20Hard Copy Output Media

Hard Copy Output

At the end of the imaging chain (see Figure 1.3), image data, however it was captured, has to be output in such a way that the image can be viewed. In this chapter we will consider output principally in the form of hard copy (images that are tangible, usually as reflection or transmission print media, rather than transient images on a screen or display). In conventional analogue systems this is achieved mainly by the negative–positive process by photographic printing. In hybrid systems output may be laser or LED writing of digital data to photographic paper or other material. In digital systems hard copy output may be by direct output to photographic paper, to plain or special papers via electrophotographic or ink-jet printing, or using special thermal transfer and other materials (see Figure 21.1). The processes and media for hard copy output from digital systems are in a stage of continuing development and systems are being improved and evolving at a very high rate.

Photographic Papers

For producing hard copy output by conventional photographic means, photographic printing papers behave in exactly the same way as negative materials, using a similar light-sensitive material, i.e. a suspension of silver halides in gelatin. Many different types of printing paper are made for applications from conventional photographic printing to computer output via laser or LED printing devices. They may be classified into two overall types: monochrome papers and colour papers. For convenience we may classify the characteristics of photographic monochrome papers under the following headings:

• Type of silver halide emulsion.
• Contrast of the emulsion.
• Nature of the paper surface.
• Nature of the paper base.

In addition to these monochrome negative–positive photographic papers, colour print materials are widely used and are available for different applications, such as:

• Papers for printing from colour negatives.
• Reversal papers for printing from colour positives or transparencies.
• Dye-bleach print materials also for printing from colour transparencies.
• Dye-diffusion materials for printing from colour negatives or transparencies.
• Papers for direct writing (laser, LED).

Type of Silver Halide Emulsion

Monochrome papers vary in terms of the type of the silver halides used in the sensitive layer. These are chloride, bromide and chlorobromide emulsions, chlorobromide emulsions being a mixture of silver chloride and silver bromide in which the individual crystals comprise silver, bromide and chloride ions. Bromide and chlorobromide emulsions may also contain a small percentage of silver iodide. Most emulsions for papers contain varying ratios of silver chloride to silver bromide, from 1:3 to 3:1. The nature of the silver halide employed in a paper emulsion largely determines its speed, rates of development and fixation, image colour and tone reproduction qualities.

Papers differ widely in their speed, chloride papers being the slowest and bromide the fastest, with chlorobromide papers in between. Similarly, silver chloride, although less sensitive than silver bromide, develops faster. Generally speaking chlorobromide papers are of greater speed and softer in contrast as their bromide content increases. The fastest of them are in the same speed range as bromide papers. These attributes are made use of in the manufacture of modern monochrome and colour papers to provide materials that not only have the optimum photographic attributes of image colour, speed and contrast but also develop and fix as rapidly as possible.

The speed of photographic papers is defined according to the standard ISO 6846, and like other current standards for speed (see Chapter 18) is based on a fixed density criterion of 0.6 above the minimum density. Details of the conditions under which the determination is made are given in ISO 6846 but the principle of the method is shown in Figure 20.1.

In Figure 20.1 the value of log₁₀ H_m is the exposure required to produce a density of 0.60 above D_min read from the graph. The ISO speed values are obtained from a table of log₁₀ H_m and corresponding values. The speed values are derived from the formula:

where H_m is the exposure (lx s) that gives a density of 0.60 above D_min. ISO speed numbers for photographic papers are prefixed by the letter P.

Spectral sensitivity of emulsions for photographic papers is also affected by their halide content. As the bromide content increases the spectral sensitivity extends to longer wavelengths. Whereas monochrome negative materials are spectrally sensitized to enable the reproduction of colours to be controlled, printing papers that are used only to print from monochrome negatives have no need of spectral sensitization other than as a means of increasing their sensitivity to tungsten light or as means of obtaining differing spectral sensitivities for variable contrast papers. However, some monochrome papers are spectrally sensitized so that they can be used for producing black-and-white prints with correct tones from colour negatives. Such papers are therefore sensitive to light of all wavelengths and require handling in total darkness or by a dark amber safelight. Also papers spectrally sensitized for laser and LED printing devices are available.

The colour or tone of the image on a photographic paper (untoned) depends primarily on the state of division of the developed image, i.e. on its developed grain size, although it is also affected by the tint of the base. The grain size of the image depends on the nature and size of the silver halide crystals in the original emulsion, or any special additions which may be made to the emulsion and on development.

