Colour Processes

Autochrome

Autochrome was the first popular colour process and proved an immediate success when put on the market in 1907. It used a mosaic screen composed of starch grains dyed red-orange, green and blue-violet in front of a fine-grain panchromatic emulsion. The need to use a fine-grain emulsion and the filtering of the screen resulted in a very slow plate having a speed of around 1 - 2 Watkins, 10 - 14 Wynne.

Autochromes were available in most sizes up to whole-plate. When introduced the cost of a box of four half-plates was 10/- this was reduced after a year or two to 7/6 (a dozen ordinary plates cost around 3/-). As well as Autochrome plates Lumière supplied the developer and other chemicals needed to process the plates and a paper, Virida, to be used as a safelight filter. For those not wishing to undertake the developing, firms, such as Sinclair, charged 1/6 to process a plate.

A popular way of displaying the images was with a viewer having a horizontal mirror which reflected the image of the Autochrome held in a frame at 45° to the mirror, cloth or wooden sides blocked out extraneous light. Other viewers used a mirror to reflect light onto the back of the Autochrome which was viewed directly from the front. Stereo Autochromes could be viewed in an ordinary stereo viewer but the image would be laterally reversed unless the plate was cut and the images transposed. Richard produced a viewer specifically for Autochromes with prisms behind the eye-piece that reversed the images. T.K. Grant, the London agents for Lumière, advertised stereo viewers for Autochrome with long-focus lenses.1

Production of plates ended around 1932 with the introduction of Filmcolor, a celluloid based cut-film, in 1934 Lumicolor roll-film was introduced.

Outline of the process
  • The photograph is taken in the normal way except that the plate is reversed with the emulsion furthest from the lens so that the light passes through the coloured screen (to focus it was usual to reverse the focusing screen).2 Special dark-slides were later sold with a layer of black card or velvet which pressed against the emulsion, these may also have their film-plane altered to compensate for the reversed plate. A yellow filter was used on the lens as the plate was over sensitive to blue (this was the case for all panchromatic emulsions of the time). Exposure meters came to be marked with a setting for Autochrome and dedicated meters were produced by manufacturers such as Watkins.3
  • The plate is developed as normal; Autochrome developer was sold as Quinomet (Metol Hydroquinone).4 At the time panchromatic plates were little used and the requirement to process the plate in total darkness, or at best using a very dark safelight, would have been new to many photographers. A darkroom timer fitted with a bell was produced to time the first development of the plate rather than holding the plate up to a safe-light. A greenish tint resulted from the plates being loaded or processed in an unsafe light. Frilling at the edges was common, gum was sometimes applied to the edges of the plate during processing or a special developing dish could be used fitted with a removable inner tray that prevented the developer reaching the edges of the plate.
  • The negative image was then dissolved with a permanganate solution. The plate was exposed to white light which acted on the previously unexposed silver halide and was then developed for a second time and fixed. A clearing bath was sometimes used to enhance the colours. The plates were varnished or bound as lantern slides.

Manufacture
The dyed grains were dusted on to a glass plate having a tacky coating, loose grains were removed by brushing. The plate then passed under a roller to flatten the grains which tended to close the gaps between them, any remaining gaps were filled with fine black powder. The intended size of the grains was 0.01 - 0.02 mm, but in practice the variation was larger. With only three colours, the screen suffered from the purely statistical problem of 'clumping' where several grains of the same colour would lie next to each other forming a larger patch of colour.

Patents
The first British patent was dated 1904, in France it had been submitted the year before. The first patent proposed that the plate be coated with grains coloured red, yellow and blue, the plate was then again coated with a tacky layer followed by a second coating of grains of the same colour, these would overlap in places producing a screen of six colours - red, yellow, blue, orange, green and blue-violet. The second patent of 1904 proposed a single coating of grains but of multiple colours. The 1906 patent referred to the grains being flattened by rolling. The patents refer to grains but also mention other substances that could be coloured and remain transparent. A series of patents was issued between 1909 and 1911 covering the production of other types of coloured screens.

