Patent GB-999698-A (GB)
Title: Focusing system for use in reducing cameras
IPC: G02B7/28
Filed: 1964-03-10
Abstract: 999,698. Automatic positioning control for camera focusing. NATIONAL CASH REGISTER CO. March 10, 1964 [March 19, 1963], No. 10048/64. Headings G2A, G2C and G3R. In a focusing system for a copying camera, a steady stream of air is jetted into the space between the objective lens and the photosensitive film, any alteration in air pressure consequent upon a change in the spacing between lens and film being caused to operate a servo system which restores the original spacing. In the form shown, the apparatus is built on to a rigid supporting structure 91 fixed to a bed 10. Within support 91 is mounted another support 92 which carries the photosensitive film, a system of lead screws being provided so that support 92 can be moved (in its own plane) in two orthogonal directions. The photosensitive film is a layer of photochromic material 80 coated on to a glass plate 82, the plate being secured between support 92 and a hinged securing section 93. The objective lens is a microscope 36 having a pupil 36a, resting on a supporting platform 191, and having a coarse and fine adjustment 188 and 189 respectively. The latter can be controlled by a servomotor 202 through a gearing arrangement. Since an image is to be produced on film 80 of considerably reduced size compared with the original, the lens opening has to be large and the focal length very small. The depth of focus is therefore so small (e.g. ¢ micron) that the distance between lens and film is extremely critical, if the successive images on the latter are all to be properly in focus. To maintain this distance automatically at the correct spacing, a steady stream of air is provided through a line 196, being led to a hood 195 surrounding the lens pupil 36a and through a number of nozzles to impinge against the film 80. The air pressure is indicated on a gauge 214, which is constructed as an electrical switching device. The indicator pointer 216 occupies a central position between two terminals 218, 219. If the lens-to-film spacing increases, the air pressure falls, pointer 216 consequently turning anticlockwise until it eventually contacts terminal 218. This energizes a relay 226, closing a contact 226a, and so connecting the power supply 223 to servomotor 213. This is connected by gearing 210, 211 to the microscope 36 (36<SP>1</SP>) and moves it nearer to film 80 to restore the spacing. Similarly, if the air pressure rises, in consequence of a decrease of the spacing, pointer 216 contacts terminal 219. In this case, through relay 221 and contacts 221a, the servomotor is driven in the opposite direction to restore the original spacing. Means are provided to prevent an exposure being made while the servomotor is cycling or hunting for a focus position, and also while the film is being advanced. Means are also provided for preventing the servomotor from moving the lens while an exposure is being made, since this may take two seconds. The image formed on the film 80 can be inspected through an objective 40. If not satisfactory, it can be immediately erased by flooding with visible light, and re-formed. The picture images formed on the photochromic film are eventually transferred permanently on to ordinary silver halide film by contact printing. Specification 942,639 is referred to.