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last updated:
October 7, 2014 2:55 PM

J. Trant. "Framing the Picture: Standards for Imaging Systems." ICHIM/MCN, San Diego, CA, Oct. 1995.


2. Image Capture

Image capture is the process of employing a device (such as a scanner or digital camera) to create a digital representation of an image that can then be stored and manipulated by a computer. Image capture can take place either directly from a work of art or artifact, or from an existing photographic image of a work. The circumstances of image capture will have a critical effect on image quality, and therefore on the future utility of a digital image. Key decisions to be made in the image capture phase include capture methodology, digital image resolution and image bit-depth. The cultural heritage community has not developed standards or guidelines to govern image capture activities.

Capture Methodology

Projects such as VASARI12 and MARC13 have focused on the direct capture of digital image data from the work of art itself, a process which is becoming more a more realistic option with the development of digital camera backs.14 Others, such as Luna Imaging, have focused on the conversion of existing photographic information into digital form.15

As collective experience in digitization increases, we will be able to develop guidelines for image capture in particular circumstances. Peter Robinson has begun to identify "Digitization Strategies for Particular Materials", in a study for the Office for Humanities Communication, in the United Kingdom.16 While focusing primarily on text, this study provides a model for the kinds of guidelines which would be useful for visual materials.

Bit Depth or Dynamic Range

Bit depth governs the maximum number of colours it is possible for a digital image to represent. While not stated formally, there appears to be a minimal requirement for 24-bit (> 16 million) colour for "quality" images of works of art, i.e. those used for more than reference or identification purposes.17 This exceeds the 8-bit (256) colour images now routinely distributed on the WWW or made available in many CD-ROM titles. Recent technological developments have focused on 32-bit (or even 36-bit) image capture. These images are then sampled and stored as 24-bit images. Key to a decision regarding an adequate bit depth for images of cultural heritage is a further understanding of image quality.

Resolution

Resolution refers to the number of pixels which make up a digital image, and can be expressed as a ratio such as 1,000 x 2,000 or in terms of dots per inch (dpi). DPI (a term inherited from print technology) is a complex and often confusing concept - used to characterize scanning devices, image files and various display devices. It has also been used to compare the pixel dimensions of a digital image to the dimensions of an original object.

Martinez and Hamber18 present a series of theoretical resolution requirements, based upon the actual dimensions and viewing conditions of a painting. They acknowledge, however that such a measure of scanning resolution requirements may not be appropriate, for "those working in the restoration and conservation departments of galleries and museums often require higher levels of resolution than perceivable by the naked eye".19 Regardless of the theory, however, compromises are inevitable when creating a working system. "Scanning at the resolution required for scientific studies may be impractical for very large paintings for time/storage reasons, and may necessitate a reduction in scanning resolution."20 Image capture and storage technologies still limit the resolution it is possible to capture.

Issues in Image Capture: Image Quality

The cultural heritage community has neither a common definition for image quality, nor standards for measuring it. We need to develop benchmarks for measuring quality in the image capture process; The AIIM/ANSI standards for document imaging,21 and the Commission on Preservation and Access Tutorial Digital Resolution Requirements for Replacing Text-Based Materials,22 provide useful models. The Kodak Q60 and other colour targets could provide fixed points of reference for ensuring the quality of digital image capture.

Guidelines are also needed regarding the creation of representations of works of art. 23 What makes good documentary photography?24 For example, colour and scale reference are not routinely included in many museum photographs. Control over the conditions surrounding the acquisition of visual information would certainly help to create a stable and predictable information source.

Image quality standards in the cultural heritage community need to take account of both the technical quality of the image, and its appropriateness for particular uses. Ester has conducted a series of experiments aimed at identifying when additional image information makes an imperceptible difference in the quality of a digital image.25 We need to build on work regarding viewer perception, to understand more about who uses visual information and in what way. With this knowledge, we can develop guidelines to ensure that the appropriate amount of image information is delivered as it is needed.

Typologies of image types, representing a series of quality thresholds defined by functional requirements, are widespread use.26 Each of these types correlates with a particular function of the system. Image types could be identified as thumbnail or browse image, (displayed along with a text record, in-line on a WWW page or with other images on a "lightbox"); a full-screen image (the image displayed to the full pixel dimensions of a user's workstation - alternatively defined according to VGA, SVGA or higher resolutions); a high-resolution image (an image which allows some zooming in on details without pixelation) and an 'archival image' (an image of resolution/dynamic range exceeding today's' display technology, kept off-line and to be used in the future which may also be used to produce high-quality print material.) Lower resolution images are derived from a high-resolution scan.

Image quality is more complex than an equation balancing bit-depth and resolution. Further study is required to represent the particular needs of specific users and systems. Initiatives such as the Museum Educational Site Licensing (MESL) Project will provide more information about the real users of images and their quality thresholds.27 As evaluation results are often not reported in the system profiles in the literature, the Imaging Initiative of the Getty Art History Information Program is undertaking a series of Case Studies, which will examine the use of imaging technology in specific institutions and project. With more research into the users of digital images and the functions for which they use image databases, we will be able to balance the different perspectives on imaging and find some common ground upon which to frame guidelines.28

Next Section: 3. Image Storage

Informatics: The interdisciplinary study of information content, representation, technology and applications,
and the methods and strategies by which information is used in organizations, networks, cultures and societies.