Multi-colour Screen Photography
Additive three-colour photography entailed making three separate exposures through three coloured filters (usually red, green and blue), the images were then combined on a projection screen or in a viewer. With a multi-colour screen the colour filters are reduced to microscopic dimensions and placed immediately in front of the emulsion. A single exposure therefore records the red, green and blue images on a single plate. In itself this was an easier process to use and, in addition, required little or no special equipment either when making the exposure or when viewing the image.

There were two types of screen, one used a regular pattern of lines and squares, to form the filters, the other used a random arrangement of coloured dots. Regular screens were often brighter as the whole plate was used as a filter without the need for black infill between filter elements. The screen was either permanently combined with the plate (as it was with Autochrome) or the screen could be separate from the sensitive plate and put in place during the exposure and again when viewing. Separate screen processes had the advantage that the image could be duplicated by contact printing and were processed as ordinary plates, the difficulty lay in producing screens with exactly the same positioning of the filter elements and registration of the taking and viewing screens.

Having a colour screen next to the emulsion was first proposed, as was much else related to colour photography, by Ducos du Hauron in 1868. The first commercially available process was by John Joly where a separate screen of parallel lines was used.5

The image on the right shows the colour screen, taken with a microscope magnification of 48x.

Autochrome Images

These quarter-plate images were taken by Herbert Young in the late 1910s.

References & Notes

BP 22988/1904. BP 25718/1904. BP 9100/1906. BJA 1905, p. 823. BJA 1908, p. 22. N&G Cat. 1908, p. 97. Coote, History of Colour Photography, p. 36. Coe, Colour Photography, p.49. Neblette, Principles and Practice, p. 546.

[1] Richard patent BP 26265/1911. The Grant advertisement is from the 1928 BJA.

[2] Alternatively the rear standard of the camera could be moved back, after focusing, a distance equal to ⅔ the thickness of the plate. If the light filter was used behind the lens the camera could be focused without the filter, positioning the filter behind the lens then compensated for the thickness of the plate. Zeiss produced filters - Ducars - incorporating a negative lens that displaced the image, the filter was fitted to the front of the lens after focusing. On some cameras, such as the Veragraphe, there was a separate focusing scale for Autochrome with a switch to move from one to the other.

[3] Tests by Alfred Watkins showed that with the very low level of light reaching the plate the law of reciprocity did not hold. He advised the use of a special meter having non-uniform scales.

[4] Originally a pyro-soda developer was used.

[5] BP 14161/1894, 19388/1895.

Dufaycolor

Dufaycolor films were introduced for still photography in 1935 and for cine use a few years earlier. It was an additive combined screen process with a regular screen pattern. Dufaycolor was comparatively easy to use and so proved popular, it remained on sale until the late 1950s. The process was developed from the earlier Dufay Dioptichrome process. It was the last additive screen-film process to be marketed.

In the late 1920s Spicer, a paper making and printing company, acquired the Dufay process and formed Spicer-Dufay to manufacture the film. The first product was cine film, in 1935 Ilford became partners in the concern and the product range was extended to include roll-film. Ilford's involvement in the company ended in 1938 though they continued to coat films.

Outline of the process
The photograph is taken in the normal way except that the film is reversed with the emulsion furthest from the lens so that the light passes through the coloured screen. A yellow filter was generally not needed for roll-film but was used with cut-film. The speed was around 17 - 19° Sch.

The film was processed by the reversal process and could be undertaken by the amateur photographer.

Manufacture
  • The film base of cellulose acetate was dyed blue.
  • Parallel lines of greasy ink (a resist) were printed diagonally across the film, there were 20 lines per millimetre.
  • The space between the lines was bleached to remove the blue dye and then dyed green.
  • The lines of greasy ink were removed.
  • A second set of diagonal lines were printed in greasy ink at right angles to the first.
  • A second bleaching removes the blue and green dye not protected by the greasy ink.
  • The clear space was dyed red and the greasy ink removed.
  • The screen was varnished and coated with emulsion.

Products
Dufaycolor was produced in roll-film, 35 mm cartridge, film-pack, cut-film and cine formats. Versions were made for daylight or tungsten, cut-film was produced in different contrasts.

