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Digital Imaging for Archival Preservation and Online Presentation Working Paper Executive Summary The following information aims to provide a general overview of digital imaging, specifically the presentation of visual images on the World Wide Web and the digital conversion of records for the purposes of Archival preservation. In terms of digital imaging for Libraries, Archives and Museums, web-access is obviously not the only major issue to be dealt with. The burgeoning field of digital preservation is becoming a vital area of expertise within these types of institutions. A central problem faced by those undertaking digitization projects has been lack of consistent or clear information regarding the best practices or standards for digital imaging. Nevertheless, recently institutions such as the National Archives and Records Administration (NARA) and working groups from the Colorado Digitization Project (CDP) and the California State Library (CSL) have suggested a series of recommendations for digitizing archival materials. This document has drawn from these sources in order to provide a condensed reference guide for those undertaking a digital imaging project. Divided into eight parts, sections I & II explain the purposes for the digitization of materials, suggesting a series of issues to be considered when selecting sources for digital conversion. Section II gives a general overview of the more technical decisions to be made before the digitization process takes place, including the classification of material according to document type, and the need for both Master and Derivative file formats. Section IV provides a detailed explanation of the bit-depth, resolution, image size, and file format recommended for the digitization of original sources into Master, Access, and Thumbnail Files. Recommendations for image editing and scanning software, and hardware such as scanners and PCs are made in sections V & VI. The glossary of basic terms in section VII explains in detail the central terms and concepts used in digital imaging. Finally, section VIII suggests a number of online resources for further reading. I. Purposes for Digitization Projects II. Source Selection Criteria III. General Standards for Creating Digital Images Classifying Document Type Master and Derivative Files IV. Specific Guidelines for Digitization of Images V. Hardware Recommendations VI. Software Recommendations VII. Glossary of Basic Concepts VIII. Bibliography I. Purposes for Digitization ProjectsThe disciplinary field of Digital Preservation is an area still very much "in flux," with no clear set of standards for the digital preservation of materials, and with some experts still very skeptical of the advantages. It is nonetheless apparent that as archivists, curators and librarians deal with problems of degradation and increasingly delimited space and storage facilities, professionals in these fields will need to turn to the potential of digital technologies for solutions. In terms of preservation, digital conversion can certainly extend the life of a particular artifact, limiting material access and providing instead a high-quality surrogate. In addition, digitization dramatically enhances access to that artifact. Not only can the image be seen on the Web by users across the world, patrons can also arrange to have the higher-resolution, uncompressed Master File sent them for offline viewing. II. Source Selection Criteria Before the digitization process takes place, it is first necessary to consider which materials will be selected for the project. These decisions will directly affect the size, level of resolution, and download time of a particular file. Doing a virtual exhibit and creating an image database can be two very different ventures, with the latter being by far the most complex of the two. The criterion for document selection will change according to the specific goals of individual institutions, however we have provided the following questions in order to facilitate the decision-making process:
Classifying Document Type According to the guidelines established by the California State Library and NARA, materials selected for scanning can be divided in the following classifications. However, decisions for scanning documents may need to be made on an individual basis, depending on the intended goals for digitization (see above):
Master and Derivative Image Files In accordance to the guidelines established by NARA and the Colorado Digitization Project, we propose that three versions of the image be created: A Master Image, Access Image, and Thumbnail Image. The following information is taken from CDP's General Guidelines for Scanning (http://coloradodigital.coalliance.org/scanning.html). Master Image:
Access Image:
Thumbnail Image:
IV. Specific Guidelines for Digitization of Images The following guidelines can be used as the minimum standards for the digital conversion of text, photographs, maps, and graphic materials: Textual Materials
PHOTOGRAPHIC MATERIALS
Maps, Plans, and Oversized Documents
Graphic Records
V. Hardware Recommendations Scanner Suggestions according to the Colorado Digitization Project (http://coloradodigital.coalliance.org/scanning.html#Scanners): Scanner Types Common Scanner Types:
Flatbed scanners are one of the most popular scanners used in libraries and archives and are suitable for scanning papers, flat photographs, and other printed materials. Flatbeds can be purchased with an optional attachment called a transparent media adapter, which allows you to scan directly from slides or negatives. However, transparency adapters do not always produce as high a quality of image as a slide or film scanner. If you plan to scan predominantly transparent materials that are smaller than 4 x 5, you may want to consider a slide or a film scanner (there are some slide/film scanners that can handle larger transparent formats). Scanners that combine flatbed scanner capabilities and 35mm slide capabilities are also on the market. Some slide scanners can deliver a better dynamic range than flatbeds; however, the resolution may not be sufficient to create digital masters or meet the resolution requirements of some users. If your collection contains predominantly oversized materials, you may want to consider outsourcing the scanning to an imaging vendor or purchasing a high-end digital camera that can capture oversize materials, which works much like a copystand setup. There are also flatbed scanners that handle originals that are 12" x 17", and some flatbed scanners can accommodate even larger sizes, although they tend to take up considerable space and produce enormous file sizes. Some participants in the CDP Project have asked about drum scanners. In general, the CDP does not recommend them for formats of significant value or that are fragile or brittle in any way, as drum scanners can cause a great deal of stress to the document. The original is also taped to the rotating cylinder, so consider how this may also affect the document. Drum scanners are designed for the graphic arts community and, as such, provide an extremely high level of resolution. Drum scanners can scan transparent as well as reflective media, in grayscale and color
Not all scanners take the same amount of time to scan the same image at the same resolution. If high production levels are important, it will be necessary to look at the time it takes for both preview and full scan images of materials similar to what you intend to scan. In general, flatbed/slide scanners accommodate a higher production rate than digital cameras, but they also are limiting in the size and type of media formats they are able to scan. All electronic devices suffer from "noise," which often
appears on scans as blotchy or matte-like areas in the dark shadow
parts of an image when these areas are lightened or have their contrast
range increased. Noise can obscure details in the shadows. Higher
quality scanners, with higher bit depths, will give better results,
as they tend to use higher quality (lower noise) components. At this point in time, we feel that commercially available, hand-held digital cameras are not suitable for archival scanning, excepting the high-end digital cameras (Kontron, Zeutschel, Leica) used by several larger institutions and imaging vendors. High-end digital cameras have no scanning limitations when it comes to size and shape, and can scan at an extremely high resolution (up to 15,000 pixels across the long dimension). They do require certain lighting requirements and a high level of operator skill. However, if you can afford a high-end, overhead digital camera, they present great potential for scanning oversize materials, media in all formats, bound materials with the aid of a book cradle, and present a lower risk to fragile materials by allowing face-up, contact-free scanning. Digital Camera Reviews http://www.steves-digicams.com/digresources.html#reviews VI. Software Recommendations (Directly from the Colorado Digitization Project: http://coloradodigital.coalliance.org/Scanners): Scanner softwareThere are two types of software that you will need for most digital imaging projects. The first is the scanning software that comes with the scanner. The second type of software is the image editing software, normally applied to the image after it has been scanned. Some software, such as Adobe Photoshop°, can serve as both the scanning software and the image editing software. The scanning software is usually limited in its functionality. You should choose scanning software that is at least capable of saving image files into standard formats such as TIFF, JPG, GIF, etc. This functionality will help production and also ensure a wide range of image delivery options. Software that converts image files from one format to another may also be useful. To produce images of acceptable quality, it is important to invest in image editing software, which is normally used for "cleaning up" an image (removing dust spots, for example) and for correction (adjusting the level of brightness and contrast, for example). Image editing software should come with the capability to crop, deskew, and rotate; adjust brightness and contrast levels; sharpen (if needed); zoom in and out; accommodate different file formats; provide controls for gamma, black and white, and color (RGB); provide a histogram and look-up table; support compression types; and possess the capability for the user to create and save customized settings, among other functions. The choice of image editing software is based on the level of image manipulation desired for your project and the level of expertise of staff. Some image editing software, such as Adobe Photoshop°, is very advanced, and may require some time and training to learn. Other software is more basic and allows for only limited operations, such as cropping and rotating, and is not difficult to master. Consider the range of operations you will normally need to perform. The cost of this software can range from free (freeware) to several hundreds of dollars. When considering cost, think about not only the cost of the product, but also how easy it is to use¼and factor in additional costs for training, accordingly. In addition to considering the capability and usability of image editing software, make sure that your current technology can support the software. Do you have the appropriate amount of memory, hard drive space, processor power, and display capabilities (a 24-bit color display card is recommended for image editing work)? The amount of image editing performed on the images should be defined in your project goals, possibly decided in consultation with the collection curator or an archivist or librarian who is knowledgeable about the materials being scanned. Some digitizing projects aim to create a "pleasing image" that may require a great deal of editing. Other projects may be more concerned with the fidelity of the digital image to the original (this may be important to scholars), and may require very minimal editing. Do you intend to match the digital image as closely as possible to the original? Are