In the recent past, most large-scale document scanning projects, constrained by the desirability of high throughput and low storage costs, produced only bilevel1 images; this is now yielding rapidly to multilevel and color scanning. Digitizing resolutions (spatial sampling frequency) for textual documents typically range today between 300 and 400 pixels/inch (ppi). 600 ppi is less common, but is rapidly taking hold as scanner speed and disk storage capacity increase

Tests of commercial OCR machines in 1995 showed that accuracy on bilevel images fell for documents digitized at less than 300 ppi, but did not appreciably rise at 400 ppi. They also showed that some systems were able to exploit the extra information in grey level images to cut error rates by 10%–40%. Researchers have suggested that grey level processing will allow OCR to read documents whose image quality is now well below par. Of course, many document images are printed in color and the costs of color scanning and of file storage and transmission are falling rapidly: the DIA research has, within the last five years, begun to take this challenge seriously – but, in my view, not as fast as it should. Some attempts have been made to issue refined scanning standards. The Association for Information and Image Management (AIIM) publishes standards for the storage of textual images, including ANSI/AIIM MS-44-1988 “Recommended Practice for Quality Control of Image Scanners” which defines “procedures for the ongoing control of quality within a digital document image management system.” It is designed predominantly for use in bilevel imaging. MS-44 test targets include:

IEEE Std 167A-1987, a facsimile machine test target that is produced by continuous-tone photography, with patterns and marks for a large range of measurements of moderate accuracy

AIIM Scanner Target, an ink-on-paper, halftoneprinted target

RIT Process Ink Gamut Chart, a four-color (cyan, magenta, yellow, and black), halftone-printed chart for low accuracy color sensitivity determinations