References & Notes

BP 11698/1908. BP 18744/1908. Dufay Color Data Book. Dufaycolor Process. Spencer, Colour Photography in Practice, p. 203. Coote, History of Colour Photography, p. 47. Coe, Colour Photography, p. 72.

Joly

The Joly process was the first screen-plate process, it was introduced commercially around 1895 but without much success due mainly to the lack of fully panchromatic plates and the width of the lines ruled on the screen. It was a separate screen process; different screens were used for exposing and viewing the image, the exposing screen having a slightly wider bandwidth.

Outline of the process
The exposure was made on an orthochromatic plate in contact with the colour screen, with the screen nearest the camera lens (light was filtered by the screen before reaching the emulsion). A yellow filter was used to cut out ultra-violet light. The plate and screen were separated and the plate developed in the normal way. A contact copy was then made to produce a positive image. The positive was viewed in contact with a viewing screen.

Manufacture
The screen was produced by ruling parallel lines of red, green and blue-violet aniline dye mixed with gum on a gelatine-coated plate, there were 200 lines per inch.

Patents
The 1894 patent describes the process and gives details such as the ink that might be used in making the screen. Joly describes a parallax problem with oblique rays when making the exposure and when viewing the finished transparency, he suggests that a microscopic gap may be left between the ruled lines. The patent is non-specific in parts and mentions alternative patterns for the screen and the possibility of a dusting-on process using coloured particles. The second patent describes the ruling machine.

The image on the right shows the colour screen, taken with a microscope magnification of 48x.

References & Notes

BP 14161/1894. BP 17900/1897. Bolas, Phot. in Colours, p. 80. BJA 1899, p. 143. Newman & Guardia advertisement.

Kodachrome

Kodachrome was a subtractive, reversal, colour process for slides and projection.1 It was introduced in early 1935 and was the first commercial integral tri-pack process where three, colour-sensitive, emulsion layers were coated on the same base.

Kodachrome was developed by Leopold Mannes and Leopold Godowsky working at Eastman Kodak in Rochester. The film incorporated new sensitising dyes that had been introduced at Eastman which tended not to wander between emulsion layers. Colour couplers were included in the developing baths which produced the subtractive colour dyes. A critical contribution from Mannes and Godowsky was the 'controlled diffusion' process where the bleaching stages could be limited to affect only the top one or two emulsion layers.

When introduced in 1935 Kodachrome was available in 16mm cine format, a year later an 8mm cine format became available and in September 1936 35 mm cartridges (at first for 18 exposures) and 828 roll-film sizes for still photography became available, sheet film was introduced in 1938.

Processing Kodachrome was complex and only undertaken by Eastman. Originally the developed film was returned as film strips, later (1938) individual images were placed in carboard mounts.

Two types of film were available - outdoor or artificial light (Kodachrome A). The original speed was about 8 weston, 35 mm film was listed at 19° Scheiner. Kodachrome was improved over the years leading to Kodachrome II which was introduced in 1961 rated at 25 ASA. A 64 ASA version was released in 1962. Kodachrome 25 and Kodachrome 64 were released in 1974.

Tri-packs
In a tri-pack colour process images in the three primary colours, red, green and blue, are recorded on three emulsions coated on separate glass plates or films which are held together at the the time of exposure. In an integral tri-pack the three emulsions are coated on a single base. Problems with seperable tri-pack systems were the difficulty in obtaining optical contact between the layers and light diffusion as it passed through the emulsion layers.

Kodachrome, and other integral tri-pack processes, had the three colour-sensitive emulsions coated in layers on a single base.

The usual layout for a tri-pack was for the blue sensitive emulsion to be on top, green in the middle and red at the bottom with a yellow filter under the blue layer. The problem with this arrangement was that the cyan image (red sensitive layer) which contributes most to the line or drawing of the final image was the most diffused as it was at the bottom. The yellow image at the top contributes the least.2

Colour developers
Karl Schinzel made the first proposal for an integral tri-pack in 1905, dye was contained in each of the three emulsion layers. During processing, the film was treated with hydrogen peroxide which, it was hoped, would bleach the dye adjacent to the silver image, unfortunately the bleaching effect was general throughout the image.

