented by NARA and the CDL have different needs and requirements, there are some differences between the two sets of guidelines.

1.2. Scope

This document provides specifications for all new digital image files prepared by producers for submission to the CDL. Digital images prepared according to these guidelines meet the criteria for CDL's Enhanced Service Level. This level of service includes the presentation and aggregation of digital assets via CDL websites, and via locally-maintained websites using CDL site building services. It is also sufficient for the ingest of digital objects into the UC Libraries Digital Preservation Repository.

Under some circumstances, institutions may submit images that do not adhere to these requirements, such as legacy or born digital images. These digital images may be subject to technical assessment by the CDL on a case-by-case basis.

These guidelines are not intended to address all of the administrative and technical issues surrounding the creation of digital image collections (including technical aspects of image files, such as resolution, pixel array, bit depth, etc.). In addition, these guidelines do not describe operational procedures for digitization (such as imaging workflow, storage, quality control, etc.). For detailed information on these and other issues related to digitization projects, consult the NARA guidelines.

These guidelines primarily define approaches for creating digital master files to facilitate affordable reprocessing. These files can be used for the creation of additional derivative files for distribution and/or display via a monitor and for reproduction purposes via hardcopy output at a range of sizes using a variety of printing devices. Many of the technical approaches discussed in these guidelines are intended for this purpose. They may not be adequate for preservation reformatting to create surrogates that will replace original records. For further discussion of the differences between these two approaches, see the NARA guidelines.

• Digital image: A digital image is defined for the purposes of this document as a raster based, two-dimensional, rectangular array of static data elements called pixels, intended for display on a computer monitor or for transformation to another format, such as a printed page.
• Digital master files: When analog materials are converted to digital through a digital reformatting process (such as scanning, photographing with a digital camera, etc.), the digital master file is the file created as the direct result of image capture. The digital master file should represent as accurately as possible the visual information in the original object. However, if the original object can not be digitized directly due to its size or other attributes, it may be necessary to use a photographic intermediary. Care should be taken that the photographic intermediary is well documented and represents the original object as accurately as possible. Files should use color rather than grayscale when color is an integral part of the original object, and any compression applied to the file should be lossless.

  In the case of analog materials reformatted to digital, the primary function of digital master files is to serve as a long-term archival record and as a source for derivative files. A digital master file may serve as a surrogate for the original, may completely replace originals, or may be used as security against possible loss of originals due to disaster, theft and/or deterioration.

  For "born digital" objects that were not created through a digital reformatting process, the digital master file comprises the original, source digital file itself.

  The long term preservation of digital master files requires a strategy of identification, storage, and migration to new media, as well as policies about image use and access. It is essential that master files remain unaltered over time. Lossy compression techniques such as JPEG should not be applied to master files, and migration procedures should include quality control procedures to ensure that the integrity of the files is maintained throughout the entire process.

  The specifications for derivative files used for image presentation may change over time; digital master files can serve an archival purpose, and can be processed by different presentation methods to create necessary derivative files without the expense of digitizing the original object again. Because the process of image capture is so labor intensive, the goal should be to create a master that has a useful life of at least 50 years. Therefore, collection managers should anticipate a wide variety of future uses, and capture at a quality high enough to satisfy these uses. In general, decisions about image capture should err towards the highest quality.

• Derivative files: These files are created from digital master files for editing or enhancement, conversion of the master to different formats, and presentation and transmission over networks.