ARHIVSKI VJESNIK 41. (ZAGREB, 1998.)

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B. Justrell, Digital imaging in archives, Arh. vjesn., god. 41 (1998), str. 23-28 - greyscale scanning using multiple bits per pixel to represent shades of grey; the preferred level of grey scale is 8 bits per pixel, and at this level the image displayed select from 256 different levels of grey which is the maximum of average human perception - colour scanning using multiple bits per pixel to represent colour; 24 bits per pixel is called true colour level, and it makes a possible selection among 16.7 million colours. The recommendation is to choose a bit depth that is in accordance with the characteristics of the source documents: bitonal scanning for document consisting of black ink on white paper, greyscale (8 bits) scanning for documents containing significant grayscale information and colour scanning for documents containing colour information. Image enhancement processes can be used to improve image capture, but the use of them raises concerns about fidelity and authenticity. Typical enhancement features include filters, tonal reproduction curves and colour management tools. The recommendation is to apply enhancement processes cautiously and document all processes that have been used. Compression is normally used to reduce file size for processing, storage and transmission of digital images. The quality of an image can be affected by the compression techniques used and applied level of compression. Compression techniques can be either "lossless" when no information is thrown away in reducing the file size, or "lossy" when the least significant information is averaged or discarded in this process. The recommendation is to use loss less, standard compression techniques which today mean ITU Group 4 or JBIG compression for 1-bit images and lossless JPEG or LZW for multi-bit images. The equipment used and its performance has an important impact on image quality. Different equipment can perform differently even if they offer the same technical capability. The recommendation is therefore that manufacturers'claims of system capabilities should be investigated carefully and confirmed through sampling and references. Operator judgement and care always have a tremendous impact on image quality. In the end decisions taken by humans decides which quality will be achieved. A continuos quality programme is recommended to verify consistency of output from the scanning process. Conclusion I have in this paper pointed out some fundamental aspects on digital imaging. Archives are initiating digital imaging programmes to meet urgent needs in archival 27