Design and Testing of a Pneumatic Focus Servo for a High-Speed Microdensitometer

• Published in: Optical Engineering 1975-08, Vol.14 (4), p.144366-144366
• Main Authors: Cronin, David J, Mirabito, Anthony F, Pepi, Jerome S
• Format: Article
• Language: English
• ISSN: 0091-3286; 1560-2303
• DOI: 10.1117/12.7971846

The capability of high-resolution microdensitometers is cur-rently limited by the inability to focus accurately and to maintain focus under dynamic conditions. This paper describes a device for establishing and maintaining submicrometer focus tolerances under dynamic operating conditions that would normally result in gross defocus. The device utilizes pneumatic operating principles in a closed-loop feedback system. The precision and repeatability of establishing the focal position is such as to virtually eliminate the need for visually obtaining best focus. The manual through-focus positioning traditionally done by the operator is eliminated. The focus position is established through a gauge indicator needle that is positioned in submicrometer increments from slightly above the target surface to 20 micrometers below. The system is capable of dynamically maintaining focus over a range of more than 150 gym, and it can accommodate object thick-nesses of up to 1/4 inch. Since the pneumatic servo system employs an air nozzle, it not only provides automatic focusing, but also a film hold-down method that was determined to be the best of a number of possible candidates.