The size of silver halide crystals in paper emulsions is very small, so that no question of visible graininess due to the paper ever arises, even with the fastest papers. As grains become progressively smaller, the image, which with large grains is black (colder tones), becomes first brown, then reddish and then yellow, finally becoming practically colourless in extreme circumstances. When a paper is developed, the grains are small at first but grow as development proceeds; it is, therefore, only on full development that the image gets its full colour. The finer the original crystals of the emulsion, the greater the possibility of controlling the image colour, both in manufacture and by control during development. Bromide papers yield relatively coarse grains and, when developed normally, yield images of a neutral-black image colour. Chloride emulsions are of finer grain. Some papers are designed to yield blue-black images by direct development, others to yield warm-black images by direct development, the colour depending on the treatment the emulsion has received in manufacture. For example some paper emulsions contain toning agents that change the image colour on development, probably by modifying the structure of the developed silver image so that it appears bluer in colour. Chlorobromide emulsions are intermediate in grain size between chloride and bromide papers. Some chlorobromide papers are designed to give only a single warm-black image colour; others can be made to yield a range of tones from warm-black and warm-brown to sepia.

Paper Contrast

Most monochrome printing papers are available in a range of contrast grades, to suit negatives of different density ranges. With any given negative, therefore, it is usually possible to make a good print on any type of paper, provided that the appropriate contrast grade is chosen. There will, however, be certain differences in tone reproduction between prints on different types of paper. The differences arise because the characteristic curves of different papers have subtle variations in shape.

Although the type and ratios of silver halides present in the emulsions of photographic papers are of considerable importance, the amount of silver per unit area (coating weight) and the ratio of silver to gelatin also have important effects on their properties. Emulsions for papers have a much lower silver-to-gelatin ratio than negative emulsions because of the surface characteristics required in prints.

Most manufacturers offer high-quality papers of the traditional type, i.e. fibre-based papers of high coating weight. These are designed for fine-art or exhibition work, where extremely high image quality and archival permanence are required. Because of their high coating weight they are capable of achieving high densities (about 2.4) and give rich blacks and excellent tonal gradation on double-weight papers with results of exhibition quality.

To print satisfactorily from negatives of different density ranges, papers are required in a range of contrast grades. Glossy-surfaced papers, the most widely used variety, are usually available in five or six grades, although some other surfaces are available only in two or three grades. The selection of a suitable grade of paper for a given negative is a matter for personal judgement based on experience, or for practical trial. If a trial is considered necessary, it is important that development should be standardized at the recommended time and temperature. If the correct grade of paper has been selected, and exposure adjusted so that the middle tones of the picture have the required density, then the lightest highlights in the picture will be almost white (though with just a trace of detail) and the deepest shadows black. If the highlights show appreciable greying, and the shadows are not black, the paper selected is of too soft a contrast grade for the negative; a harder paper should be tried. If the highlights are completely white with no detail at all, and not only the shadows but the darker middle tones are black, the paper selected is of too hard a contrast grade for the negative; a softer paper should be tried.

Different contrast grades of photographic papers are now expressed in terms of log exposure ranges, according to ISO 6846. In Figure 20.1 it can be seen that the log exposure range is defined by log₁₀ H_S – log₁₀ H_T which are determined from the points S and T on the characteristic curve and expressed as values from ISO R40 to ISO R190. To avoid decimal points in expressing ISO ranges the differences in log exposure values are multiplied by 100:

The lower the number the higher the contrast of the paper. From Figure 20.1 it can also be seen that the average gradient or contrast of a paper is given by:

This is the slope of the line joining points S and T on the characteristic curve.

Variable-Contrast Papers

In addition to the conventional printing papers so far described, special types of paper are manufactured (e.g. Agfa Multicontrast, Ilford Multigrade, Kodak Polycontrast) in which the effective contrast can be changed by varying the colour of the printing light. With these variable-contrast papers it is possible to produce good prints from negatives of any degree of contrast on one paper which thus does the work of the entire range of grades in which other printing papers are supplied, and makes it unnecessary to keep stocks in a variety of grades.

A variable-contrast paper depends upon the use of two emulsions with differing spectral sensitivities and contrasts, mixed together in the coated layer. For example, one emulsion may be blue-sensitive and high in contrast and the other green-sensitive and low in contrast. Thus exposing the paper to blue light will give a high-contrast image and exposing it to green light a low-contrast image. Varying the proportions of blue and green light in the enlarger will give intermediate grades. The colour of the exposing light is controlled by the use of specially calibrated filters, by the use of a colour enlarger or by the use of a purpose-made enlarger head with an appropriately filtered light source. Variable contrast papers have the advantage of needing only one box of paper rather than a number of boxes, one for each paper contrast. It is also possible to fine-tune the range of the paper by appropriate selection of filters. Filters are usually numbered from 0 to 5 to cover various specified ISO ranges from low to high contrast.

