“NEW THREE-COLOR CAMERA
By Jack H. Coote
British Tricolour Processes, Ltd., London, England
Summary— There is described here a beam-splitter color camera which utilizes the principle of a single-reflection prism block to expose three films in the combination of “bipack and one”.
DESPITE MANY prophesies made, to the contrary, during the past decade, present indications are that the importance of color cameras in association with subtractive color printing processes is likely to continue.
It was because of a firm belief in the superiority of the “three-strip” method of color photography that British Tricolour, after developing its subtractive printing process to a pilot-plant stage in 1946, decided to design and build a beam-splitter color camera, the outline elevations of which are seen in Figs. 1 and 2.
The camera decided upon utilizes the well-known principle of a single-reflection prism block to expose three films in the equally well-known combination of “bipack and one”.
The two film transport mechanisms are of the Vinten “Everest” type and are standard black-and-white movements both threading from the same relative direction.
Employment of two standard movements effects obvious economies, but involves certain problems of design. It will be clear from Fig. 3 that it is likely to be difficult to thread No. 2 gate while the movement is in its operating position; particularly since with a gold dividing surface to the prism block the bipack is used with greatest light efficiency in the reflected light beam.
In order to avoid such threading difficulties it was decided to make the whole of No. 2 mechanism a movable assembly, swinging into and out of its operating position as required. It was considered necessary to be absolutely certain that the design and construction of such a “swinging gate” would prove satisfactory under all working conditions.
Various measurements, therefore, were made on the camera in order to test the consistency of register of the relevant moving parts of the camera mechanism, and also to check for variation of register between the two film gates at various temperatures.
The swinging gate was repeatedly opened and closed, measurements of position being made at each closure after clamping tightly with the clamp. A suitable target was mounted on the gate and viewed with a microscope mounted on the fixed frame of the camera.
Movement in a vertical or horizontal direction did not exceed 0.0001 inch.
Measurement of angular position was made with an autocollimator and reflector. An angular variation of 1/2 minute about a vertical axis was obtained, because of slight variations in the clamping pressure. This amount represents about 0.00015 inch over the width of the film gate.
The method of locating the prism block with a dovetail slide and end stop, gave location both for angle and position, with variations of not more than 0.0002 inch.
A crossed line target graticule was mounted in each film gate so that the images were approximately superimposed when viewed through the prism system with a long-focus microscope.
The whole camera was placed in a refrigerator and allowed to settle to a steady temperature of approximately zero degrees centigrade.
The camera was then removed and the microscope was set up to view the targets through the prism; measurements were then made of the relative positions of the targets both horizontally and vertically.
The temperature was gradually raised to approximately 100 degrees Fahrenheit (38 degrees centigrade), microscope observations being taken at intervals. Some difficulty was experienced at the lower temperatures because of condensation on the glass surfaces, but variations measured in either direction did not exceed 0.0005 inch.
The way in which the No. 2 assembly can be swung away from and returned to its operating position should become apparent after reference to Fig. 4. The main drive shaft runs along the base of the camera, driving No. 1 movement through a combination of helical and spur gears and No. 2 movement via a train of three gears located ahead of both the mechanism and the shutter. The outer surfaces of the forward bearings of the main shaft provide the journals on which the whole of the movable assembly is pivoted.
It will now be realized that although the closed or exposing position of the movable gate has been referred to as its operating position, the mechanism can, in fact, be rotated while in its open or loading position, a feature which is essential for efficient threading.
The bearings at the base of the movable assembly operate in conjunction with a tongue at the top of the gate which enters a slot for lateral location and butts against an anvil set at right angles to the direction of swing. The assembly is locked into its closed position by means of a spring catch and single thumbscrew.
Accurate placement of the two film-moving mechanisms of a color camera represents only a part of the problem of ensuring precise registration between the images formed at the two gates. Since it is essential in practice for the prism block to be removed easily for cleaning, means must be provided for replacing it quickly with at least as high a degree of accuracy as that set for any other associated movable component.
The most convenient method of checking the position of the prism block in relation to the two exposure apertures is to illuminate two identical target images while located in the gates by the register pins, and to view them from the lens position by means of a simple microscope. The target images may be made photographically upon normal film stock, or may consist of a pattern of holes drilled coincidentally through two strips of thin metal carrying accurate negative perforations.