Benno Homolka showed that some substances would react in combination with the development of the silver images to produce a dye; no commercial use was made of this but the idea was taken up in future processes.

An integral tri-pack process was proposed by Fischer in 1911 which included dye-forming colour-couplers where a chemical reacts with the oxidised developing agent to produce a dye. The process could not be realised at the time as the couplers could not be prevented from wandering between the emulsion layers, however, in essence it was very similar to later processes.3

The colour-couplers could be held in the developer as it was with Kodachrome, the alternative was to hold the colour-coupler in the emulsion which considerably simplified processing, this method was used in Kodacolor and Agfacolor Neu.

Kodachrome Film Structure
Three thin emulsions were coated on a film base with a yellow filter layer under the top, blue sensitive, layer and a gelatin layer between the second and third, green and red sensitive, layers. The gelatine layers separating the emulsion provided a buffer for the controlled bleaching process. Each layer is responsive to one of the three primary colours, the yellow filter layer prevents further penetration of blue light.

The final image is made up of dyes, yellow, magenta and cyan the silver image is completely removed.

Processing
In procesing Kodachrome an initial development produces a negative silver image in each layer, these are bleached out and the film exposed to white light to produce latent positive images. The film is then developed three times in separate colour developers that introduce dyes, complentary in colour, into each of the three layers.
  • The exposed film is developed to give a negative silver image in each layer.
  • The silver image is bleached out along with the yellow filter, the un-exposed silver bromide is not affected.
  • The film is exposed to white light which acts on the un-exposed silver bromide.
  • A second development is given in a developer containg a colour-forming coupler. A cyan dye is produced that affects the now positive image in each layer. The film is then dried.
  • The film is bleached which converts the silver image back into developable silver chloride and destroys the dye. This process is accurately controlled to affect only the top two layers, the bottom layer (red sensitive layer, cyan image) is not affected.
  • An exposure to white light is given.
  • The film is developed with a magenta colour-forming coupler. This gives a magenta and silver image in the top two images. The film is then dried.
  • The film is again bleached long enough to destroy the dye in the top layer and convert the silver back into silver chloride.
  • An exposure to white light is given.
  • The top layer is dyed yellow by another development containing a yellow colour-forming coupler.
  • A bleaching stage converts the remaing silver images to a silver halide which is removed by fixing.

The process was simplified in 1938:

  • The film is developed to give a negative silver image in each layer.
  • This is re-exposed to red light from the back which affects only the bottom (red sensitive) layer.
  • The film is developed in a cyan dye-forming developer.
  • The film is re-exposed from the top to blue light.
  • The top layer is developed in a yellow dye-forming developer.
  • The middle layer is developed in magenta forming developer containing a chemical fogging agent.
  • The silver images and yellow filter are bleached out.

References & Notes

Coe, Colour Photography, p. 120. Henney, Colour Photography for the Amateur, p. 98. Coote, History of Colour Photography, p. 140. Mees, 'From Dry Plate to Ektachrome', p. 214. Friedman, History of Colour Photography, p. 108. Wall, Three-Color Photography. BJA 1935 p. 251. US pat 2113329.

[1] The Kodachrome name had already been used for an unrelated two-colour process.

[2] In a process marketed as Colorsnap this format was reversed with the blue sensitive layer at the bottom, this was not such a good idea as blue light had to pass through the red and green layers and these emulsions were also sensitive to blue.

[3] BP 15055/1912.

Paget Duplicating Method

This is a colour process using a regular pattern screen developed by George Sydney Whitfield of Paget. A separate screen is used allowing the negative to be processed in the normal way and for the transparency to be made by contact printing. A different screen, with modified colours, was used for viewing the transparency. The process was introduced in 1913, re-introduced following World War I and discontinued around 1925. From 1925 the old-established firm of C. Baker in Holborn advertised a very similar process. The process had a speed of around 15 Watkins, F24 Wynne.