Paper Surface

The surface finish of a paper depends on the texture, or mechanical finish of the paper, and its sheen. The texture of a paper depends on the treatment that the paper base receives in manufacture. Glossy papers, for example, are calendered, to produce a very smooth surface, while grained papers are usually embossed by an embossing roller. Rough papers receive their finish from the felt on the paper-making machine.

Surface texture governs the amount of detail in the print. Where maximum detail is required, a smooth surface is desirable, whereas a rough surface may be employed to hide graininess or slight lack of definition. A smooth surface is also desirable for prints that are required for reproduction and have to be copied using a camera and various textures are available for special effects. Paper surfaces therefore may be specified according to texture and surface and appear in manufacturers catalogues under names such as: Smooth/Glossy; Smooth/Semi-Matt; Fine-Grain/ Lustre.

Surface finish arises largely from the super-coating, the thin layer of gelatin that is applied over the emulsion of many papers in manufacture, to provide protection against abrasion. This layer gives added brilliance to the print, by increasing the direct (specular) reflecting power of the paper surface. Glossy papers are smooth with a high sheen; a higher maximum black is obtainable on glossy papers than on others. Matt papers are smooth but have no sheen; starch or powdered silica is included in the emulsion to subdue direct reflection. At one time a large variety of surfaces was available, but rationalization in the photographic industry has greatly reduced the number. Papers with a fine irregular patterned surface are called variously ‘lustre’, ‘stipple’, etc., and those with a regular pattern are called ‘silk’ or a similar name. There are also semi-matt surfaces which are smooth but not glossy.

Paper Base

The colour, or tint, of the base paper used for photographic papers may be white or one of a variety of shades of cream or blue. In general, cream papers tend to give an impression of warmth and friendliness; they are very suitable for prints of warm image colour. A white base may be used to simulate coldness and delicacy. Many white-base papers contain optical whiteners which fluoresce when irradiated with blue or UV, thus increasing their apparent whiteness. Two thicknesses of base are commonly available, designated single-weight (150–200 g/m², thickness approximately 150 μm) and double-weight (250–300 g/m², thickness approximately 260 μm) respectively. Both double-weight and single-weight papers are used for enlargements; single-weight is often sufficient if the print is to be mounted, but in the larger sizes double-weight paper is to be preferred because with thinner papers there is a danger of creasing in a wet state.

Resin-Coated Papers

Most printing papers now consist of a paper base coated or laminated on both sides with an impervious layer of a synthetic organic polymer such as polyethylene (polythene). Such papers are termed RC (resin coated) or PE (polyethylene) papers. They offer a number of advantages for the user. Because the base on which the emulsion is coated is impervious to water and most processing chemicals, washing and drying times are considerably shorter than with conventional papers. For example, washing times are reduced from 30–45 minutes for fibre based papers to about 2 minutes for RC papers, which saves both time and water. RC papers cannot be heat-glazed, but the glossy-surface variety dries to a gloss finish, and the papers lend themselves very readily to rapid machine processing. They also dry almost completely flat. Processing can be considerably quicker than with fibre based papers if the chemicals devised specifically for RC papers are used. However, they also have their limitations: processing a number of sheets at one time in a processing dish may lead to damage of the emulsion surface by the sharp edges; different retouching techniques and materials are required; and there is a tendency for papers to ‘frill’ or de-laminate at the edges, especially if dried too rapidly. Dry mounting of RC type papers generally requires the use of special low-melting-point mountants. RC papers are available in a limited number of surfaces. These include glossy, ‘silk’ and ‘pearl’. Only one weight of RC paper, intermediate between single-weight and double-weight, is usually available. Figure 12.5 shows the layer structure of a typical photographic paper. A modern polythene-coated paper has a thickness of around 250 μm and a weight of approximately 270 g/m².

Colour Photographic Papers

Colour papers are provided for direct printing of colour-negative (print) and colour-slide materials as well as for output from hybrid systems via laser or LED printing devices. All colour-print papers are polythene-coated and, unlike monochrome papers, are available only in one or at most two contrast grades. For negative–positive colour printing two contrast grades are provided by some manufacturers. The higher contrast grade gives increased colour saturation with some sacrifice in latitude; the lower contrast grade is more appropriate for negatives of high-contrast subject matter.