Except when the prism block is perfectly adjusted, the two targets will not appear coincident when viewed through the microscope, and it seemed desirable that any adjustment of the prism block should be observed continuously through the microscope without the necessity of removing the block from the camera in order to effect adjustment in any of the desired directions.
The adjustable mount which was adopted is shown in Fig. 5. There are three directions of movement provided (Fig. 6), two rectilinear, A – A1 and B – B1, and one pivotal C – C1. All these movements are controlled by micrometer screws and locked with setscrews.
The three prism-block movements, together with the prior location of the two exposure planes in a precisely vertical position at right angles to each other with registration pins in a common horizontal plane, provide the means of obtaining perfect registration.
It has already been mentioned that the reflecting surface of the prism block is metallized with gold in order to take advantage of the well-known dichroic effect which is peculiar to thin films of that metal.1 With the gold surface a single high-speed orthochromatic emulsion is exposed behind a yellow filter in the transmitted beam while a bipack with a high-speed noncolor-sensitized element in front and a highly red-sensitive element in the rear provides the blue and red records, respectively. The emulsion of the front element of the bipack carries a red-filter layer.
An unavoidable result of inserting any light-dividing means, except rotating reflectors, between the camera lens and the film planes is a serious increase in the minimum separation which it is possible to have between the rear element of the lens and the film planes. As this minimum distance is usually about 50 mm, even the advantage to be gained from the use of a glass with a high index of refraction for the prism block does not reduce the effective distance sufficiently to permit the use of normal 25- or 35-mm lenses.
This difficulty had been met by the use of lenses of the negative-telephoto type, which have an effective focus considerably less than their back focus.2 Unfortunately this solution of the problem gives rise to distortion and cannot, therefore, be considered satisfactory. With this in mind, C. G. Wynne of Wray, Ltd., succeeded in computing a 35-mm objective without recourse to a supplementary lens,3 although a negative supplementary still has to be tolerated for a lens of 25-mm effective focus.
However, it cannot be suggested that the performance of the objective is the only factor influencing definition in any camera using bipack, for the inferior resolution which results on the rear element of any bipack sets a most serious limitation upon the definition of the final composite print. Loss of definition from this cause cannot be avoided, but it can be reduced to a minimum in two ways: by the use of the most suitable emulsion (coated at an optimum coating weight) for the front element of the bipack, and the maintenance of the most perfect contact obtainable between the two films of the pack consistent with freedom from excessive drag or scratching during exposure.
Both roller pressure pads and solid-metal pressure pads with slightly convex surfaces have been and are being used,4 but two glass pads having a 0.003-inch crown were chosen for the British Tricolour camera. This means of providing good contact between the two films has resulted in the rear element resolving 25 lines per mm, at the same time as the front element resolves 45 lines per mm.
Focus adjustment may be made at the camera by means of a control knob, or remotely by means of Magslip motors, the latter method always being used when the camera is in its blimp. For focusing directly onto the single green record film, a magnifying “look-through” system is provided, and the image is observed through the glass pressure pad (Fig. 7).
Parallax compensation is effected automatically by the operation of focusing the camera lens. A Mitchell erect-image self-focusing finder is used, but it has been found possible to reverse the optical head and thereby reduce the separation between camera and finder lenses to about 4 inches.
The means by which parallax is automatically corrected for all lenses is thought to be novel and should merit detailed description. Each lens mount carries its own permanently attached annular cam, which upon insertion of the lens into the camera, engages with one of two rollers attached to a pivoted arm (Figs. 1 and 8). A flat bearing surface attached to the inside forward end of the finder is urged by spring pressure into contact with the second roller on the pivoted arm. Upon rotation of the lens sleeve during focusing, its accompanying cam serves to alter the position of the pivoted arm and with it the angular relationship between the finder and the camera.
The Magslip motors required for the dual purpose of focusing the lens and adjusting the view finder are somewhat larger than would be necessary for focusing only, but it is considered that the inconvenience of the additional weight is more than offset by the advantages of having combined focusing and parallax compensation under all operating conditions.
