Objective Evaluation

One way to perform an objective evaluation would be to determine conversion requirements based on the process used to create the original document. Take resolution, for instance. Film resolution can be measured by the size of the silver crystalline clusters suspended in an emulsion, whose distinct characteristics are appreciated only under microscopic examination. Should we aim for capturing the

properties of the chemical process used to create the original? Or should we peg resolution requirements at the recording capability of the camera or printer used?

There are objective scientific tests that can measure the overall information carrying capacity of an imaging system, such as the Modulation Transfer Function, but such tests require expensive equipment and are still beyond the capability of most institutions except industrial or research labs.^[9] In practical applications, the resolving power of a microfilm camera is measured by means of a technical test chart in which the distinct number of black and white lines discerned is multiplied by the reduction ratio used to determine the number of line pairs per millimeter. A system resolution of 120 line pairs per millimeter (lppm) is considered good; above 120 is considered excellent. To digitally capture all the information present on a 35 mm frame of film with a resolution of 120 Ippm would take a bitonal film scanner with a pixel array of 12,240.^[10] There is no such beast on the market today.

How far down this path should we go? It may be appropriate to require that the digital image accurately depict the gouges of a woodcut or the scoops of a stipple engraving, but what about the exact dot pattern and screen ruling of a halftone? the strokes and acid bite of an etching? the black lace of an aquatint that becomes visible only at a magnification above 25³ ? Offset publications are printed at 1200 dpi-should we choose that resolution as our starting point for scanning text?

Significant information may well be present at that level in some cases, as may be argued for medical X rays, but in other cases, attempting to capture all possible information will far exceed the inherent properties of the image as distinct from the medium and process used to create it. Consider for instance a 4" ³ 5" negative of a badly blurred photograph. The negative is incredibly information dense, but the information it conveys is not significant.

Obviously, any practical application of digital conversion would be overwhelmed by the recording, computing, and storage requirements that would be needed to support capture at the structure or process level. Although offset printing may be produced at 1200 dpi, most individuals would not be able to discern the difference between a 600 dpi and a 1000 dpi digital image of that page, even under magnification. The higher resolution adds more bits and increases the file size but with little to no appreciable gain. The difference between 300 dpi and 600 dpi, however, can be easily observed and, in my opinion, is worth the extra time and expense to obtain. The relationship between resolution and image quality is not linear: at some point as resolution increases, the gain in image quality will level off. Benchmarking will help you to determine where the leveling begins.