you more concerned with the photographer's/creator's intent when editing the digital image (i.e., high contrast; scanner operator makes decisions about tone and color values of the digital image)? Or are you more concerned with reconstructing the appearance of the original as it would have existed when first created (to digitally reconstruct deteriorated originals)? What constitutes a "good image" for the purposes of your project--a faithful reproduction or a pleasing image--should be defined prior to scanning. VII. Glossary of Basic ConceptsIMAGE CAPTURE PIXELS RESOLUTION Bit-depth, dynamic range, or tonal-depth all refer to the number of colors or shades of gray that can be represented by a single pixel. Bits are the smallest unit of data stored in a computer, and bit-depth refers to the number of bits used to represent each pixel in a digital image. The more color an image has, the more bits it will require. Using 8-bit color means that each pixel can represent one of 256 shades of color; using 8-bit grayscale means that each pixel can represent one of 256 shades of gray. Image Editors can now work in True Color or 24 bit-color, and pixels can represent any one of 16 million shades of color. Master Files will require the optimum bit-depth in order that the integrity of the original source be maintained. As increased bit-depth will necessarily create a larger file and download time, it is important that Access Files be optimized for the web by stripping redundant color not visible to the human eye. IMAGE SIZEWhen asked to define Image Size, pixels are typically used as the unit of measurement for the long and short dimensions of a particular digital image, although you can also see the dimensions of an image represented in inches or centimeters. For an 8" x 10" photograph would typically have the pixel dimension of 2400 x 3000 pixels, although how large this would look on your screen would depend on how many pixels per inch, or what resolution is defined. IMAGE EDITORSAn Image Editor is the software program used to create graphics and format the spatial dimension, resolution, bit-depth, and overall visual appearance of a scanned digital image. The industry standard among Image Editors is Adobe Photoshop, the most versatile program on the market. FILE FORMATThe most common image file formats for Access and Thumbnail Files
are JPEG and GIF, which are both supportable by the Web. For the
higher resolution, uncompressed or lossless Master Files, TIFF is
the standard format. GIF (Graphic Interchange Format) JPEG (Joint Photographic Expert Group) TIFF (Tagged Image File Format) MASTER IMAGE The Master or Archival Image file represents as closely as possible the information contained in the original source. This uncompressed, high-resolution, unedited file serves as a long term source for derivative access file, and is usually stored in the TIFF format.
ACCESS IMAGE The Access Image File is usually derived from the Master File, and is used for general web access. It will have a smaller file size for faster download time, may be edited and compressed, and will be stored in the JPEG or perhaps GIF format. THUMBNAIL IMAGEThis is a very small image usually presented with the bibliographic record of a source. It is desinged to display quickly online, allowing users to determine whether they want to view the larger access image. It is usually stored in the GIF or JPEG file formats. VIII. Bibliography of Online Resources Conservation Online "Using
Kodak Photo CD Technology for Preservation and Access: A Guide for
Librarians, Archivists, and Curators" (Anne
Kenney and Oya Y. Reiger) Image Permanence Institute (RIT) The Image Permanence Institute is a university based, nonprofit research laboratory devoted to scientific research in the preservation of visual and other forms of recorded information. (see pdf file on Digital Imaging) Archives and Preservations by National Archives and Records Administration (NARA) This page is designed to offer guidance concerning archival preservation, management, and training to all levels of archivists and preservation professionals from the at-home record-keeper, to the family genealogist, to professionals seeking technical guidance from the National Archives and Records NARA Guidelines for Digitizing Archival Materials for Electronic Access Although NARA is careful to state that these guidelines should not be seen as a set of standards for digital imaging, this report offers excellent advice for digitization projects. Safeguarding Digital Library Contents and Users: Digital Images of Treasured Antiquities An IBM collaboration with the Biblioteca Vaticana Apostolica is an early experiment to explore the technical, financial, and practical challenges of making illustrated mediaeval manuscripts accessible via the Internet. The Vatican collection is important to literary scholars, historians, and art historians because it contains not only seminal texts but also magnificent illustrations. We convey what the Vatican Library project and several other IBM joint studies--studies with el Archivo General de Indias de Sevilla (Spain), the lifetime collection of Andrew Wyeth's paintings, a collection held by the Hebrew Union College, and the Yale Beinecke Library--teach about administering intellectual property rights. The question is how to maintain both the intellectual integrity of disseminated images and also the reputations of the institutions making their collections digitally accessible. Preservation of
Library and Archival Materials: A Manual Selecting
Research Collections for Digitization by Dan Hazen, Research Library
Group: Diginews (December 1997) "Digital
Libraries: A Selected Resource Guide" by K. Klemperer &
S.Chapman. "Manuscripts
and Archives in a Digital Age" by Steven Douglas Miller
"Selection
for Digitizing: A Decision-Making Matrix" California State Library Scanning Standards by Liz Bishoff (http://www.library.ca.gov/assets/images/scandocrev1122.PDF) Colorado Digitization Project: General Guidelines for Scanning |
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