While a single magazine could have been made to house the three films, it was decided that certain advantages could be obtained by using one bipack and one single-film magazine. The chief advantage of this arrangement is that with one magazine loading from each side of the camera, it becomes possible to reload while the camera remains in its blimp. Furthermore, the problem of minimizing film flap, particularly serious when three films must be transported simultaneously, is to some extent simplified when the two films comprising the bipack run through the camera in combination, although fed from separate 1000-foot rolls and wound up side by side in the same magazine.
In order to restrict undesirable movement of the bipack loops, while still permitting the two films to turn through a right angle and so enter and leave their exposing position without undue tendency to twist, a removable trapping roller is used, and this is rotated by the progress of the films themselves (Fig. 9).
This reduction of film flap and the extremely quiet Vinten movements combine to produce a color camera with a reasonably low noise level, although it is still necessary to employ a blimp which is large when judged by black-and-white standards.
The image formed by the Mitchell finder, which remains attached to the camera when the latter is in the blimp, is conducted through the wall of the blimp by means of a simple series of mirrors.
A range of driving motors with integral gear boxes permits the camera to be used in conjunction with the usual sound systems including Western Electric 220-volt interlock and 12-volt direct-current lock; Radio Corporation of America and British Acoustic 220-volt synchronous, as well as for “wild” shooting.
The exposure rating of the camera, when the combination of stock already described is used, is equivalent to Weston 8 to daylight or corrected high-intensity arc light. This rating means that a key-lighting level of 350 foot-candles is required when exposing at full aperture.
The author is indebted to Gilbert Murray, the engineer who was responsible for all of the construction and most of the design of the camera.
(1) J. H. Coote, “The evolution of the single exposure colour camera”, Phot. J., Vol. 81, p. 293; June, 1941.
(2) British Patent 355,452; 1931.
(3) British Patent 575,075; 1946.
(4) J. W. Boyle and B. Berg, “Studio production with two-color bipack motion picture film”, J. Soc. Mot. Pict. Eng., Vol. 48, pp. 111-116; February, 1947.”
(Coote, Jack (1948): New Three-Color Camera, In: Journal of the Society of Motion Picture Engineers, Vol. 50, June 1948, pp. 543-553.)
“(Made by Dufay-Chromex Ltd., London, England.)
Classification.— Three-colour subtractive printing process.
Two- or three-colour print on non-sensitized emulsions containing substantive colour couplers, involving resensitizing steps, separate printing, and single processing.
(a) Beam-Splitter (Fig. 232). Prism beam-splitter embodying a prism block consisting of two 45° prisms cemented together to form a cube, one of the 45° faces being coated with a partially reflecting surface of gold. The prism diverts part of the beam originating from the lens to a film gate at 90° to the lens axis, the balance of the beam passing through the prism to a gate in the normal position.1
(b) Three-strip Negative Arrangement. Bi-pack records the diverted (viz., reflected) beam. (Blue and red records.) Single film records the direct beam. (Green record.) The bi-pack consists of a non-sensitized, hence blue-recording, front film, the emulsion surface of which is coated with a red filter excluding green and blue light from the rear film which is coated with a highly red sensitive emulsion.
The single film bears a green sensitive emulsion which is exposed behind a minus blue (yellow) filter. This combination of films is known as “Three-Strip”, and is supplied by Eastman Kodak to the special requirements of Dufaychrome.
(c) Successive Frame Negative. Alternatively to the three-strip system, for static subject matter, separation negatives are recorded on a single film as successive sets in the order blue, red, green. A disc carrying the filters rotates in gear with the camera drive, each exposure being covered by a rotation of 120°. Eastman Background X is generally used. Exposure is balanced to yield a density of 1.50 for the “white step” of a standard chart. The “white step” density should not vary by more than plus or minus 0.10 between the three colour records. To achieve this the luminance of the exposing light is adjusted or the exposure of one or other of the colour records reduced. Generally it is the red which must be reduced by some 75% and the green by some 25-30%. This is accomplished either with neutral filters of density 0.60 and 0.15 for the red and green filters respectively, or the disc sectors can be masked proportionately. It is preferable to mask equally at either end of a sector rather than to mask at one end only. The filters used are Chromex 608 (Blue); 609 (Green); 657 (Red).
