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Evaluation of film digitizer's spatial resolution by slit image computational analysis

This work presents a computer algorithm to evaluate film digitizers in terms of its spatial resolution by using image processing techniques. A testing pattern containing slits of different widths, not-equally spaced, was developed. When digitizing this pattern, the algorithm automatically analyses the digital image and evaluate spatial resolution capabilities of the digitizer. These analyses were made by calculating computationally digital image slit width and the distance between the slits. Sampling distances in both directions (parallel and perpendicular to scan direction) are determined by comparing calculated values with previous measurements made by using a calibrated microscope. Evaluation also includes the determination of the presampling modulation transfer function (MTF). The algorithm was used for the measurement of effective pixel size and presampling MTF of a Lumiscan 50 laser digitizer and an Umax Powerlook II optical scanner in both directions. Results showed that the laser digitizer presents significative difference between parallel and perpendicular MTF and the optical scanner presents better MTF considering high frequencies components. Results obtained with the proposed algorithm confirm the possibility of evaluating spatial resolution limitations of any film digitizer using an automatic and simple method.