All colour photographic papers, whatever their chemical principles of image formation, depend on the subtractive principle of colour reproduction (see Chapter 2), and have individual emulsion layers that are sensitive respectively to red, green and blue light. Chromogenic materials comprise layers containing cyan, magenta and yellow colour couplers which form dyes after colour development. Silver-dye-bleach materials – Ilfochrome Classic (formerly Cibachrome) – contain cyan, magenta and yellow dyes that are bleached in correspondence with the image. Dye-diffusion materials, such as Fujifilm Pictrography, contain dyes which upon activation and release diffuse, in correspondence with the image, into a receiving layer. Table 20.1 summarizes the various colour hard copy output photographic materials.

In the case of the most widely used chromogenic systems for printing from colour negatives there has been standardization amongst the manufacturers, who all provide compatible materials and processing solutions. There are also many independent suppliers of processing chemicals. All chromogenic negative– positive papers are processed by the Kodak RA process or equivalent system of other manufacturer.

Processing Photographic Paper

The developers used today for monochrome papers of all types are commonly MQ or PQ formulae. We saw earlier that the colour of the image is influenced by development. The three components of the developer that have the most important influence on image colour are the developing agent, bromide restrainer and organic anti-foggant. For example, developing agents such as glycin or chlorohydroquinone (chlorquinol) give a warm image tone in the absence of organic anti-foggants but are little used now in print developers. Organic anti-foggants such as benzotriazole tend to give a cold or bluish image. Thus PQ developers will give a bluish image because they usually contain an organic anti-foggant. As the bromide content is increased the image becomes warmer in tone, so MQ developers tend to give warmer images than PQ developers. Chromogenic colour development depends upon colour couplers in the paper reacting with oxidized developing agent to form the image dyes, whereas silver dye-bleach materials involve the bleaching of dyes already present, and dye-diffusion materials involve the diffusion of dyes to a receptor layer and require special processing equipment.

Resin-coated (PE or RC) papers generally have shorter process times than their fibre-based equivalents, and lend themselves to machine processing where speed of access and throughput warrant the expense of a processing machine. Table 20.2 summarizes the approximate processing times for fibre-based and resin-coated papers.

Most colour-print materials require the temperature to be kept within ± 0.3 °C whilst the silver dye-bleach process is more tolerant and allows variation of ± 1 1/2 °C. The process times for colour photographic papers have been reduced considerably since they were first used, as shown in Figure 20.2. These changes have been brought about by changes in the materials, processing solutions and by the use of higher processing temperatures.

Development techniques For dish processing, the exposed print should be immersed in the developer by sliding it face upwards under the solution. Development of papers is a straightforward operation, but prints must be kept on the move and properly covered with solution. The busy printer will find it convenient to develop prints in pairs, back to back, feeding them into the solution at regular intervals, keeping the pairs in sequence and removing them in order when fully developed. Several prints may be developed at one time in the same dish provided there is sufficient depth of developer and that the prints are interleaved throughout. The action of interleaving the prints, i.e. withdrawing pairs sequentially from the bottom of the pile and placing them on the top, will help to dislodge any airbells which may have formed on the prints.

When making enlargements of very large size, the provision of large dishes or trays sets a problem. Where very big enlargements are made only occasionally, the dishes need be only a few inches larger than the narrow side of the enlargement, because development can be carried out by rolling and unrolling the paper in the processing baths, or by drawing it up and down through the solution. In exceptional cases, development can be carried out by placing the enlargement face up on a flat surface and rapidly applying the developer all over the print with a large sponge or swab. Previous wetting with water will assist in obtaining rapid and even coverage of the print by developer. Fixing can be done in the same way. Alternatively, makeshift dishes can be made from large pieces of cardboard by turning up the edges, clipping the corners (which should overlap), and lining them with polythene sheet.

For processing prints, especially colour prints, many small-scale tanks and machines are available and are becoming increasingly popular. These may be operated in the light once they have been loaded with the paper in the darkroom. They range from the simple and relatively inexpensive tube or drum processors which are capable of processing both films and prints, to the more costly and somewhat larger roller transport processors (see Figures 20.3 and 20.4).