(d) “Monopack”. It is feasible to use separation negatives derived from any reversal original colour film such as Kodachrome, Ansco Color, Agfacolor, Gevacolor.
Printing.— The blue-record negative of a separation set is printed on to a positive (non-colour-sensitized) emulsion layer containing an immobile yellow colour former. The latent image formed by the first printing operation is not immediately developed, but, instead, the film is resensitized with positive emulsion – this time containing an immobile magenta coupler. The magenta-forming layer is printed behind the green-record negative. Neither of the two latent images so far formed is yet developed, and the film is again sensitized with a positive emulsion – this time containing an immobile cyan coupler. When the cyan layer has been printed from the red record negative, the pack is ready for a single colour development operation and the subsequent removal of the three resulting metallic silver images (E.P. 608,507).
A sound track is added by coating the transparent track area with whatever type of positive emulsion may be required for the best sound quality. After exposure, the track is processed to a metallic silver image in a normal manner, no protection of the picture area being necessary.
Processing.— Identical to Agfacolor positive. (See Agfacolor.) Namely, a developer, such as diethyl-p-phenylene diamine hydrochloride, followed by washing, hardening, bleaching for removal of silver, fixing and washing.
General Description.— This process presents a “set-up” very like Technicolor, namely, a beam-splitter camera of generally similar design: a straightforward three-colour printing job by successive printings all on one side of the film base and all surface contact printed; and a normal silver sound track. The credit for this remarkable achievement goes to Mr. Jack Coote, F.R.P.S., the well-known author of “Making Colour Prints”. For once we have a process conceived and carried through to the commercial stage by a man who before he began developing the process was master of the general subject of colour photography; whereas most processes are misshapen and generally illegitimate offspring of doubtful parentage, this time we know the parent and had cause to expect furthermore the child to be gifted and exceptional. And so indeed it turned out to be.
Coote always agreed with the writer entirely that the right and proper first (recording) stage in a colour process is a good set of separation negatives. Coote has paid some attention to stripping integral tripacks, but soon found that to perfect a new product of this type would mean prolonged research, so that he decided to take the plunge and evolve a beam-splitter. Considering that Technicolor have always done their best to scare others away from this field, Coote was very brave to go through the gate and challenge the fearsome creature within. In this trial of strength he has proven himself a worthy opponent and the champion must look to his laurels (Fig. 232).
At the time of writing the commercial exploitation of this process is limited to Gt. Britain, but rapid developments are bound to occur. Studio tests have proven the camera to be an efficient instrument and the printing process yields a wide chromaticity range (Fig. 233). Table 62 shows the position of the Dufaychrome subtractive primaries on the chromaticity diagram, Illuminant C. being used, and the additive primaries modulated by these. Figs. 234-6 give the spectral transmission of the Dufaychrome subtractive primaries.
1 See Journ. Soc. Mot. Pic. Eng., 50 (June 1948), pp. 543-553; also Brit. Journ. Phot., 66 (May 1949), pp. 218-219.”
(Cornwell-Clyne, Adrian (1951): Colour Cinematography. London: Chapman & Hall, pp. 414-418.)
“Nach einer neueren Mitteilung (290) wird die Firma Dufay-Chromex unter dem Namen Dufaychrome einen neuen subtraktiven farbenphotographischen Prozeß herausbringen. Möglicherweise bedeutet das eine endgültige Abkehr auch dieser Firma vom additiven Verfahren.”
(Schultze, Werner (1953): Farbenphotographie und Farbenfilm. Wissenschaftliche Grundlagen und technische Gestaltung. Berlin/Göttingen/Heidelberg: Springer 1953, p. 36. (in German)
“British Tricolour (1943-47)
Three-colour subtractive process
British Tricolour was devised by a British inventor named Jack Coote. In 1943 he began working on a system in which the camera ran at 72fps and captured three-colour images successively on a film strip. This method, however, suffered from fringing problems endemic to all successive-frame processes. In 1944 a private company, British Tricolour Processes Ltd, was set up and in 1945 work began on developing a new system. That same year Tricolour was used to film scenes of the VE day celebrations which were released as Victory Comes to London. By 1947 his company had designed and built, in association with Wray Optical Works Ltd, a camera similar to a three-strip Technicolor camera which used a beam splitter to reflect some of the light through filters and onto a bipack film, which composed the red and blue records, while the remaining light registered on a single film strip through a green filter, giving a three-colour negative record.
