Patent GB-608134-A (GB)
Title: Improvements in or relating to photographic printing apparatus
IPC: G03B27/72
Filed: 1945-08-02
Abstract: 608,134. Photo-electric exposure-determining means in photographic printing machines. REID, J. B., and BRIGHTMAN, C. G. Aug. 2, 1945, No. 19899. [Class 40 (iii)] [Also in Group XX] In apparatus of the kind comprising a station for grading the negative in terms of its mean density and a printing station, the intensity of light from the grading lamp is adjusted by a variable series resistor until a standard response is obtained in a light-responsive device, and said resistor is subsequently connected in series with the printing lamp when a print is made, means being provided for compensating for differences in spectral sensivity between the light-responsive device and the printing material, so as to obtain prints of constant mean density from negatives of differing mean densities. The operating mechanisms of the apparatus comprise a series of lamp-intensity controlling resistors which are switched successively by selector mechanisms into the grading and printing lamp circuits in step with the passage of negative from grading station to printing station. As shown, a strip of negatives carried by a supply spool passes beneath a roller to the negative density grading station which comprises a ground glass screen, beneath which are the grading lamps, out over a second roller and under a third roller into the printing station which it leaves under a roller and passes to the take-up spool. The printing paper, which is also carried on a spool and which is driven by means incorporating a motor, passes beneath the printing head where, during printing, it is pressed against the negative by a pad, and then into processing baths. The operation of the electrical controlling means, starting with the apparatus at the beginning of a cycle. as in Fig. 2, is as follows: with the negative size switch set to NS1, the apparatus is ready to deal with negatives of such size that the first negative is in the printing head, and the fourth negative is at the grading station with two idle second and third negatives between the two stations. On connecting the primary of T1 to the A.C. mains, the grading lamps GL1, GL2 and the variable resistor pilot lamp GRL1 are lit and the left-hand coil of the relay A and the relay coil A<1> are energized by the D.C. supply taken from bus-bars BB1, BB2, thus closing contacts A<1> and A<1>1, A<1>2 respectively, the latter pair switching resistor R into the grading circuit and resistor R2 into the printing circuit. A photo-cell 6 is then placed above the negative in the grading station and the cell current is brought to its standard value by varying R1 and hence the intensity of the grading lamps, grading the fourth negative. The first negative is then printed by.closing the switch S1 with the resistor R2 used to grade the negative in series with the printing lamp PL. Simultaneously, the A.C. relay M is energized, closing contact M2, making the circuit to the right-hand coil of the relay F before breaking the circuit to the movable contact of the relay and thus closing contacts F1, F2, which latter lights the print number indicating lamp P1. At the end of a predetermined time, S 1 opens, de-energizing relay M and hence the right-hand coil of relay F whilst energizing the left-hand coil of relay F so that contacts F, F2 remain closed throughout and also energizing the righthand coil of relay A. Successive closures of S1 cause successive exposures and cyclical lighting- up of print number indicating lamps P1 ... P5. When the desired number of exposures has been made, the step on key is closed, energizing the slow-acting relay X which closes X2, energizing relay coil E which closes contacts E1, E2 and makes the contact X1 before breaking the circuit to contact M2, cutting out the relays F ... K and extinguishing the print number indicating lamps. Closure of E2 de-energizes left-hand coil of relay A, energizes right-hand coil of A, lights lamp GRL2 and energizes lefthand coil of B which closes contacts B2, B3, the closure of the latter breaking the circuit to the pilot lamp GRL1 so de-energizing relay A1 and hence opening contacts A<1>1, A<1>2; and closing B<1>1, B<1>2, connecting R3 into the printing lamp circuit and R2 into the grading lamp circuit. If the switch S1 is still open when S2 is closed, the contact M1 remains open and the operation of relay E is delayed until the printing operation is complete, when M1 closes. The negatives are then wound on one frame and the sequence repeated employing the relays B, B1, resistor R2 being used to grade the fifth negative and R3 to print from the second negative. Similar operations apply to the printing and grading of the third and sixth, fourth and seventh negatives after which the apparatus is again in the initial condition illustrated. For dealing with larger size negatives with which there is only one idle negative between the grading and printing stations, the negative size switch NS is moved to position 2, cutting out resistor R4 and its associated relays. To compensate for differences in the spectral sensitivity of the photo-cell-chiefly sensitive to green if of the selenium barrier layer type-and the printing paper-chiefly sensitive to blue-the wattage of the grading lamp is made larger than that of the printing lamp so that on placing the same resistance in the two circuits the colour temperature of the grading lamp is decreased by a greater amount than that of the printing lamp. For fine adjustment of the spectral compensation, a resistor R5 is connected in parallel with the grading resistors R1 ... R4 when these are switched into the grading circuit. To allow adjustment of the mean print density, a variable resistor R7 is connected in series with the grading resistor when this is switched into the printing lamp circuit, and to maintain the spectral matching previously achieved a variable resistor R8 is connected in parallel across the resistors R1 (or R2, R3, R4) and R7 in the printing circuit. The calibration of the apparatus is described in the Specification. Print density may also be varied by changing the galvanometer sensivity control GSC, keeping the sums of the logarithmic values of R7 and GSC constant.