Present-day colour photographic processes are tricolour systems, reproducing different colours that occur in nature by suitable combinations of three primary-coloured stimuli. Each of these primary colours—blue-violet, green, and red—covers roughly one-third of the visible spectrum. Tricolour impressions can be produced by combining coloured lights (additive synthesis) or by passing white light through combinations of complementary filters, each of which holds back one of the primary colours (subtractive synthesis).
In subtractive synthesis yellow, magenta, and cyan filters or dye layers subtract varying proportions of the primary colours from white light. The yellow filter absorbs the blue component of white light and so controls the amount of blue present in a white-light beam that has passed through the filter. Similarly, the magenta filter controls the amount of green light left, and the cyan controls the amount of the red component. A cyan and a magenta filter superimposed in a white-light beam hold back both the red and the green component, making the emerging beam blue. Similarly, a cyan and a yellow filter together yield green, and a yellow and a magenta filter together yield red. Superimposing such filters or dye images of different densities in a white-light beam can therefore re-create any colour impression in the same way as superimposing light beams of the primary colours.
There are two basic types of color film in general use: color reversal and color negative. When processed, a color reversal film produces a positive transparency in which the colors match those of the original subject. It can be viewed by the transmitted light of a hand viewer or light box, or it can be mounted as a color slide for projection.
A color negative film, on the other hand, produces a color negative in which the colors are complementary to those of the subject. This negative is used to make color prints and enlargements for viewing with reflected light.
So-called “white” light can vary considerably from bluish to yellowish. The human eye adapts to these variations easily, and so most persons are unaware of differences in lighting. Color film, however, cannot adjust to such differences. It is balanced, or matched, to provide the most natural-looking colors with the use of one particular light source. For example, a daylight-type color film is designed to be used with daylight, which is bluish, or with an electronic flash, which creates a bluish light. Another film, Type A film, is balanced for the much more yellowish illumination given by a photoflood bulb. Tungsten, or Type B, film is matched to the light of tungsten-halogen bulbs. For the most natural-looking results, photographers use color film with the light source for which it is balanced, or they use the proper conversion filter.
Instant, or self-processing, film is available for taking both color and black-and-white photographs. In this type of film, the chemicals necessary to develop the latent image on the emulsion and produce a finished print are included in the film itself. The chemicals are automatically activated when the film is removed from the camera. Development may take place very rapidly—within a matter of seconds—depending on the particular make of film and the ambient temperature.
Negative colour materials work in a similar way but yield a negative dye image by direct development. Blue subject tones record in the blue-sensitive film layer to produce a yellow negative image. Green colour components yield a magenta dye image in the green-responding layer, and red components yield a cyan dye image in the red-recording layer. With respect to the subject, the colour negative therefore reverses the tones in brightness as well as in colour. Printing the colour negative on a colour paper with three differentially responding layers reverses the process once more, reconstituting the original subject colours in a positive print.
Some colour films come with different combinations of colour sensitivity and dye colour formed in the individual layers for deliberately falsified colour rendering (false-colour films) in special applications.
Colour-positive (print) materials have a paper or white, opaque film base instead of transparent film and have no antihalation layer. The emulsion sequence may be different from the above scheme; their spectral sensitivities may be keyed to the transmission characteristics of the negative dyes for better colour reproduction.
The dye images are reasonably lightfast but fade on prolonged exposure to ultraviolet-rich radiation. Colour transparencies and prints intended for continuous display may be protected by ultraviolet-absorbing coatings or filter layers.
To ensure correct “white-light” colour reproduction with different types of lighting, the sensitivities of the three film layers must be matched to the colour temperature of the light. Colour slide (reversal) films are therefore made in different versions balanced for faithful rendering either with 5,500 to 6,000 K light sources (such as daylight or electronic flash) or with specified tungsten lighting (3,200 to 3,400 K).
Such accurate film balance matching is less vital with negative colour films since the colour rendering of the print can be modified during colour printing. “Universal” amateur negative colour films are usable with any light, from tungsten to daylight. For high quality, professional negative colour films are still preferentially balanced to either daylight or tungsten sources.
Strongly coloured filters are suitable only for special effects; they overlay the colour image with the filter colour. Pale correction filters can match a film to a light source other than that for which it is balanced—e.g., pale blue, with a daylight-type film used in tungsten lighting, to raise in effect the colour temperature. Pale pink or amber filters similarly reduce the colour temperature for using artificial-light-balanced films in daylight. Colour-film manufacturers publish detailed recommendations of actual filters required for such conversion.
The processing sequence for colour materials is longer than for black-and-white films and requires more solutions. Development needs very precise timing and temperature control. Colour films can be processed in amateur developing tanks; professionals use sets of tanks in temperature-controlled water jackets with provision for standardized solution agitation.