After 1945 tire process achieved modest exposure in the field of advertising shorts, and in 1947 it was announced that the process was ready to go into full commercial production. The roll-out was, however, held up pending planning permission for the construction of a commercial lab. Coote had found a suitable site at Maidenhead, but the regional Town and Country planning officials at Reading refused permission, and the matter was to be sent to the government for arbitration. This delayed the start of construction, which was scheduled to take a year to complete. In the intervening period Dufay-Chromex approached British Tricolour about acquiring the patents. Coote agreed, and went with the patents to Dufay-Chromex’s plant in Thames Ditton in south-west London. British Tricolour was then developed under the name of Dufaychrome, and technically the two processes were identical (see Dufaychrome for details of the printing process).
Victory Comes to London (1945)
Cornwcll-Clyne, Adrian, Colour Cinematography (London: Chapman & Hall, 3rd edn, 1951), p. 414.
Journal of the Society of Motion Picture Engineers, June 1948, pp. 543-53.
Kinematograph Weekly, 3 April 1947, pp. xix-xx.
Kinematograph Weekly, 22 May 1947, p. xix.”
(Brown, Simon (2012): Technical Appendix. In: Sarah Street: Colour Films in Britain. The Negotiation of Innovation 1900-55. Basingstoke, Hampshire: Palgrave Macmillan, pp. 259-287, on p. 265.)
“Dufaychrome (1948-c. 53)
Three-colour subtractive process
Dufaychrome was a subtractive colour system developed by Dufay-Chromex to replace its additive Dufaycolor system. Dufaychrome used a prism in the camera which allowed part of the light to pass through to a film gate along the axis, and diverted the remaining light from the source by 90 degrees to a second gate. The diverted image, consisting of the blue and red records, was recorded on bipack film, the non-diverted image – the green record – on a single film. An alternative successive frame version was also proposed, specifically for animated films, which recorded blue, red and green images successively on a single film strip. Each record was printed onto the same piece of film, one at a time, the film having been treated with the complementary subtractive colour. So the blue record was printed first onto an emulsion layer which had been treated so it contained a yellow coupler, a colourless substance in the emulsion which is transformed into a colour dye by the developing process. This was not processed, and the film was then resensitised using a magenta coupler (magenta being the complementary colour to green) onto which the green record was printed. Finally, the film was resensitised containing a cyan coupler onto which the red record was printed, creating a three-layered colour record which was then processed, bringing out the colour dyes in the couplers.
Dufaychrome was launched by Dufay-Chromex after World War II. With the success of Technicolor in the mid-1930s, it became clear that subtractive colour was to become the industry standard. At that time Dufaycolor was achieving some success in the field of commercial shorts but only limited use in feature films owing to various technical problems and issues of cost. Seeking to develop a subtractive process, in 1937 Dufay acquired the rights to Cinecolor, but any development was stalled after the company gave over many of its factories to essential war work in 1940. In 1943 Dufay-Chromex proposed a deal with Kodak in the UK to pool resources to develop a colour process but the discussions came to nothing.
By the end of World War II Dufay-Chromex was in financial trouble, with losses of almost £15,000 in the financial year 1946-47. The board responded with an aggressive expansion plan, prompted in part by a perceived gap in the market left by the demise of the German Agfacolor company after the war. Supported by the Board of Trade, Dufay was given the opportunity to develop a colour process to rival Technicolor. In 1948 it acquired the rights to the British Tricolour process, which would form the basis of Dufaychrome. In July 1948 Adrian Klein announced that development of Dufaychrome was underway but that organising the provision of lab facilities was slowing down the process of commercial exploitation. Klein reported that the labs would be fully functional within eighteen months, but that a limited service would be available in eight months. Part of the plan for the labs was for Dufay-Chromex to bring the coating of the Dufaychrome film stock in-house, this having been contracted out to this point. At the same time he announced that Rank was to make a feature using Dufaychrome.
