Scanning Specifications
Selecting the appropriate scanning specifications for the particular project workflow is critical for generating high-quality, cost-effective scan products. While resolution is important for producing high quality scanned products, significant consideration to bit depth, color space, file format types, and reliability are also required.
Scanning Optical Resolution – the selected resolution will depend on the source material and quality, archiving standards, intermediate processing steps, and end-use needs. As shown in the table below, the declining resolution with enlargement of photographic slides is significant and therefore, a scanning resolution of at least 4000 dpi is recommended. Standard documents and line drawings can typically be scanned at 300 to 600 dpi, depending upon original quality and optical character recognition (OCR) needs.
Bit Depth – the selected scan bit depth depends on the source materials, intermediate processing steps, and end-user needs. Single bit depth (monochrome) can be used for some text-only documents or line drawings and for generating a white page background in the final digital file. Documents with graphics, photographs, and other features should be scanned at 16 bit minimum. Photographs, aerial images, and similar materials should be scanned at 24 bit; although for specialized processing, 48 bit scans are used. If using the ColorPerfect Adobe Photoshop plugin, 48 bit input files are preferred for processing (such as, negative to positive conversion, color correction). Post processing, the files can be converted to the more commonly used 24 bit depth. It is not unusual to generate a higher scanning bit depth for image processing, followed by a conversion to a digital file with a more typical (lower) bit depth.
Color Space/Profile – color images should use the AdobeRGB (1998) color space. Grayscale images should use the common Gray Gamma 2.2 color space.
Output File Format – an appropriate file format should be selected based on the item type (document, map, photograph), item state (archive or distribution), and processing needs (post-scan processing, clean up). Archival raw files and processed files should be stored in uncompressed or loss-less compressed, common formats, including TIFF or PDF. JPEG format is not an acceptable archival format, due to compression artifacts that increase each time this file format is saved. Proprietary, closed, or patented formats, such as MrSID (SID), should be avoided, due to licensing and restrictions, limited software availability, and potential for limited future format support and access to appropriate decoding software. TIFF files can be saved using lossless LZW or Delfate/ZIP compression to significantly reduce file storage requirements, while preserving the original detail before compression. Deflate/ZIP compression generally results in smaller files than the older LZW compression.
Reliability – is the expected number of cycles or length of time between failures, often expressed as mean time between failures (MTBF). Commercial grade scanners often have orders of magnitude greater MTBF estimates than consumer-grade scanners.
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