Patent GB-1156061-A (GB)
Title: Light Projecting Device for use on an Automatic Plotting Machine or the like
IPC: G01D15/14
Filed: 1966-06-17
Abstract: 1,156,061. Optical apparatus. GERBER SCIENTIFIC INSTRUMENT CO. June 17, 1966 [Oct. 1, 1965], No.27197/66. Heading G2J. In apparatus wherein a spot of light from a projector is movable over a light-sensitive surface 212, Fig. 13, by moving the light projector in a plane parallel to the surface 212, the intensity of the spot of light is controlled in accordance with its linear speed with respect to the surface 212 so that the degree of exposure per unit area of surface 212 remains constant whatever the speed. The light projector may be mounted on the carriage of an automatic plotting machine for movement in two co-ordinate directions, and for standing still, under the control of a computer so as to record or reproduce data, draw lines, or produce dots, letters, numbers, symbols or other characters. The light source may be energized with a voltage which is varied in accordance with the speed of movement of the carriage, Fig. 14 (not shown). Alternatively the light from source 210, Fig. 13, may be focused by lenses 24a-24c to form an image at an aperture 41 in a fixed plate, a shutter arm 46 controlled by a rotary solenoid 48 being selectively movable to allow light to pass through or be blocked at the aperture 41. Adjacent the shutter is positioned a variable density filter disc 56 whose angular position determines the filter density and thus the intensity of the light transmitted therethrough, the angular position being controlled by a motor 82 through driving gears and a cam and slot arrangement, the motor 82 being fed with the output of a device (85, Fig. 2, not shown), adding vectorially the outputs of a pair of rate sensors (83) which are responsive to the speed of the carriage in the two co-ordinate directions. The light after passing through filter disc 56 is reflected by a cold light mirror 92 to pass through condenser lens 91, a character selection wheel 98 and projection lenses 30, 31, to the surface 212. The mirror reflects only a narrow band of wavelengths so that not only are the heat rays eliminated but also the wavelengths of the light projected on to surface 212 are matched to a desired point on the film sensitivity curve to make the exposure time less critical and also reduce the colour correction necessary in the projection lens system. The character selection wheel 98 has a series of apertures 95, Fig. 4, each of which is covered by a plate 100 provided with an appropriately shaped aperture. For the symbols b, which are intended to be projected when the light projector is stationary, provided that the light illuminates the symbol uniformly, the exposure at the film surface will be constant so that either the symbols b have no filter associated therewith or alternatively if a filter is provided it will be similar for each symbol. With the symbols a, however, which are intended to be projected when the carriage is moving so as to draw lines, the exposure will increase with the thickness of the line, i.e. with the size of the symbol a, and so a filter 105 is provided for each symbol a to ensure that the intensity of the light projected on to the film is greater, the thinner the line. The wheel 98 is connected by gears 104, 106, to a commutator which produces a signal indicative of the angular position of the wheel. The computer providing a command signal for wheel 98 thus compares the position of the wheel with its desired position and drives the wheel 98 through motor 114 and gears 113, 104, until the appropriate position is reached, exact location being ensured by a pawl 116, which, when pivoted about shaft 121 towards the wheel 98 engages one of a series of pins 120 by its notch 119. Instead of the wheel 98 having numerous symbols, differing in size it may be provided only with different shaped symbols, variations in size being accomplished by replacing the fixed focus projection lenses 30, 31 by a zoom lens under the control of the computer. In another form the wheel 98 is replaced by a variable aperture mechanism consisting of a pair of sliding plates (188, 190, Fig. 11, not shown) defining a variable aperture (196). The light projector may be adjusted in height relative to the carriage, which is movable in the two coordinate directions, by a pair of knobs 124. This enables accurate focusing of the projector to be made without the sensitive film in position the projector then being raised by an amount, as indicated on a dial indicator 140, which corresponds to the thickness of the film.