The proposed film with Rank does not seem to have emerged, and neither did the ambitious expansion plans. Instead, Dufay-Chromex’s fortunes did not improve. The following year the company’s losses increased to £101,223 and in 1949 Dufay-Chromex began a process of overall retrenchment. Many factories were closed and the work transferred to the main factory at Elstree, while the focus of the company shifted to the making of colour and black-and-white roll film, mostly for the amateur market, and also the making and selling of its line of inexpensive box cameras for still photography.
At the end of 1949 Eric Lightfoot, the chairman of Dufay-Chromex, announced that it was the intention of the company to continue to maintain and develop both Dufaycolor and Dufaychrome, but not to go into large-scale production until such time as market conditions improved. Although clearly Dufaychrome did achieve limited commercial exploitation there is very little to suggest it was used regularly or with any great success. A series of commercial advertising films, dating from 1950, are held in the BFI National Archive. These are very short, lasting only a few seconds and mostly consisting of a single shot of a product over which a title is placed. Whether this was a compilation of commissioned films or films which were specifically taken to sell the process is unknown. In 1952 a screen test of Audrey Hepburn was recorded, so attempts to promote the commercial use of the process were still ongoing by then, but as commercially exploitable products both Dufaycolor and Dufaychrome were effectively finished by 1955.
Dufaychrome Advertising Compilation (1950)
The Audrey Hepburn Screen Test (1952)
Cornwell-Clyne, Adrian, Colour Cinematography (London: Chapman & Hall, 3rd edn, 1951), pp. 414-18, 522-8.
HM Treasury Capital Issues Committee File No T266 161, Dufay-Chromex Ltd, National Archives.
Today’s Cinema, 2 July 1948, p. 3.”
(Brown, Simon (2012): Technical Appendix. In: Sarah Street: Colour Films in Britain. The Negotiation of Innovation 1900-55. Basingstoke, Hampshire: Palgrave Macmillan, pp. 259-287, on pp. 268-269.)
BY JACK H. COOTE, F.R.P.S.
It is safe to say that if British Tricolour had a commercial scale processing plant they would be operating at full capacity to-day. As it is, owing to difficulties with building licences, materials, permits and location of industry, they have nowhere to operate the process and consequently their colour cameras remain idle and their dozen key technicians are compelled to continue small scale experiments on a process which has already been shown capable of reaching commercial standards.
British Tricolour began in the usual basement laboratory some four years ago. At that time only five people were on the job – Jenkins, who was responsible for the design of most of the processing and printing equipment; Hornsby, who was the chemist; Plumb, who built and erected the equipment; Potter, who made emulsions, and the writer.
In those days test negatives were either made in separate lengths as red, green and blue records of inanimate subjects, or in the form of successive frames on a single length of film exposed in an old DeBrie fitted with rotating filters and running at about double speed. […] A print of any subject could not exceed fifty feet at that time, as copies were made by means of a mixture of machine and hand processing, and the manual stage involved the use of-fifty foot Leica type developing grids.
Nevertheless, a considerable number of specimen prints were made under these experimental conditions, and by the end of 1943 a roll of some four hundred feet had been completed and was considered very promising both by those who made it and – more important – by those who had paid for it.
The next step was to design and install a continuous processing pilot plant and work on this began in early 1944. It was also clear that negatives could no longer be made with rotating filters (except of course, on the animation bench) and that a beam splitter colour camera would be necessary, for it has always been and still is an essential part of British Tricolour’s policy to operate their printing process via “silver” separation negatives obtained directly from the subject, rather than by means of a colour transparency as an intermediate step. Adherence to this principle does not mean that Tricolour are unaware of the very real need for greater flexibility in colour cinematography, in fact throughout the period of development of the printing process they have also carried out continuous work on a form of “stripping monopack” which will at once satisfy their requirement for direct “silver” separation negatives and the cameraman’s desire to use a single film in any ordinary camera.
It took some time to secure the services of the man who was to build the Tricolour cameras, as the war was still in progress and he was working in the aircraft industry; but at the beginning of 1945 Murray began work with very limited machine shop equipment and even more limited basement space, and single-handed, within twelve months, had designed and made the first camera.