Processing of nonsubstantive colour films, in which the couplers are in the colour developer, is more complex because each emulsion layer is reexposed by appropriately coloured light and colour-developed separately. This operation requires automated processing machinery.
Negative colour films are practically all of the substantive-coupler type. Most again follow a standard processing sequence consisting of colour development (forming a negative silver image in each emulsion layer together with a corresponding dye image), a rinse, and a bleaching and fixing stage to convert the silver image into silver halide and dissolve that (plus residual halide) out of the emulsion. A final rinse and drying conclude the process, which, excluding drying, takes about 12 minutes. Substantially the same procedure is followed for processing positive colour papers.
Positive prints may be obtained from colour negatives by enlarging the colour negative onto a positive colour paper. Colour control consists of modifying the colour of the printing light by yellow, magenta, and cyan filters (typically by inserting high-density filters of these colours to varying degrees in the light path) to obtain a print of the correct or desired colour balance. The light is thoroughly mixed in a diffusing box before reaching the negative. An alternative method is to have three light sources behind the yellow, magenta, and cyan filters and to adjust their relative intensities or switch them on for different exposure times.
This subtractive, or white-light, printing method depends on subtracting or holding back colour components of white light. Commercial photofinishing printers often use an additive system in which prints are given successive exposures through high-density red, green, and blue filters. Each of these exposures forms the image in one of the emulsion layers of the paper; colour balance depends on the proportions of the individual exposures. In automatic colour printing systems the exposures are controlled by photocells that evaluate the red, green, and blue components of light transmitted by the negative.
Colour transparencies can be printed on a reversal colour paper similar to a reversal film and processed in an analogous way. The same kind of colour control with filters is again possible, but the colour effect of the subtractive filters or of the additive filter exposures is reversed.
Dye-destruction processes differ from chromogenic colour materials (where colour images are produced during development) in starting off with emulsion layers containing the final dyes. During processing these are bleached in proportion to the silver image formed. Straightforward processing of a dye-destruction or dye-bleach material yields a positive image from a positive original and consists of: (1) development to form a silver image; (2) stop-fixing to arrest development and remove unexposed silver halide; (3) dye bleaching to bleach the dye in the areas containing a silver image; (4) silver bleaching to convert the silver image into silver halide; and (5) fixing to remove residual silver halide. Washing is done between all the processing stages.
Obtaining a positive image from a negative requires a more elaborate processing sequence, analogous to reversal processing in a chromogenic system. Dye-bleach materials use far more light-stable dyes than those produced by colour-coupling development. The positive–positive procedure also yields duplicate transparencies on dye-bleach materials with a transparent film base.
Materials derived from instant-picture diffusion-transfer processes have been adapted to colour print production. They are more expensive than traditional colour print materials but considerably easier to process. In their simplest form they require only a single highly alkaline activator bath followed by a water rinse, the whole sequence lasting about five to 10 minutes, with considerable processing latitude. Such materials exist for prints from either negatives or transparencies. The colour printing and filter control principles are the same as with the traditional processes described above.
The original method of producing colour prints was based on separation negatives obtained by photographing the original scene on separate black-and-white plates or films through a blue, green, and red filter, respectively. This method analyzes the subject in terms of its tricolour components in the same way as the initial negative images in a three-layer colour film. Positive prints from the separation negatives, converted into colour images (e.g., by toning) and superimposed on top of each other, yield a subtractive tricolour print.
Up to 150 prints can be obtained from a set of gelatin-relief positives simply by redyeing them and repeating the transfer. In the late 20th century, the development of a panchromatic matrix film made it possible to produce the relief positives directly from a colour negative.
Many amateur colour pictures are in the form of transparencies, particularly on 35-mm film. These are usually mounted in plastic or card frames or bound between glass for projection on a screen in a darkened room. The projector consists of a lens, a holder for the slide, and a lighting system (lamp, reflector, and condenser lenses to concentrate the light onto the slide). Modern slide projectors take the slides in magazines or trays holding 30 to 50 or more slides. An automatic slide transport feeds each slide from the tray into the light path of the projector and may be operated from a remote control unit or by pulses from a tape recorder, which can also record a commentary to the complete slide series. Some projectors feature remote-controlled or automatic focusing to keep each successive slide image sharp on the screen.
The size of the projected image depends on the distance of the projector from the screen and the focal length of the projection lens. Projectors may also project from the back onto a translucent screen; such rear-projection setups are more compact, and the image is often bright enough for viewing in daylight. The rear-projection system is used in schools and for commercial displays. Elaborate slide shows are produced by linking two or more projectors aimed at the same or adjacent screen areas. With a suitably assembled slide set, the pictures can be made to change, overlap, and assemble, according to a predetermined program.