These are all small-scale processing devices used by both amateurs and professionals. Larger-scale machines are described in Chapter 17. The simple drum processor is the print analogy of the daylight film developing tank, and is available in a number of sizes ranging from one suitable for processing a single sheet of paper 20.3 cm × 25.4 cm (8 inches × 10 inches) to one large enough to process a single sheet of paper 40.6 cm × 50.8 cm (16 inches × 20 inches). An appropriate number of smaller sheets can also be accommodated in this larger drum. The exposed paper is loaded into the tank in total darkness with its emulsion surface facing inwards and its other side in contact with the inner surface of the drum. All subsequent processing operations are then carried out in the light. Temperature control is achieved by two main methods: either by rotating the drum in a bowl of water at the appropriate temperature, or by a pre-rinsing technique in which water is poured into the drum at a temperature above that of the surroundings and the final processing temperature.

Details of the required temperature of this pre-rinse are provided by the drum manufacturers in the form of a nomogram or calculator. These simple processing drums use very small quantities of solutions for processing each print but suffer from the disadvantage that they have to be washed thoroughly after each use to prevent contamination. Washing of prints is carried out outside the drum. An alternative approach to the simplification of print processing is by using a table-top self-threading roller transport processor. This finds application in the processing of both monochrome resin-coated papers and colour-print materials. Machines of varying degrees of sophistication are available. The more advanced versions are microcomputer-controlled, with built-in sensors and automatic replenishment systems, solution recirculation and agitation by pumps, together with variable speed, adaptable to the processing of monochrome or various types of colour papers. The majority of machines require separate washing and drying of the prints, but they may be provided with additional modules so that dry-to-dry processing can be carried out. Two examples of table-top roller transport processors are given in Figure 20.4. Machines of this type are capable of processing up to approximately 200 8 × 10 inch prints per hour, and no plumbing in is required.

Fixation

Prints are fixed in much the same way as negatives and acid fixing baths are invariably used. The thiosulphate concentration of print fixing baths does not normally exceed 20 per cent, compared with a concentration of up to 40 per cent for films and plates. A fixing time of up to about 5 minutes is used for fibre-based papers, and only around 30 seconds is needed for resin-coated papers (see Table 20.2). To avoid the risk of staining, prints should be moved about in the fixing bath, especially for the first few seconds after immersion. Prolonged immersion of prints in the fixing bath beyond the recommended time must be avoided. It may result in loss of detail, especially in the highlights, because the fixer eventually attacks the image itself.

The print is the consummation of all the efforts of the photographer; thus everything should be done to ensure its permanence, and proper fixation is essential. Improper fixation may lead not only to tarnishing and fading of prints, but also to impure whites on sulphide toning. For the most effective fixation of papers, it is probably better practice to use a single fairly fresh bath, than to use two fixing baths in succession. With the latter method, the first bath contains a relatively high silver concentration. Silver salts are taken up by the paper base in this bath, and tend to be retained in the paper even after passing through the second, relatively fresh, bath. This, of course, does not apply to RC papers.

Where processing temperatures are unavoidably high, a hardening–fixing bath should be employed. Several proprietary liquid hardeners are available for addition to paper fixing baths. Use of a hardener is often an advantage even in temperate climates if prints are to be hot glazed or dried by heat. Never use a fixing bath for papers if it has previously been used for fixing negatives. The iodide in the solution may produce a stain.

Bleach-Fixing

In chromogenic colour-print processing the removal of the unwanted silver image and unexposed silver halides is usually carried out in a single bleach-fix solution (see Table 20.1). Bleach-fix solutions contain an oxidizing agent that converts metallic silver to silver ions, together with a complexing agent (thiosulphate) that forms soluble silver complexes with the oxidized silver and with any unexposed silver halide remaining in the emulsion.

Bleach-fixing is carried out for the recommended time at the recommended temperature, and in small-scale processing is discarded when the specified number of prints has been passed through the solution or when its storage life has been exceeded. In larger scale processing it is normal practice to recover silver from the solution and reuse the bleach-fix after aeration and replenishment. Electrolytic or metallic displacement techniques of silver recovery are the preferred methods. In the case of the latter method the presence of iron salts in the solution after silver recovery does not have the disadvantage it has when this method is used to recover silver from fixer solutions, because the oxidizing agent is ferric EDTA, which can be formed by the addition of EDTA (ethylene diaminetetra-acetic acid) to the de-silvered solution. Thus the recovery and replenishment operation forms extra oxidizing agent. It is important to bleach-fix the print material for the specified time and temperature because incomplete removal of silver and silver halide results in a degraded print, especially noticeable in the highlight areas. Bleach-fixing is a less critical stage than development and a larger amount of temperature variation is permitted (usually ±1 °C).

