Patent GB-307181-A (GB)
Title: Improvements in or relating to apparatus for controlling the exposure of a unit to asource of electric light
IPC: G03B27/16
Filed: 1928-02-08
Abstract: 307,181. Pictorial Machinery, Ltd., and Clock, B. Feb. 8, 1928. Projection - < >printing apparatus. - Apparatus for controlling the exposure of a photographic negative o r positive to a source of electric light, particularly in step-and-repeat projection - p r i n t ing machines, comprises a member controlling the opening and closing of a shutter in such a way that the product of duration of exposure and intensity of illumination of the unit by the source of light during exposure is constant. This constancy is obtained by operating the shuttercontrol member by an electric motor &c. which is energized by the same source of supply as the light, the shutter being held open for a predetermined number of revolutions of the motor. The shutter-control means comprises a slidable bar 23 which has a slot 24 co-operating with a fixed pin 125 to limit its movements and is connected by a Bowden wire 124 to the shutter. A starting-key 25 which can be depressed against a spring 27 is connected to the bar 23 by a weaker spring 28 so as to urge the bar to move to its shutter-opening position when the key is depressed, and in the opposite direction when the key is released. For retaining the bar 23 in either operative position, two pawls 30, 31 are provided which are mounted on a common pivot 33, are connected at their outer ends by a spring 35, and coact with a stop 34 on the bar 23. The pawls are operated by an electromagnet 21, the pivoted armature 22 of which carries an arm 39 connected by an arm 32 to a lever 36 which is mounted on the pivot 33 and has a pin 38 extending between the pawls. The means for timing the exposure comprises a shaft 45 which can be coupled to the driving electric motor by a clutch 46, 47 and is geared to a spring-controlled timing- shaft 48 through another clutch 49, 50. The shaft 48 carries a pointer 51, Fig. 6, movable over a dial 520 away from an adjustable stop 53,. and a switch arm 54 adapted to operate contacts 55, 56 to return the bar 23 to its shutter-closing position when the pointer 51 reaches the zero of the scale 152. The movable part 47 of the clutch 46, 47 is operated by a lever 58 secured to a shaft 60 which also carries a lever 70 having at its outer end a pin 71 extending into a slot in the slide-bar 23. The shaft 60 also carries an arm 77 operating the lever 78 connected to the movable element 50 of the clutch 49, 50 in such a way that when the bar 23 moves into its shutteropening position the clutch 49, 50 is engaged. The upper end 79 of the lever 78 carres a pin 88 supported in slots in a bracket 89, Fig. 5; a plunger 90, Fig. 9, bears on this pin under the action of a spring 80. A bell-crank lever 82, 83 is formed with a detent 81 adapted to engage the end 79 of the lever 78 and normally held in this position by a spring 191. The shaft 48 carries a cam plate 89 which releases the detent at or just before the pointer 51 reaches its zero position. Fig. 11 shows the electrical connections. The motor 15 is connected to the same supply as is the lamp in the light box and is associated with two sets 91, 92 of three contact-springs, of which each of the centre members 93, 94 has two contacts co-operating with the contacts 95, 96 and 97, 98. The member 93 is operated by the key 25 and the member 94 by the armature 22. In the rest position, the magnet 21 is shunted by the contacts 95, 93 and 94, 97, and the motor circuit is open. Partial depression of the key 25 closes contacts 93, 96 without opening contacts 93, 95, and thus completes the motor circuit without energizing the electromagnet 21 provided that the contacts 101, 102 controlled by the circuit control device 103 are closed. Further depression of the key separates the contacts 93, 95 and causes the magnet 21 to be energized. The electromagnet then maintains the motor circuit closed at the contacts 94, 98 so that the motor will continue to run when the key 25 is released until the magnet is shortcircuited by the contacts 55, 56 operated by the switch-arm 54, Fig. 6, on the shaft 48. In the event of a failure of the electric supply, the movement of the timing- device is arrested by the control-device 103 which opens the contacts 101, 102. The load on the motor can be varied by an adjustable brake 115, Figs. 3 and 4, bearing on a disc 116 on the shaft 45. By adjusting this brake the speed of the motor can be controlled so that the product of the duration of each exposure and the intensity of illumination during exposure may be a constant in spite of fluctuations of the source of light during the exposure.