Patent GB-788920-A (GB)
Title: Improvements in or relating to exposure control devices for photographic printing apparatus
Inventor: NEALE DENIS MANKTELOW
IPC: G03B27/16
Filed: 1954-11-18
Abstract: 788,920. Photo-electric control of photographic printing. ILFORD, Ltd. Nov. 16, 1955 [Nov. 18, 1954], No.33488/54. Class 40(3). [Also in Group XL (c)] Photographic printing apparatus in which the average intensity of the printing light transmitted through the negative and the printing paper is measured continuously during the exposure interval comprises means for causing the time integral of the transmitted light needed to cut off the printing light to increase with the time necessary to correct exposure of the negative so that a degree of compensation is obtained for the effect of after-glow of the printing lamp with short exposure times or of any other lag in switching off the printing light, e.g. in a shutter. As shown in Fig. 1, the light transmitted through the negative and printing paper is sensed by a photo-cell V1 connected through a potential divider R1, R2 to a source of positive potential and having its cathode connected to a capacitor C1 and to the trigger electrode of a cold cathode gas discharge valve V2 arranged to operate a relay A3 energized through a switch S. An exposure is initiated by opening switch S to de-energize relay A3 so that relay contact a3 thereof closes to switch a printing lamp L while relay contacts al and a2 open to charge up the capacitor C1 and shortcircuit a bias feed resistor R6. This causes a rise in the cathode potential of valve V2 necessitating the accumulation of a larger charge on the capacitor C1 before valve V2 is triggered to operate relay A3. The rise in cathode potential is delayed by a capacitor C4 so that the charge on capacitor C1 required to trigger the valve increases with the negative density. When valve V2 fires the relay A3 is re-energized so that contact a3 opens to switch off the lamp L and contacts a1, a2 close. In another form, valve V2 may be a thyratron having the capacitor C1 connected directly between its grid and cathode, this thyratron being connected in circuit so that after the commencement of an exposure period the anode voltage tends to fall, this fall being delayed by a capacitor so that the thyratron is more sensitive to triggering at the beginning than at the end of an exposure interval. A further form is shown in Fig. 3 comprising two pentodes V3 and V4. Before an exposure is made, double throw switch S is closed to short-circuit capacitor C1 through resistor R4. Valve V3 conducts and the voltage developed across resistor R11 maintains the cathode of valve V4 at a positive bias, but, owing to the large voltage across resistor R10, the grid of valve V4 is sufficiently negative to prevent this valve conducting. When switch S is opened, the capacitor C1 charges and the positive bias on the grid of valve V3 is removed. Valve V3 thus ceases to conduct, but the potential on the grid of valve V4 rises until this valve conducts and energizes relay B1 to close switch b1 and energize the printing lamp L. Valve V3 remains cut off until capacitor C1 has been charged to a potential slightly below that of the cathode of the valve when valve V3 conducts, a capacitor C5 being so connected as to necessitate a progressive increase of the charge on capacitor C1 needed to trigger the circuit. When valve V3 conducts the valve V4 is cut off to de-energize relay B1 and switch off lamp L.