Image formation and the photographic process
INTRODUCTION
The foundation of silver-based photography is the light sensitivity of silver salts named silver halides: the chloride, bromide and iodide salts. The latter is usually a very small percentage additive to silver bromide crystals. The discovery by Richard Leach Maddox in 1871 of a method of coating a gelatin dispersion of silver halide crystals on glass plates heralded the universal use of gelatin as a colloid in photographic emulsions.
Silver salts are usefully light sensitive, or can be made so by spectral sensitization. They are also developable to amplify the invisible effect of small exposures – the photographic latent image. An amplification factor of 109 to 1012 is achievable by photographic development – some 1010 (10,000 million) silver atoms are developed for each photon of light absorbed by an emulsion crystal.
Silver halide photography is shown diagrammatically in Figure 13.1. The immediate effect on the emulsion, at exposure, is the formation of a latent image. This is made visible by development. Light areas of the subject appear as dark developed silver, which appears black when finely divided – a negative image. Other areas contain unchanged silver salts, which would become dark over time if left in the gelatin. Residual silver salts are removed by fixing in a solution, which removes them as soluble complex salts. The fixing agent present is often sodium thiosulphate, the photographers’ hypo. Another chemical commonly present in the solution is mildly acidic, to neutralize alkaline developer solution remaining on the negative. Once the negative is washed and dried it is printed through a similar negative-working process, to produce a positive. Print emulsions are usually coated on paper and viewed by reflection.
SILVER HALIDES
Silver halides are effectively insoluble in water and formed in photographic emulsions by preparing the salts in the presence of gelatin. This can be achieved by adding a solution of silver nitrate to soluble salts such as ammonium bromide or potassium chloride in a dilute solution of gelatin. Further treatments encourage crystals of the precipitated silver salts to grow into crystals of the required size and shape. Other treatments then achieve the required sensitometric speed, contrast and spectral sensitivity. Figure 13.2 illustrates spectral sensitivities of emulsions containing two pure silver salts, silver chloride and silver bromide, as well as technologically useful silver chlorobromide (silver chloride containing a percentage of silver bromide) and silver iodobromide (silver bromide with a small content of silver iodide). These blends result in mixed crystals, containing both silver salts in carefully controlled proportions. Silver bromide may be present in a wide range of concentrations in emulsion crystals, but silver iodide is usually present as a small proportion only. The silver halides used, and their relative proportions, differ considerably depending on the use of the materials concerned. Negative emulsions of camera speed usually contain silver iodobromide emulsions containing, as the name implies, crystals composed of silver bromide and of the order of 5% silver iodide present in each crystal. Such formulations can give high inherent emulsion speeds and are suitable for spectral sensitization to give panchromatic monochrome materials, or the selective sensitivities necessary in colour films. Negative emulsions of camera speed generally possess lowish contrast. They are subsequently printed on slower, high-contrast printing materials. They have fine grain and may or may not need spectral sensitization. They are usually chlorobromide emulsions containing silver bromide and chloride present in each crystal (but no iodide), and are formulated to give high contrast, appropriate printing times with conventional enlargers, and minimal emulsion fog to give clear whites and the maximum tonal range in prints.
Figure 13.1 Outline of the silver-based photographic process.
Figure 13.2 Wedge spectrograms of silver chloride, chlorobromide and iodobromide emulsions – without spectral sensitization (to tungsten light at 2856 K).
Silver chloride (AgCl) absorbs mostly in the ultraviolet region of the spectrum, while a small silver bromide (AgBr) content extends the spectral absorption further into the blue. Silver bromide itself has much more absorption of blue light and this may be extended towards the green by a small content of silver iodide (AgI). Only light absorbed by an emulsion can make it developable, so increases in spectral absorption and its extension to more of the spectrum are important in determining the range of spectral sensitivities.
