The Cine Technician (1939)

March-April, L938 T HE ( I N I'. -TECHNICIAN 0( (O Subtractive Cclour Processes All subtractive colour processes produce their effects, not l>\ coloured or optical accessories such as screens or filters placed in front ot the emulsion, but b\ a dyeing dt the emulsion itself. A simple example will show the general method used in subtractive colour production. Prints are made oi three negatives which have been exposed behind red, green and blue priman filters. II these prints are projected bv a triple lantern on a screen so that the images coincide, and the filters used for exposure placed in front of them, an additive colour picture results. If. however, the black silver image of each positive is converted h\ a dye-toning or a mordanting process to a colour complementary to that oi the filter used in taking the negative (red, green and blue filters corresponding to blue-green, magenta and yellow prints), and if the three prints are laid over each other in register and the composite images projected by white light, then a coloured image is produced bv the subtractive method. (Charts I I 1 and IV.) The coloured screen image is produced by the dyes in the prints subtracting from the white light of the projection lamp all colours which the original object did not possess. A red surface, for example, would be colourless in the print made from the red filter negative, but in the prints from the bine and green filter negatives it would be strongly darkened on development, and would thus be magenta and yellow in the dyed prints. Yellow and magenta together absorb blue and green from the white light, only red light passing through them: the red of the original is thus produced on the screen. Where all three dyes are superimposed in the three prints, no light passes and the screen image will be black (cf. Chart IV). In practice, this arrangement o| three films with dil ferenf linages placed over each other would never be used for subtractive colour reproduction. The actual method in use iua\ be roiighh classified as iollows : 1. So kmg as two priman colours onh are used (usual 1\ exposing on bipack film), the separation negativi s are printed on either side oj a positive film with a sen S I t i V C C I II II I S I ( I I I <>ll Cadi Side | \ M | ; | | ) I | M , 1 1 I i I I I o f e \ a 1 1 1 p I e I . with the positive images in proper register. The changing oi the silver positive images to dye linages ma} be done bv ordinary chemical toning methods (iron and uranium baths) or actually bv the dye-toning, i.e., mordanting processes using silver iodide or copper salts and subsequent dyeing with basic dyestufrs. (Chemical and dye toning processes) '2. The separation negatives are printed, the prints developed as gelatine rebels. The rebel films or matrixes being rolled up with three Pinatype dyes and printed on a silver-free emulsion bv a dye-transfer or imbibition process. (Technicolor process). .'i. —Positives are made from the separation negatives and these are printed on to a special material. Agfa Tripo film, which has one emulsion on one side and two emulsions on the other, these three emulsion layers being sensitised to three different spectral regions. The single emulsion on the one side is dyed blue, while the upper emulsion on the other side of the films is lived yellow and the under magenta. Each separation print is printed on to the emulsion whose colour is complementary to the filter colour used to produce it. The printing light is adjusted in each case by a filter to the spectral sensitivity oi the particular printing layer in question at the time. A iter develi >pmg and fixing this film, the silver images MOLE RICHARDSON'S HIRE SERVICE From 500-watt Spots to 36 in. Sun Arcs Sound Booms Generators Fog. Wind and Rain Machines Everything in Auxiliary Equipment 24-HOUR SERVICE PHONE: WILLESDEN 68Z4 & 5 (DAY) ELSTREE 1513 (NIGHT) GRAMS: " MOLERENG. HARLES. LONDON n H.I. Arc Works LONDON. N.W.10