The Cine Technician (1939)

199 T h ]•; N B-TE C H N I C I A N March-April, 1938 Summary of Colour Cinematography By Dr. F. W. PETERSEN, Agfa's Colour Expert. According to the Helmholtz theorv of colour, the visible region of the spectrum maj be divided into the three primary colours — red, green and blue, suitable admixture of these three primaries yielding at will practically any desired colour hue or neutral grey tone. Thus, by using a photographic process based on three colours, a natural colour reproduction may be obtained. A considerable number of different lines may be produced by using two primary colours, which must be complementaries if a correct neutral grey scale is to be obtained, but the use of two colours can never give a truly natural reproduction of colour. The restriction of the colour process to two primary colours, though, reduces the technical and economic difficulties in film work so considerably that in spite of the faulty reproduction of colours, such two-colour processes are frequently used for cinematography, while the full three-colour processes, which have been used for many years in "still" photographic work, and for 16 m.m. sub-standard films, are now only just ready to be applied to the 35 mm. film. The following review of processes will be limited to the confines of cinematography, and all comments refer to three-colour processes. The problems of colour cinematography can be reduced to the following three fundamental questions : 1. How are the multifarious hues of nature to be recorded in terms of three primary colours'.' (Taking or recording process). 2. How are these three records to be assembled into a suitable coloured positive for projection? (Projection or reproduction process). '■'>. What about the possibility of duplicating the primary records? The Recording Process in the Camera The simultaneous production of three colour separation records may be arranged in a number of ways, namely : 1. The beam-splitting method, which, it must be admitted, has not yet developed to a point where it works perfectly in practice. 2. The multi-layer method by using emulsions which are selectively sensitised. Examples are the Agfa Bipack film (for two colours), and the 16 mm. three-colour processes of Agfacolour and Kodachrome. In the last two cases, the three emulsions are coated over one another and cannot be separated mechanically, as they form a single film. 3. The Teclmicolour method which combines the beam-splitter and bi-pack principles. 4. The lenticular film process, first invented \>\ Berthon-Keller-Dorian, and developed commercially by Agfa (Agfacolor), Kodak (Kodacolor), and Siemens (Opticolor). .'>. The Agfa lenticulated bipack process, combining the principles of lenticulation and bipack. A number of other possible combinations need not concern us here. 1. — Beam-Splitter Processes The lipiht passing through the lens of the camera is split up by a special optical system of prisms or mirrors into three sections, a filter being placed in the path of each beam which transmits one of the primaiy colours, This filtered beam of light is then recorded in the usual way on a panchromatic film. i.e.. one sensitive to all colours A record of the red primary is obtained behind the red filter, and blue and green records behind the filters transmitting these colours. The optical and mechanical problems involved in the construction of a successful beam-splitter camera lor three-colour records particularly so far as time or special parallax is concerned have not yet been satisfactorily solved so far as tin. cinema is concerned. Two-colour beam-splitter devices tor two-colour processes are much more satisfactory (cf. the Buscb beam-splitter), and a device of this type is used in the Technicolour camera. 2. — Multi-layer and Bipack Method The separation of the colours in these processes is produced by using selectively sensitised emulsions, which may be separated by filter layers. The various emulsions and filter layers may either be coated one over the other on the film base, thus forming a solid whole which one may term a "monopack," or else they may be coated on separate film bases, and placed together with the emulsion sides in contact (bipack or tripack). An example of the "monopack" process may be seen in the new Agfacolour film, which has three emulsions forming one mechanical whole. A red-sensitive emulsion is coated on the celluloid base, over it a green-sensitive emulsion. Over this is coated a yellow filter layer, and on top of all a blue-sensitive emulsion. Agfacolor film is only being marketed for the present as an amateur material in 16 mm. gauge. The exposed film is treated by a special reversing process and becomes a positive, and the three emulsions sensitive to red, gr< en and blue are dyed in a coupling developer to the complementary shades of blue-green, magenta, and yellow respectively. The same film is thus used in the camera and in the projection — but, in principle, it will also be possible to use separate films for taking and projecting, by developing one Agfacolor multi-layer film exposed in the camera into a complementary coloured negative, printing on a second Agfacolor film on which by colour development the colours yellow, magenta and blue-green are. produced. In cinematographic practice considerable use has been Fig. 1. Feeding mechanism of a Bipack Camera