The method of producing the screen was patented in 1908, other related aspects such as the use of a registration device on the screen and the production of prints were also patented.1

The colour screen consisted of red, green and blue squares with twice as many blue squares as red or green. J.H. Pledge measured the squares as being, on average, 1/400 inch for the side of the blue square and 1/300 inch for the green and red squares.2

For a short period around 1914 a combined plate version was sold as was a process for producing paper prints.

In use a panchromatic plate is exposed behind the Taking Screen. After development a positive plate is made, by contact printing, which is viewed in register with the Viewing Screen. Special plates were sold by Paget for the negative and positive plates. A light filter was used over the lens when making the original exposure.

The image on the right shows the colour screen, taken with a microscope magnification of 48x.

References & Notes

BP 9044/1908. BJA 1913, pp. 706, 1258. BJA 1914, pp. 705, 1262. BJA 1920, pp. 500, 640. BJA 1922, p. 73. Coe, Colour Photography, p. 61.

[1] BP 5144/1912, 18900/1912, 24566/1913.

[2] BJA 1914, p. 705.

Sanger-Shepherd Process

This was a subtractive printing process, introduced around 1900, for the production of lantern slides and transparencies. Colour separation negatives were produced in the normal way through red, green and blue filters. The negative taken with the red filter was printed on to a photographic plate and chemically converted to a blue/green colour. The green and blue negatives were printed on to bichromated gelatine, containing silver bromide, coated on a celluloid base. The negative being in contact with the celluloid. These were developed in warm water to remove the unhardened gelatine and then dyed in the complimentary colour to the filter used at the negative stage (the image from the green negative was dyed magenta, that from the blue negative was dyed yellow). The three images were then superimposed and bound together, Canada Balsam was sometimes used between the layers.

The Sanger-Shepherd company sold all of the equipment and supplies required for both the printing process and to produce three-colour separation negatives.

References & Notes

BJA 1901, 1317. BJA 1905, p. 1320.

Trichrome Carbro

The Autotype company was the leading supplier of carbon and carbro materials. The Trichrome process is the adaptation of the Carbro process for three-colour work producing prints or transparencies.

The process uses bromide enlargements from red, green and blue separation negatives. Each bromide print is placed in contact with a pigmented gelatine sheet, coloured cyan, magenta and yellow, (referred to as pigment paper or carbon tissue). During the process the gelatine sheet is sensitised with potassium bichromate making the gelatine insoluble after contact with silver.

Outline of the process
  • Bromide prints are produced from three-colour negatives (red, green and blue). It was important to ensure that the bromide print did not have an additional gelatine coating.
  • Pigment paper, which was pigmented gelatine attached to a paper support, was sensitised by soaking, separately, in the A and B solutions.
  • The pigment paper was placed in contact with the appropriate bromide print and left for 15 minutes. The 'red' bromide print was matched with the cyan pigment paper, the 'green' print with the magenta paper and the 'blue' print with the yellow paper. On separating the two, the image on the bromide print will be found to have been bleached and no longer visible, it could be recovered by re-developing the print. The sensitised gelatine becomes insoluble in proportion to the silver in the bromide print, i.e. shadow areas in the print will produce a thick layer of insoluble gelatine.
  • The pigment paper was then placed on a waxed celluloid sheet (with the surface previously in contact with bromide print next to the celluloid).
  • The celluloid and pigment paper were placed in warm water to 'develop' the image, the paper support could be removed and the soluble gelatine would dissolve leaving an image consisting of pigmented gelatine of varying thickness.
  • The above was carried for each of the three prints.
  • The three images were then mounted on a single temporary support (gelatine coated paper). The order of mounting on the temporary support was cyan, magenta and yellow. The use of a temporary support was to give the correct left-right orientation in the final print. After attaching each image to the paper support the celluloid support (to which the pigmented gelatine was still attached) was removed.
  • The last operation was to mount the images on a final sheet of paper (single transfer paper), which leaves the cyan image on top.

References & Notes

Autotype Colour Printing Processes. Henney, Colour Photography for the Amateur.

Colour Processes

Autochrome

Dufaycolor

Joly

Kodachrome

Paget Duplicating Method

Sanger-Shepherd Process

Trichrome Carbro


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