Washing

The purpose of washing prints is to remove all the soluble salts (fixer, complex silver salts) carried on and in the print from the fixing bath. Where only a few prints are being made they may be washed satisfactorily by placing them one by one in a large dish of clean water, letting them soak there for five minutes, removing them singly to a second dish of clean water, and repeating this process six or eight times in all. A method that is equally efficient and much less laborious, though requiring a larger consumption of water, consists of the use of three trays, arranged as in Figure 20.5, through which water flows from the tap. Prints are placed to wash in the bottom tray of this cascade washer. When others are ready for washing, the first prints are transferred to the middle tray and their place taken by the new ones. When a further batch is ready, the first prints are put in the upper tray and the second batch in the middle one, leaving the bottom tray for the latest comers. In this way, prints are transferred from tray to tray against the stream of water, receiving cleaner water as they proceed. For washing large quantities of prints, this type of washer is made with fine jets for the delivery of water to each tray. These afford a more active circulation of water and dispense with the occasional attention required with the simpler pattern.

Other examples of efficient arrangements for washing prints are shown in Figure 20.6. One method consists of a sink fitted with an adjustable overflow; water is led in by a pipe which is turned at right angles to the sink and terminates in a fine jet nozzle. The effect of this is to give a circular and also a lifting motion to the water and the prints which overcomes the tendency of the prints to bunch together. The overflow is a double pipe which is perforated at the base. This extracts the contaminated water (which has a higher density than fresh water) from the bottom of the sink and prevents it from accumulating. If the top of the outer pipe is closed off, the overflow becomes a siphon which periodically empties the water away, down to the level of the perforations. It is possible to buy siphons that convert processing dishes into washing tanks (Figure 20.6b).

A form of print-washer which solves the problem of prints sticking together is by holding them in a rocking cradle like a toast-rack (Figure 20.6c). The in-flow of water causes the rack or prints to rock backwards and forwards so ensuring that the prints are in constant motion and that there is an efficient flow of water around them. Numerous mechanical print washers are sold which are designed to keep prints moving while water passes over them. In purchasing any print washer it is necessary to be satisfied that prints cannot come together in a mass so that water cannot get at their surfaces, nor be torn or kinked by projections in the washing tank. Commercial print washers are in general suited only to prints of relatively small size. Washing in automatic print processors is more efficient than that for manual or batch processing. The reasons for this may be summarized as follows:

• Efficient removal of fixer from the print surface by the use of squeegees or air-knives.
• Control of the temperature of the water.
• Uniform time of immersion in the processing solutions.
• Control of exhaustion of the fixer by replenishment and possibly by silver recovery.
• Efficient separation of the prints, allowing free access of the wash water.
• Reduction of carry-over of chemicals from one solution to another by the use of squeegees between the solutions.

All the above features of automatic print processors result in a shorter wash time than is required in manual or batch processing in order to yield prints of comparable permanence. However, some of these features can be incorporated in manual processing. For example, tempered wash water can be used, fixers can be replenished at the proper intervals, and wash tanks can be designed to give efficient washing. The efficiency of washing can be increased by introducing a solution of 1 per cent sodium carbonate between the fixer and final wash. This is recommended for fibre-base papers and improves their life expectancy.

Drying

Once they have been thoroughly washed, prints intended to be dried naturally can be placed face upwards in a pile on a piece of thick glass. The excess of water should then be squeezed out and the surface of each print wiped gently with a soft linen cloth or chamois leather. The prints can be laid out on blotters or attached to a line with print clips. Where a large volume of work is being handled, fibre-based prints may be dried by heat using flat-bed or rotary glazing machines, or special rotary dryers. It is usually an advantage to use a hardening–fixing bath when prints are to be dried by heat. The drying of matt and semi-matt papers by heat gives a higher sheen than natural drying, because raising the temperatures of the paper causes bursting of starch grains included in the emulsion to provide the surface texture. With semi-matt papers the higher sheen may in some instance be preferred, but with matt papers it will usually be considered a disadvantage, in which case natural drying should be employed.

Resin-coated papers may be dried by hanging up the prints in the air or in a drying cupboard in the same way as films or by the use of one of the many commercially available purpose-made dryers for RC papers. They dry rapidly without curling. They may also be dried on glazing machines provided that the following precautions are observed:

• All surface moisture is removed.
• The base is towards the heated ‘glazing’ surface.
• The temperature of the glazing surface is below 90 °C.

RC papers are impervious to water so that it is essential that the emulsion is not placed in contact with the heated surface and that surface moisture is thoroughly removed by squeegeeing.

