Patent GB-762303-A (GB)
Title: Improvements in and relating to apparatus for the optical copying of photographic negatives
IPC: G03B27/80
Filed: 1954-03-29
Abstract: 762,303. Photo-electric exposure timers. LANDER, F. March 29, 1954 [April 1, 1953], No. 9107/54. Class 40 (3). [Also in Group XX] In apparatus for timing the exposure of photographic paper for enlarging from negatives a condenser is charged in accordance with the light penetrating the copying paper and switches off the light source after a period determined by the mean density of the negative. As shown, the unit comprises a base portion 10 housing a photo-electric cell and associated switching mechanism and a hinged lid having two raised edge-portions 13, 14. Part of the lid is a ground-glass screen on which the sensitive paper is placed and the area exposed is adjustable by movable masks 17, 18 having portions which slide in slots 15, 16. Movement of the masks also adjusts the settings of sliders moving over potentiometers 36, 37 situated in the raised portions of the lid thus adjusting the circuit to take account of different areas through which light is received. A flap 22 which presents a white upper surface when pivoted into the dotted position is provided for focusing the image of the negative to be printed. The photo-electric cell 40 situated in the bottom housing immediately beneath the raised portion 13, and a reflecting surface 56 (Fig. 3) is provided which is of decreasing white content as it nears the cell. This surface may have white stripes of decreasing width, while a grid may be situated between the ground-glass plate and the photo cell to permit the uneven passage of light to the cell. Circuit (Fig. 3).-A photo-multiplier tube 40 has its voltage of square wave form connected between the dynode nearest the anode and, through a resistor 49 and condenser 50, the anode itself. The other dynodes are fed with voltage derived from the secondary of a transformer 48 the primary of which is fed with a voltage derived from the network comprising resistors 36, 37, 52, 53. Resistors 36 and 37 are variable to take account of the size of the print while resistor 53 variable by knob 24 (Fig. 1.) takes account of the transmissivity of the printing paper and the sensitivity of the emulsion. Resistor 52 is variable by a switch 33 which is spring loaded to return to a central position so as to arbitrarily increase or reduce exposure time. These controls affect the sensitivity of the photo-tube and hence the photo-cell current for a particular size, sensitivity, and transmissivity of print. In operation, mains switch 26 is closed when subsequent closure of press button switch 34 energizes relays 72 and 75, relay 72 locking over contacts 73. Relay 75 charges over its contacts 76 so as to energize the printing lamp 77 and de-energize the dark-room lamp 78. The dark-room lamp may be switched on if the operator so desires by switch 32. Condenser 50 now charges on alternate half-cycles of the supply voltage finally triggering a glow discharge tube 58 when the potential across the condenser indicates a certain exposure. The pulse produced in the glow discharge tube is inverted in a pulse transformer 60 and triggers a thyratron 64 which energizes an anode relay 67 which opens the anode circuit of the thyratron and de-energizes relays 72 and 75. Relay 75 puts out the printing lamp and switches on the dark-room lamp while relay 72 discharges the charging condenser over contact 57. In the arrangement of Fig. 4 (not shown) the rectangular wave form voltage for the dynodes of the photo-cell is produced by inverting a voltage developed across a resistor in series with the primary of a mains transformer and introducing it into the circuit of the potentiometers supplying the stage voltages.