GB653539A - Improvements in or relating to colour photography - Google Patents

Abstract

653,539. Exposure meters for colour photographic apparatus. HALL, J. M. Aug. 10, 1944, No. 15246/46. Convention date, Aug. 21, 1943. [Classes 97 (i), 98 (i) and 98 (ii)] A densitometer for evaluating a light beam projected towards a surface by a photographic enlarger comprises a mirror which has a light transmitting spot and forms a part of a casing containing a light source of variable intensity, and an optical condenser. This apparatus is utilized in methods of correctly exposing enlargements and colour balanced separation negatives. A transparency 30 is carried in a projector 11, the bellows 17 of which support a filter carrier 18 and a lens and diaphragm 22, the aperture of which is controlled by a rotating member 23. A comparison photometer comprises a casing 24 mounted on the base of the projector containing a lamp 25, reflector 26, condenser 26a, diaphragm and filter support 28 and a hood 33 supporting a mirror 34 which has a small clear spot 35 therein. A diaphragm member 29 with a series of openings of different sizes is mounted on support 28 along with a filter disc 32 carrying red, green and blue filters and a fourth clear opening. The light from the projector passing through the transparency 30 and a selected filter in the slide 19 falls on to a chart placed on the base board 50 of the meter and its intensity upon reflection in the mirror 34 is compared with the light transmitted by the transparent spot 35 and the appropriate filter behind it. The diaphragms are adjusted to cause the spot to disappear, the adjustment values enabling the exposure factor to be calculated. The exposure factors for threecolour filters are obtained by this method, and the three-colour separation negatives are prepared by aid of the projector in exposing sensitized plates or films placed on the table 50. In a modification, Fig. 3, the parts of the densitometer are more compact, the lamp 43 being backed by a reflector 46 and having an iris 47 with scale, a colour-correcting plate 54 and a light diffusing member 54a. The mirror 51 has a small clear spot 52. A scale 53 is mounted on the top of the casing for calculating the exposure for certain film and aperture factors.