Patent GB-576725-A (GB)
Title: Photographic scanning method and apparatus for the production of orthogonal photographs of three-dimensional objects
IPC: G03B19/02
Filed: 1943-08-10
Abstract: 576,725. Photographic map reproductions. AERO SURVEY CORPORATION. Aug. 10, 1943, No. 12935. Convention date, Aug. 8, 1942. [Classes 98 (i) and 98 (ii)] An orthogonal photograph free from parallax errors of an object whose surface elements do not lie in a single plane is produced in a camera adapted to photograph at any instant only a small part of the object, progressive relative movement being produced between the camera optical system and the object to scan the object and a light-sensitive element being maintained in fixed optical relationship with the projected image to photograph progressively an image of each part of the object on the corresponding part of the light-sensitive element, the optical axis or axes of the optical system being maintained parallel to a fixed axis or axes. For an image of equal size, the light-sensitive plate may be maintained stationary and the object moved at twice the speed of the lens, or the object maintained stationary and the plate moved at twice the speed of the lens, or the lens may be maintained fixed and the plate and object moved at equal speeds in opposite directions. An. image erecting optical device, (e.g. a pair of erecting prisms), may be used and the plate and object maintained stationary and the lens and erecting system moved together as a unit. Formulae are given for the movements in the case of reduction or enlargement. The object may be photographed by an optical system including lenses in telecentric relationship and having a field of view smaller than the extent of the object to be photographed. As shown in Fig. 9, a scanning camera comprises an electric motor provided with reversing control mechanism, vertical standards 102, 103 supporting upper and lower horizontal guide rods 106, 107 on which is mounted a frame 108 carrying a carriage 116 for the objective on vertical guide rods 114, 115. The square rod 115 carries spaced notches 119. Mounted in the carriage 116 is the objective mount 121 fixed to which is a tube projecting torwardly towards the object and carrying at its outer end a telecentric lens and also a tube projecting rearwardly in the direction of the plate. A frame 127 sliding on rods 106, 107 carries guide rods 133, 134, the latter being square and provided with notches 135, sliding on which is the plate carriage 136. The carriage 136 carries a frame 139 receiving a holder tor sensitized plates or films, and is connected to the end of the rearwardly projecting tube by a light-excluding bag or bellows. The frames 108, 127 are driven by feed screws 143 and 145 respectively, the screws (of different pitch) being driven at uniform angular speed from the motor 104. The carriage 116 is fed vertically by a reciprocating rod 151 having transverse notches engaging a pawl on the carriage 116, which also carries a second pawl engaging the notches on 119. The rod 151 is normally urged downwardly by gravity and a spring 160, and carries a roller 161 engaging a cam 162 adjustably mounted on a rail 163. Every time the frame 108 makes a complete reciprocation, the carriage 116 is raised through a distance of one notch on 119. The carriage may be returned to a lower position by retracting the pawls manually. A similar arrangement is provided for the vertical stepwise feeding of the plate carriage 136. Exposure of the plate is made only during the travel of the frames from right to left, although the arrangement could be made to permit of exposure during both directions of travel. Exposure of the plate during the return travel is prevented by a shutter operated by a cable release itself operated by a slidable rod carrying a roller engaging the cam 162 for closure and a cam 179 for opening. The reversal of the motor may be controlled by hand switches or alternatively by an arrangement involving a brushshifting solenoid controlled by a switch 186 tripped by stops 187, 188 at the ends of travel. Safety switches 189a 189b are provided to cut out the motor in the event of failure of the reversal mechanism, and a similar safety switch 189c is provided to arrest the upper movement. The motor circuit is provided with a rheostat regulating the speed of travel of the frames and carriages. For adjusting the line of juncture between successive strips an adjustable mask is placed adjacent the telecentric lens. The mask, Fig. 17, may comprise two mask plates 192a, 192b slidable in a frame 193 by screws 194 and having pointed notches 195 to provide a hexagonal opening. The opening may alternatively be a rhombus or trapezium. This mask may be replaced by an adjustable mask in the tube extending rearwardly from the objective mount. In a modified scanning camera used in association with a number of multiple projector heads, the optical tube carrying the telecentric lens projects laterally beneath the projector heads and over the model and is provided with a reflecting prism. A switching device is operated by the travel of the carriages so that only that projector is in operation at any given moment in which the shadow effect of the optical tube.is obviated. Specifications 530,331 and 553,299 are referred to.