Glazing of Fibre-Base Papers

The appearance of prints on glossy fibre-base papers is considerably enhanced by glazing, a process that imparts a very high gloss. Glazing is effected by squeegeeing the washed prints on a polished surface. When dry, the prints are stripped off with a gloss equal to that of the surface onto which they were squeegeed. Glazing is best carried out immediately after washing, before prints are dried. If it is desired to glaze prints that have been dried, they should first be soaked in water for an hour or more. Glass is usually considered to give the finest gloss, although some other surfaces are also suitable. Chromium-plated metal sheets and drums, however, are far more widely used, as is polished stainless steel. Before prints are squeegeed on to the glazing sheet, the surface of the sheet must first be thoroughly cleaned, and then prepared by treating it with a suitable glazing solution to facilitate stripping of the prints. Where a considerable volume of work is handled it is usual to employ glazing machines, on which prints are dried by heat, to speed up the operation. These machines are of two types, flat-bed and rotary. Glazing can be completed in a few minutes. Flat-bed glazers accommodate flexible chromium-plated sheets on to which prints are squeegeed. The glazing sheet is placed on the heated bed of the glazer and assumes a slightly convex form when held in place by a cloth apron which serves to keep the prints in close contact with the glazing sheet. In rotary machines, prints are carried on a rotary heated chromium-plated (or stainless steel) drum on which they are held in place by an apron.

Pictrography and Pictrostat

Fujifilm have introduced a novel system which combines rapid access to dry images but takes full advantage of the high image quality associated with photographic silver-halide-based media without the use of processing chemicals. One system has been devised for high quality output from a digital printer. It enables direct printing from scanned images Photo CDs etc., by exposure via laser diodes (Pictrography). An essentially similar system has also been devised for printing negatives and slides (Pictrostat) using a conventional light source. The process is shown in Figure 20.7 and involves thermal development and transfer of dyes from a donor layer to the receiving sheet. All the necessary image forming chemicals are contained in the donor layer and processing is activated by a combination of moisture and heat. In exposed areas a latent image is formed which on development releases a dye which diffuses from the donor layer to the receptor layer. The layers are peeled apart and the process takes less than one minute for an A4-sized print. It is a complete system which has to be carried out in the manufacturer’s hardware using the appropriate rolls of donor and receptor materials.

Dry Silver Materials

Dry Silver materials were originally developed by the 3M Company more than 35 years ago and combine the advantages of the high sensitivity and image quality of silver halides as sensors with a totally dry thermal development process. Unlike the previously described pictrographic system, the Dry Silver process does not involve image transfer initiated by moisture and heat but is carried out by heat alone in a single layer. The Dry Silver system is shown in Figure 20.8.

The Dry Silver layer is around 10 μm thick and contains silver halide and spectral sensitizing dye, a supply of silver in the form of an organic silver salt (silver behenate), a developing agent dispersed in an organic polymer. The application of heat induces development in those areas where a latent image has been formed. The supply of silver to form the image comes from the organic silver salt present in the layer and is a form of solid state physical development. The developing agents used in this system are not like those normally used in conventional wet photographic systems. They have to withstand the high temperatures of around 130–140 °C used for thermal development which takes place in a few seconds. Despite having all the image forming components present in the thermally developed layer they are claimed to have a life expectancy of more than 20 years.

A colour Dry Silver process has also been devised by the 3M Company. This process depends upon the formation of dyes from leuco dyes (colourless dye precursors) resulting from the reaction between silver behenate and the leuco dye catalysed by the latent image. This reaction takes place in less than 6 seconds at around 135 °C. The amount of silver generated is insignificant and does not affect the appearance of the dye image. The mechanism of the Dry Silver colour process is shown in Figure 20.9.

These materials are suitable for many applications, which include continuous tone photographic printing and computer hard copy output using lasers, laser diodes or LEDs. They form the output in a number of medical imaging applications.

Cylithographic Materials/Cycolor

A dry colour process originally developed by the Mead Corporation, USA., in the early 1980s, Cylithography is a light-sensitive material which, when exposed, releases dyes when developed by pressure followed by heat. It is a single layer material like Dry Silver, but depends upon photoploymerization of organic chemicals, rather than the light sensitivity of silver halides.

The sensitive layer consists of microencapsulated leuco cyan, magenta and yellow dyes sensitive to red, green and blue light respectively. The walls of the microcapsules are around 0.1 μm thick and are relatively easily broken by pressure. On exposure, the microcapsules become hardened via photopolymerization and resistant to rupture by the subsequent pressure development brought about by passing through rollers. The sequence of operations and the structure of this material is shown in simplified form in Plate 24.