LATENT IMAGE FORMATION
The latent image is an exposure-induced change within a silver halide crystal that increases the probability of development from very low to very high. All emulsion crystals would eventually be reduced to silver if developed for long enough, but the rate of reduction is very much greater for those crystals bearing a latent image. The change produced by exposure is the formation at a site or sites on or within the crystal of an aggregate of silver atoms. Latent image sites are probably imperfections and impurities (e.g. silver sulphide specks) existing on the surface and within the bulk of the silver halide crystal. Light quanta, photons (see Chapter 2), are absorbed, releasing photoelectrons which combine with interstitial silver ions to produce atoms of silver. Interstitial silver ions are mobile, displaced from their normal positions in the crystal lattice, and are present in emulsion crystals prior to exposure. The general reaction can be represented by the equations:
in which hv is the energy of a photon, e− is an electron and the dots represent important electrons associated with atoms.
Energy levels relevant to latent image formation are shown for a silver bromide crystal in Figure 13.3. Discounting thermal fluctuations, absorption of a quantum of energy greater than 2.5 eV (electron volts) is necessary so that a valence electron may be raised to the conduction band. The valence band is the highest, filled, band of energy in which the electrons exist. The conduction band comprises empty energy levels; electrons excited to this band are free to move and electrical conduction can take place. Between these two levels is a band gap in which electrons cannot exist. A quantum of energy of 2.5 eV is equivalent to a wavelength approximating 495 nm, and corresponds to the long wavelength sensitivity limit of silver bromide. Also shown in Figure 13.3 are the relevant energy levels of an adsorbed dye suitable for sensitizing silver bromide to longer wavelengths. A photon of energy less than 2.5 eV may, upon absorption by the dye, promote the molecule from its ground energy state to an excited state. If the excited electron can pass from the dye to the crystal, latent image formation may proceed. In this way silver halides can be sensitized to green, red and even infrared radiation.
Although several mechanisms have been suggested to explain latent image formation, considerable evidence supports the basic principle suggested by Gurney and Mott in 1938. The essential feature is that the latent image is formed from alternate arrivals of photoelectrons and interstitial silver ions at particular sites in the crystal. Figure 13.4 shows the important steps. The process is considered as occurring in two stages:
1. Nucleation of stable sub-developable specks.
2. Subsequent growth to just-developable size and beyond.
Broken arrows indicate decay of unstable species, an important characteristic. The first stable species is the two-atom centre, although the size of a just-developable speck is three to four atoms. Thus, a crystal must absorb at least three to four quanta in order to become developable, but throughout the sequence there are opportunities for inefficiency and, generally, more than this number are required. In modern emulsions an average exposure of 10 photons is required for developability of an emulsion crystal. Photon sensitivities may extend from three or four to 30 or 40. During most of the nucleation, species can decay and liberated electrons can recombine with halogen atoms formed at exposure. If this occurs, photographic effect is lost. The photo-equivalent surplus halogen atoms formed may also attack photolytically formed silver atoms, re-forming halide ions and silver ions. Gelatin is a halogen acceptor and should remove halogen atoms before such reactions can occur, but its capacity is limited and its efficiency drops with increasing exposure.
Figure 13.3 The energy levels relevant to latent image formation.
Figure 13.4 Latent image nucleation (top row) and initial growth (bottom row).
These considerations help to explain low-intensity and high-intensity reciprocity law failure (see Chapter 8). At low intensities, photoelectrons are produced at a low rate and the nucleation stage is prolonged. Consequently the probabilities of decay and recombination are relatively high. At high intensities, large numbers of electrons and halogen atoms are simultaneously present in the crystals. Such conditions yield high recombination losses. Nucleation may also occur at many sites in a single crystal, producing large numbers of very small, unstable silver specks.
SPECTRAL SENSITIVITY OF PHOTOGRAPHIC MATERIALS
The inherent light sensitivity of silver halide emulsions is confined to the range of wavelengths absorbed by the silver halides, illustrated in Figure 13.2. This includes blue and violet regions of the visible spectrum, the ultraviolet region and shorter wavelengths extending to X- and gamma-radiation. This applies to all types of emulsion – chloride, bromide, iodobromide – although the long-wavelength sensitivity cut-off varies with the type of emulsion, as shown. The amount of light absorbed in the inherent sensitive region, and hence the useful speed of an emulsion, depends on the volumes of the silver halide crystals present. Fast emulsions therefore give coarser-grained images than slower emulsions.
