Patent GB-909743-A (GB)
Title: Shutter mechanism
IPC: G03B9/64
Filed: 1959-05-14
Abstract: 909,743. Photo-electric exposure control. INTERNATIONAL POLAROID CORPORATION. May 14, 1959 [May 15, 1958 (2)], No. 16608/59. Class 40 (3). [Also in Group XX] In a shutter mechanism, the time for which an aperture is exposed by the shutter means is determined by electromagnetically induced forces which act on at least the aperture closing part of the shutter means to retard this part, and which are variable for changing the exposure time. As shown (Fig. 1), an aperture opening shutter blade 34 is pivoted at 38 and is held in abutment with a projecting tab 48 on a aperture closing blade 36 by means such as a weak spring 50. Blade 36 is rigidly fixed<SP>2</SP>to a coil 28 which may rotate about an axis 38 in the magnetic field between a permanent magnet 24 and a flux conducting ring 30. On depression of a release 60, mechanism 54 of the type disclosed in Specification 677,288 and having a rotatable impulse member 56, imparts a rapid rotational movement to an arm 52, which is rigidly fixed to coil 28. As a result, the blades 34 and 36 are rotated for exposure of aperture 22. Due to the current induced in coil 28, blade 36 is slowed down as it rotates and the time of the exposure may be varied by adjusting either manually or automatically (see Fig. 3), the resistance in series with the coil 28. In the arrangement of Fig. 3, a resistance 66 in series with coil 28 is adjusted automatically by a variable impedance control 68 in series with a photo-electric cell 62. In a modification (Fig. 4), a single blade 80 having an opening 82 for exposing aperture 22 for a time depending on the speed of the blade is steadily urged to rotate by a spring 88 but is normally prevented from moving by means not shown. For exposure, the blade 80 is given an initial impulse by a spring 86 and is retarded by electromagnetic forces as before. For use with shutter blades having a translatory motion as opposed to a pivotal motion, a coil 116 (Fig. 6), is arranged to move axially above the internal pole 112 of a permanent magent 110. Alternatively, part 112 above may comprise the magnet and part 114 a flux conducting member. The embodiments may be modified by having the permanent magnet movable and the coil and flux conducting members fixed. In an alternative embodiment (Fig. 12), a coil 266, on a single movable blade, is in series with the normally high resistance of an electronic switching means such as a transistor 262. A second coil 264 is also carried by the blade and it is arranged that the voltage induced in this coil during movement to exposure opposes the voltage of a photovoltaic cell 260. When, during said movement resultant voltage applied to transistor 262 reaches a predetermined value, the resistance of the transistor drops and the main braking coil 266 becomes operative to control the exposure time. In a modification (Fig. 13), the photo-voltaic cell is replaced by a photoconductive cell arranged in parallel with the coil 264. In a further modification (Fig. 14), a transistor 262 is arranged in circuit with a signal winding 266 of high impedance and a second winding 272 of low impedance, both windings being carried by a single pivoted blade for interaction with the stationary magnetic field, the arrangement being such that movement of the second winding causes the transistor to be turned abruptly from a non- conducting state, in which the winding 266 is open circuited so as to be unable to provide a retarding force for the blade, to a conducting state, permitting current to flow through the winding 266 and a retarding force to be applied to the blade. As an additional means for adjusting the exposure there may be provided (Fig. 10) an iris mechanism 200 which comprises blades 202 pivotally mounted at 204 and 208 for adjustment by a drive ring 210. On displacement of a bar 240 which also releases a shutter mechanism 234 of the type shown in Fig. 1, a latch element 238 releases a spring-urged fly-wheel 224 which rotates and by means of an edge 226 thereof striking a surface 228 of a latch 218, also releases the driving ring 210. The ring 210 then rotates, due to tension of a spring 212, and the aperture formed by blades 202 in front of the exposure aperture 206 gradually increases, the size of the aperture attained when exposure takes place depending on the time to exposure of mechanism in 234. A variable aperture means may also be included in any of the other mechanisms having two blades by providing the aperture opening blade with a slit of gradually increasing width, the maximum width attained during exposure then being determined by the speed of the aperture closing blade. Specification 857,410 is referred to.