The Tricolour camera employs the usual combination of a single film (green record) and a bipack of two films (blue and red records). The prism block is divided diagonally and the dividing surface is metallised with gold to provide the required ratio of reflected to transmitted light. Two Vinten Model “H” movements of the latest type are incorporated in the camera, and to facilitate loading while avoiding the use of a hinged lens panel the bipack gate may be swung away from its operating position.
It can be seen from the illustrations that there are two separate magazines in place of a triple magazine accommodating all three films. The magazine serving the single film side of the camera is perfectly normal, and is threaded from one side of the camera, while a double width magazine houses the two films of the bipack and is threaded from the opposite side.
A Mitchell viewfinder is attached and linked for automatic focus and parallax adjustment, while remote focusing of the lenses has been provided by means of Magslip motors.
A set of special lenses were designed in conjunction with the prism block by C. G. Wynne of Wray, comprising four f.2 objectives of 35, 50, 70 and 100 mm. focus. The shorter focus lenses of this range called for a very specialised type of design in order to ensure that the clearance between the back of the mount and the film plane is sufficient to accommodate the prism block.
As already suggested, it is intended, whenever the use of a colour camera is not possible, to employ a stripping multi-layer negative material from which to obtain the required silver image separation negatives. The British Tricolour “pack” at the time of exposure consists of the three necessary colour sensitive emulsion layers, separated by thin (0.0005 in.) regenerated cellulose foils. After exposure in any ordinary camera, the film is processed so that the latent image in the outermost emulsion layer – the blue-record – is alone developed. After this, the pack is washed and dried – but not fixed – and a print is made from the negative silver image by means of infra-red rays and an infra-red sensitive stock, so that the two latent images still remaining in the green and red sensitive layers are unaffected. When a satisfactory duplicate blue record has been assured, and this can be checked by means of a routine exposure on the end of each roll of exposed film, the pack is immersed in water for a few minutes until it becomes possible to peel off the developed blue-record layer together with its supporting cellulose layer – thereby uncovering the green record emulsion ready for development and subsequent duplication in the same manner as the previous image. When the duplicate green record has been obtained and the second emulsion and cellulose layer have been removed, the third or red record can be developed and the negative either duplicated or used on its original base.
The continuous processing pilot plant was completed by the summer of 1945, and, after a few months of testing and adjustment, it was operated continuously on the production of that type of work which it could accommodate – film strips, advertising slide-on-film subjects and film flashes.
When colour camera negatives are being printed, they are in the form of three separate strips and each negative is printed in turn on an orthodox step printer using a Bell and Howell shuttle gate. Sound tracks, which are produced in silver, are printed on standard rotary printers.
The outline of the printing and processing procedure is as follows – the blue-record negative of a positive (not colour sensitised) emulsion layer containing an in-separation set is printed on to a mobile yellow colour former. The latent image formed by the first printing operation is not immediately developed, but, instead, the film is resensitised with positive emulsion – this time containing immobile magenta coupler. The magenta forming layer is printed behind the green record negative. Neither of the two latent images so far formed is yet developed, and the film is again sensitised with a positive emulsion – this time containing an immobile cyan coupler. When the cyan layer has been printed from the red record negative, the “pack” is ready for a single colour development operation before the final removal of the three unwanted metallic silver images.
A sound track is added by coating the transparent track area with whatever type of positive emulsion may be best suited to the sound system in use. After exposure the track is processed to a metallic silver image in a normal manner – no protection of the picture area being necessary.
The advantages of the process are that each colour record can be printed independently of the other two component images on to relatively heap and easily controlled positive emulsion. No colour sensitisers are required and no accurately pre-determined balance of speed or coating weight is involved between the emulsion layers. The scale and colour reproduction characteristics of the process benefit from the fact that the positive images are printed directly from “silver-image” negatives and that the component colour images faithfully follow their corresponding silver densities. Because of this latter factor, there is no “dye-spread” or “bleeding” entailed, and the definition of the finished colour image is limited only by the quality of the original negatives and the accuracy with which the component images are registered.
Articles by T. Thorne Baker, Jack Coote and J. H. Jacobs by permission of “Kinematograph Weekly“; and articles by Jack Coote and W. J. Partleton by permission of “Film Industry” magazine.”
(Huntley, John (1949): British Technicolor Films. Cornhill, London: Skelton Robinson, on pp. 205–208.)