ARHIVSKI VJESNIK 41. (ZAGREB, 1998.)

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B. Justrell, Digital imaging in archives, Arh. vjesn., god. 41 (1998), str. 23-28 The substance of a digital imaging programme The key components of an imaging programme are: - conversion - collection management - presentation - maintaining and long term access. All these components are equally important - the chain is not stronger then its weakest part - but in this paper I will focus on conversion. The goal of any imaging programme should be to capture and present in new formats the significant informational content contained in a collection of documents. For that reason the quality assessments of the digital images have to be based on a comparison between those digital images and the original source documents, not on some vaguely defined concept of what is good enough to serve immediate needs. What is sufficient for today's purposes will probably be inadequate tomorrow. This will raise the question of image quality. My point is that the problem is not to capture an image at the highest quality possible, but to match the conversion process to the informational content of the original - no more, no less. What governs digital image quality at capture? Basically it is - resolution - bit depth - image enhancement - compression - equipment used and its performance - operator judgement and care. I will look closer to each of these factors. Resolution is determined by the number of pixels used to present the image, expressed in dots per inch (dpi) or pixels per inch (ppi). Increasing the number of pixels used, will result in a higher resolution and a greater ability to record fme details, but just continuing to increase resolution will not result in better quality, only in a larger file size. The key is to determine the point at which sufficient resolution has been used to capture all significant details in the source document. The recommendation is to choose a resolution that is sufficient to capture the finest significant details in the group of documents that are intended to be scanned. Bit depth is determined by the number of bits used to define each pixel. The greater bit depth used, the greater number of grey and colour tones can be represented. There are three kinds of scanning techniques: - bitonal scanning using one bit per pixel to represent black or white 26