Response of photographic materials to short-wave radiation
Although silver halides have inherent sensitivity to all radiation of shorter wavelengths than the visible, the recording of such radiation involves special problems. Crystals of silver halide in emulsions significantly absorb radiation of shorter wavelength than about 400 nm. Consequently images produced by ultraviolet (UV) radiation lie near the emulsion surface, because the radiation cannot penetrate very far. At wavelengths shorter than about 330 nm, the radiation is also absorbed by glass, causing the short-wavelength cut-off seen in Figure 13.2. Recording beyond this region requires quartz or fluorite optics (see Chapter 10). At about 230 nm, absorption of radiation by gelatin becomes serious and conventional emulsions cannot be used. Fluorescence can, however, be employed to record in this region. Ordinary photographic films can be used, the emulsion being coated with petroleum jelly or mineral oil, which fluoresces during exposure giving a visible image, which the emulsion records. The jelly is removed using a solvent before processing.
X-ray or gamma-radiation is absorbed very little by emulsions, and it is necessary to coat very thick emulsion layers containing a large amount of silver halide to obtain an image. This is normally applied in two layers, one on each side of the film base. An alternative is to use fluorescent intensifying screens, placed in contact with each side of the X-ray film, which emit, under X-ray excitation, blue or green light to which the film is very sensitive. Modern rare-earth salt intensifying screens considerably reduce the radiation dosage to patients receiving diagnostic X-ray exposures. For very short-wave X-rays and gamma rays, used in industrial radiography, lead screens can be used. Exposed to X-rays or gamma rays, these screens eject electrons which are absorbed by the photographic material, forming a latent image. Two major types of intensification of the effect of X-ray exposure are used, depending on the wavelength: salt screens and lead screens.
Ultraviolet radiation is present in daylight and, to a much lesser extent, in tungsten light. Ultraviolet radiation from 330 to 400 nm, referred to as the near-UV region, may affect ordinary photographs, increasing haze in distant landscapes and giving blue, hazy results in colour photographs of distant, high-altitude or sea scenes.
Response of photographic materials to visible radiation
Photographic materials relying on the unmodified sensitivity of the silver halides have been referred to as blue-sensitive, non-colour-sensitive, ordinary or colour-blind materials. The most commonly used description is probably ‘ordinary’. Materials used by the early photographers were of this type. Such materials lack sensitivity to green and red spectral regions, and cannot record colours correctly; reds and greens appear too dark (even black) and blues too light, the effect being greatest with saturated colours.
For some work this doesn’t matter. Blue-sensitive or blue-and-green-sensitive emulsions are in general use for monochrome printing papers, and for negative materials used with black-and-white subjects in graphic arts applications. Even subjects with muted colours can be recorded with reasonable success on blue-sensitive materials. Many acceptable photographs have survived from the days when these were the only materials available. When colours are more saturated, blue-sensitive materials show their deficiencies markedly, and photographs in which many objects appear far darker than the observer sees them cannot be regarded as satisfactory.
SPECTRAL SENSITIZATION
Vogel, in 1873, discovered that silver halide emulsions could be rendered sensitive to green light in addition to blue by adding a suitable dye to the emulsion. Later, dyes capable of extending the sensitivity into the red and even the infrared region of the spectrum were discovered. This use of dyes is termed dye sensitization, or spectral sensitization. The dyes, termed spectral sensitizers, are added to the emulsion at the time of its manufacture. Sensitization follows from the dye becoming adsorbed to the emulsion grain surfaces. Dye that is not adsorbed does not confer spectral sensitization. The amount of dye required is extremely small, usually sufficient to provide a layer one molecule thick over only a fraction of the surface of the crystals. The quantity of dye added, and hence the sensitivity gained in the sensitized region, depends to a certain extent on the surface area and nature of the silver halide crystals.
The sensitivity conferred by dyes is additional to the sensitivity band of the undyed emulsion, and is added on the long-wavelength side. The extent to which an emulsion has been dye sensitized makes a considerable difference to the amount and quality of light that is permissible during manufacture and in processing.
