Patent GB-863606-A (GB)
Title: Apparatus for the photographic printing of colour transparencies
IPC: G03B27/73
Filed: 1957-03-22
Abstract: 863,606. Photo-electric colour printing. AGFA A.G. March 22, 1957 [March 24, 1956], No. 9561/57. Class 40(3) [Also in Group XX] Apparatus for the photographic printing of colour transparencies by three successive partial exposures each in one of three additive primary colours has a photo-electric cell 13 for regulating each exposure and a regulating circuit which incorporates three preset adjustable potentiometers 55, 58, 59 to vary the effect of the photocell at each exposure. Light from a printing lamp 4 passes through apertures in three control discs 16, 17, 18 mounted on the shaft of a motor 14 to fall on printing material (not shown) lying on a translucent focusing screen 11. Each of the control discs 16, 17, 18 contains four circular apertures, three of the apertures in one of the discs containing filters of the primary colours used. The other aperture and all the apertures of the other discs are transparent. Cams on the circumference of the discs engage fixed contacts (not shown) while rotation of the disc containing the coloured apertures is regulated by an electromagnet 27. Light from the lamp 4 passes at each exposure through the printing material and screen 11 to be reflected at 12 on to the photo-cell 13. In operation the discs are initially arranged to present a transparent aperture. The potentiometers 55, 58, 59 are preset in accordance with the sensitivity to each colour of the photo-cell and the printing material and having regard to the colour distribution of the negative. The switch 39 is closed, a capacitor 57 is short-circuited and the grid of a valve 40 is at cathode potential. A key 46 is then pressed to energize a relay 41 operating contact 47, closing contact 48 and opening contact 49, Relay 41 is de-energized when a capacitor 43 is discharged, returning contacts 47, 48, 49 to their initial positions. A resistance 51 prevents immediate re-energization of relay 41 should the key 46 be depressed longer than necessary. During this period the electro-magnet 27 is briefly energized liberating all three control discs so as to introduce the blue filter of the disc 16 into the path of the light. During rotation a cam on the disc 17 closes a contact 33, energizing a relay 42 to open a contact 52 and, by closing a contact 53, light up the printing lamp 4 to commence the blue exposure. At the same time a cam on the disc 18 closes a contact 36 whereby the potential from a transformer 54 controlled by the preset adjustable potentiometer 55 is applied to the primary winding of the supply transformer 56 of the photo-electric cell 13. When the capacitor 57 has been charged by cell 13 and the grid of the valve 40 has attained a predetermined potential, the valve 40 ignites and energizes relay 41 to operate contacts 47, 48, 49, shortcircuiting capacitor 57 and extinguishing the printing lamp 4. The process is then repeated to bring the green and red filters consecutively into line. Control discs 16, 17, 18 can be housed outside the enlarger head or the coloured apertures arranged vertically above each other in filter holders (Fig. 6, not shown). In an alternative embodiment the potentiometers 55, 58, 59 are replaced by a variable capacitor 57 with preset mechanical means to effect the variation.