An Evaluation of photo CD\u27s resolving power in scanning various-speed films for archival purposes

While the advantages of digital archiving are numerous, the process has been slow to be implemented because of several limitations, particularly high costs. Photo CD shows great promise as a technology for archiving not only because of its cost- effectiveness, but also its speed, multi- resolution format, and efficient compression. Organizations that are beginning to construct digital archives of their resources are doing so to the tune of several million images. When archiving such large quantities of images, one wants to anticipate as many future uses as possible to avoid further scanning costs. Of all potential uses for an archived image, printing on coated stock with a fine line-screen will have among the highest resolution requirements. Although the Photo CD master format offers much flexibility, there is some concern that the format does not provide enough resolution for commercial-grade printing, especially at greater enlargement percentages. In these cases, better results may be achieved with Pro Photo CD, which is more expensive and much slower, but provides four times as much resolution as the master Photo CD. However, simply having more resolution does not necessarily translate into improved image quality. The benefit of the added resolution is likely dependent on the speed of the film and whether there really is more information in the emulsion to be captured. For films above a certain speed, the graininess of the film may offset the extra resolution provided by Pro Photo CD, and no improvement in image quality will be gained. The film speed at which this would occur is currently unknown. Testing the scan quality of various film speeds at 16*base and 64*base can help define the boundary of when Pro Photo CD offers a real advantage, if any, for archiving 35mm film. The findings would supply some guidelines for organizations faced with making decisions about how to use Photo CD most appropriately for archival purposes. To this end, three films of varying speeds and resolving powers were chosen: Ektachrome Lumiere 100, Ektachrome Professional 100 and Ektachrome Elite 200. Two test objects were obtained: an RIT alphanumeric resolution target and a continuous tone photograph containing objects with fine detail. Scans of these chromes were made with both Photo CD and Pro Photo CD scanners. An objective analysis was made by observing the smallest levels resolved on the resolution target for each of the films at both the 16*base and the 64*base resolutions. A subjective analysis was conducted by a panel of respondents making pairedcomparison judgments of image quality for the continuous tone test images. It was hypothesized that differences between the 16*base scan and the 64*base scan would be detected only with the Ektachrome Lumiere 100 film in both the objective and subjective analyses. No differences were expected between the two resolutions for the other two films. This is because it was theorized that the added noise introduced by the higher grain of the faster films would offset the extra resolution provided by the 64*base scan. The results did not concur with the given hypothesis. Instead, differences were noted for all of the tested films in both the objective and subjective evaluations. The data from the alphanumeric resolution target shows an improvement in resolving power with the 64*base resolution for all three films tested. In the subjective test, an increase was also observed in the large enlargement of the Professional 100 and the Elite 200 films. Both of these indicate that none of the films tested contained enough noise to offset the benefit of the added resolution. It should be noted that the differences observed were slight. In terms of recommendations for choosing among Photo CD options for digital archives, the following guidelines can be concluded. All films with speeds of 100 or less will see some, though slight, added benefit from the extra resolution provided by Pro Photo CD. Films with an ISO speed rating of 200 that use fine- grain technology, such as Kodak\u27s TGRAIN, will also see benefit from added resolution. However, in either case the benefit does not seem to subjectively matter unless the image is enlarged beyond the standard dimensional limits of 16*base. Thus, unless large-format reproduction of archived images is likely, Pro Photo CD scans are not necessary