Plate 24 shows the formation of a red image in these materials. Exposure to red light hardens the red sensitive microcapsule containing the cyan leuco dye, leaving the green and blue sensitive microcapsules unaffected. On applying pressure the unhardened blue and green sensitive microcapsules containing yellow and magenta leuco dyes are broken and the leuco dyes released. The application of heat in the presence of a ‘developer’ completes the formation of yellow and magenta dyes to form a red image.

This system is claimed to be less expensive than other systems, requiring only inexpensive hardware, no ink cartridges and no expensive receiving sheets. It is single layer material, containing all the necessary image forming components (like Dry Silver), which only requires the application of pressure and heat to form the image.

Thermal Imaging Materials

A variety of systems that involve differing thermal technologies have been devised for application to imaging. They all involve the application of external thermal energy, which induces image formation by chemical or physical means or a combination. Their names and abbreviations are summarized in Table 20.3.

A number of other imaging materials also involve the application of heat but those previously described use heat to reveal an image that was originally formed by light (see Figures 20.7, 20.8, 20.9 and Plate 24) whereas thermal imaging materials use a heated printing head to write to the material. They have been used for many years in applications such as faxes, printing of tickets and receipts and in medical imaging.

Direct thermal imaging (D1T1) involves the use of a heat-sensitive paper but until recently has not found application for photographic quality output, neither has direct thermal transfer (D1T2), which is an on– off process, unable to produce continuous tone without the additional complexity of screening. An example of D1T1 technology is Fujifilm’s Thermo-Autochrome (TA) system, which contains thermally sensitive layers that produce yellow, magenta and cyan dyes with a resolution of around 200 dpi. Dye diffusion thermal transfer (D2T2) can produce continually varying amounts of dye and hence continuous tone. This material finds extensive application in hard copy output of photographic quality. The application of heat causes transfer of dyes from a donor web to the receptor (see Plate 25). These materials are often called dye sublimation, although this terminology is incorrect since direct contact between donor and receiver is an essential requirement for dyes to migrate and with any air gap the process does not work. It seems unlikely that sublimation is involved in the transfer. Receptor materials for dye diffusion thermal transfer papers have special surface coatings to optimize dye uptake, stability, gloss and brightness and the ability to withstand the high temperatures required for the dye transfer.

These materials are capable of 8-bit reproduction and have an optical density range similar to that of conventional colour photographic paper. Their quality is high and virtually indistinguishable from a conventional photographic colour print, but at the time of writing the equipment is expensive. The printing speed like many digital output devices is slow because in this case it requires three (or more) successive transfers of cyan, magenta and yellow dyes, together with the time needed for data handling and transformations.

Materials for Ink-Jet Printing

Papers and films for ink-jet printing must have an appropriate spread factor of the ink droplet since high image quality is obtained from circular dots of high contrast with well-defined edges. When the ink droplet hits the paper surface it has a diameter equal to that when it is in-flight. The final diameter is reached after the solvent has evaporated and the ink has spread in to the paper surface. Thus the paper surface is very important in controlling image quality. The final diameter is reached when all the solvent has evaporated to leave the ink absorbed by the paper. Also a delicate balance has to be achieved between adhesion and diffusion so that the ink droplet sticks to the paper surface and does not spread excessively. Figure 20.10 shows the cross-section of an ink droplet and receptor layer with varying amounts of diffusion and adhesion.

For photographic quality output the receptor layers are coated on resin-coated heavy-weight papers like photographic papers. The receptor layer may comprise two layers, a top image-forming layer and immediately below an ink-fixing layer which contains inorganic micro-pore particles in a starch binder. Subtle improvements are being made in these papers to achieve resolutions of 600 dpi or greater by modifications to the receptor layers and their compositions to control adhesion, diffusion and other properties such as water and fade resistance.

Bibliography

Diamond, A.S. (ed.) (1991) Handbook of Imaging Materials. Marcel Dekker, New York.

Gregory, P.L. (ed.) (1995) Chemistry and Technology of Printing and Imaging Systems. Chapman and Hall, London.

ISO 6846: 1992 Photography – Black-and-white Continuous Tone Papers – Determination of ISO Speed and ISO Range for Printing.

Proudfoot, C.N. (ed.) (1997) Handbook of Photographic Science and Engineering, 2nd edn. IS&T, Springfield, VA.

Tani, T. (1995) Photographic Sensitivity. Oxford University Press, Oxford.