Importance of Resolution and Density

The Importance of Resolution and Density for Scanning Microfilm

Normal microfilm is a high contrast recording medium that was never designed to capture halftone or continuous tone material, photographs, negatives, prints, and heavily illustrated print material. But, it is possible to link the capabilities of the Hermann & Kraemer (H&K) high resolution 35mm planetary camera that is capable of resolving more than 200 lines per millimeter to a Continuous Tone Filming (CTF) process that successfully captures continuous tom images with great fidelity and adheres to all preservation standards. Continuous tone filming is not new, but previous techniques used either non-preservation film bases (acetate) or suffered from loss of resolution both during original image capture and during duplication. The CTF process suffers from neither of these limitations. Resolution occasionally exceeding 200 lines per millimeter is achieved using this technique. While there is some loss of resolution during duplication, it is often not measurable; that is the resolution measured from the standard test target remains the same. One measure of quality is that third generation CTF images have been taken to a reputable photo shop and high quality paper prints produced.

The second factor for successful scanning of microfilm to a digital image is density variation. Preservation standards allow density to range .20 density points within a roll of microfilm. However, the narrower the range of densities on a roll of microfilm, the more readily it can be scanned without making adjustments for changes in density throughout the roll of microfilm being scanned. There is at least one system that is specifically designed to produce preservation microfilm with narrow density ranges, ExpoSure^Tm developed by MAPS The MicrogrAphic Preservation Service, Inc. A US patent was awarded for ExpoSure^Tm in 1992. ExpoSureⁿ" when combined with the resolution capability of the H&K 35mm planetary camera can produce microfilm images with a resolution of 200 lines per millimeter and a density range for a given roll of microfilm that is often less than .05 density points.

These two characteristics, high resolution and narrow density ranges, produce microfilm that is far more easily scanned into a variety of digital formats. Anecdotal evidence suggests that microfilm with narrow density ranges and high resolution can be scanned with significantly fewer adjustments during the course of the scanning process, and the resultant digital image is superior to the use of conventional microfilm techniques.