Determining Scanning Resolution Requirements for Replacement Purposes
To illustrate benchmarking for conversion, let's consider the brittle book. For brittle books published during the last century and a half, detail has come to represent the size of the smallest significant character in the text, usually the lowercase e. To capture this information-which consists of black ink on a light background-resolution is the key determinant of image quality.
Benchmarking resolution requirements in a digital world have their roots in micrographics, where standards for predicting image quality are based on the Quality Index (QI). QI provides a means for relating system resolution and text legibility. It is based on multiplying the height of the smallest significant character, h, by the smallest line pair pattern resolved by a camera on a technical test target, p: QI = h³p. The resulting number is called the Quality Index, and it is used to forecast levels of image quality-marginal (3.6), medium (5.0), or high (8.0)-that will be achieved on the film. This approach can be used in the digital world, but the differences in the ways microfilm cameras and scanners capture detail must be accounted for. Specifically, it is necessary to make the following adjustments:
1. Establish levels of image quality for digitally rendered characters that are analogous to those established for microfilming. In photographically reproduced images, quality degradation results in a fuzzy or blurred image. Usually degradation with digital conversion is revealed in the ragged or stairstepping appearance of diagonal lines or curves, known as aliasing, or "jaggies."
2. Rationalize system measurements. Digital resolution is measured in dots per inch; classic resolution is measured in line pairs per millimeter. To calculate QI based on scanning resolution, you must convert from one to the other. One millimeter equals 0.039 inches, so to determine the number of dots per millimeter, multiply the dpi by 0.039.
3. Equate dots to line pairs. Again, classic resolution refers to line pairs per millimeter (one black line and one white line), and since a dot occupies the same space as a line, two dots must be used to represent one line pair. This means the dpi must be divided by two to be made equivalent to p.
With these adjustments, we can modify the QI formula to create a digital equivalent. From QI = p ³h, we now have QI = 0.039 dpi ³h/2, which can be simplified to 0.0195 dpi ³h.
For bitonal scanning, we would also want to adjust for possible misregistration due to sampling errors brought about in the thresholding process in which all pixels are reduced to either black or white. To be on the conservative side, the authors of AIIM TR26-1993 advise increasing the input scanning resolution by at least 50% to compensate for possible image detector misalignment. The formula would then be QI = 0.039 dpi ³h/3, which can be simplified to 0.013 dpi ³h.