Some classes of sensitizing dyes reduce the natural sensitivity of emulsions to blue light, while conferring sensitivity elsewhere in the spectrum. This may not be desirable in black-and-white emulsions but is often useful in colour materials.
For practical purposes, dye-sensitized black-and-white materials may be divided into four main classes:
1. Orthochromatic
2. Panchromatic
3. Extended sensitivity
4. Infrared sensitive.
The spectral sensitivities of emulsions of these classes are illustrated in Figure 13.5.
Orthochromatic materials
The term orthochromatic is applied generally to emulsions sensitive to both blue and green light. Orthochromatic materials are, in fact, not often used for general-purpose photography, although such sensitization is used in a few special-purpose monochrome materials. The tonal rendering of colours by orthochromatic films yields monochrome images in which blues and greens are recorded as much lighter than orange and red, which appear very dark.
Panchromatic materials
Materials sensitized to both the green and red regions of the spectrum, and thus sensitive to the whole of the visible spectrum, are termed panchromatic, i.e. sensitive to all colours. It was not until 1906 that the first commercial panchromatic plates were marketed. This advance enabled the manufacture of the first commercially available colour photographic plate, by the brothers Lumière.
Usually, the red sensitivity of panchromatic films extends up to 660–670 nm. They have two main advantages over the earlier types of material: they yield improved rendering of coloured objects, skies, etc. without the use of colour filters, making possible the control of colour rendering by means of colour filters. When making a negative, and aiming to produce a correct monochrome rendering of a coloured subject, panchromatic emulsions must be used. If it is necessary to modify tonal relationships between different colours in the subject, full control requires the use of panchromatic materials in conjunction with filters.
Figure 13.5 Wedge spectrograms of typical materials of the principal classes of spectral sensitivity (to tungsten light at 2856 K).
Extended sensitivity materials
The sensitivity of the human eye is extremely low beyond 670 nm and an emulsion sensitive beyond this wavelength gives an infrared effect. Subjects that appear visually quite dark may reflect far-red and infrared radiation. This leads to a fairly light reproduction when recorded.
Panchromatic emulsions with sensitivity extended to about 750 nm are available for general photography and can be useful for haze penetration in landscapes, giving a light rendering of green foliage and a dark rendering of blue skies. The reproductions can be striking and account for the use of such materials.
Infrared materials
The colour-sensitized materials so far described meet the requirements of conventional photography. For special purposes emulsions sensitive to yet longer wavelengths can be made; these are infrared materials. These were not widely used until the 1930s. Since then, sensitivity has been extended to the region of 1200 nm. Infrared monochrome materials are normally used with an opaque filter over the camera lens or light source, to prevent visible or ultraviolet radiation entering the camera.
Infrared materials find use in aerial photography for haze penetration and for distinguishing between healthy and unhealthy vegetation, in medicine for the penetration of tissue, in scientific and technical photography for the differentiation of inks, fabrics, etc., which appear identical to the eye, and in general photography for pictorial effect. The penetration of haze depends on the reduced scattering exhibited by long-wavelength radiation. Other applications utilize the different reflectance and transmittance of objects to infrared and visible radiation.
Lenses are not usually corrected for infrared, so that using infrared emulsions requires a small increase in camera extension. This is because the focal length of an ordinary lens for infrared is greater than the focal length for visible radiation. This applies even when an achromatic or an apochromatic lens is used (see Chapter 10). Some cameras have a special infrared focusing index. With others, the correction necessary must be found by experiment, usually some 0.3–0.4% of the focal length.
Other uses of dye sensitization
Sensitizing dyes have other uses besides the improvement of the colour response of an emulsion. In particular, when a material is to be exposed to a light source that is rich in green or red and deficient in blue (such as a tungsten lamp), its speed may be increased by dye sensitization. Thus, some photographic papers are dye sensitized to obtain increased speed without affecting other characteristics of the material. Many modern monochrome printing papers use differences in spectral sensitization to define high- and low-contrast emulsion responses within the same material. Modification of the spectral quality at printing, by suitable filters, enables control of the print contrast. This eliminates the need for several contrast ‘grades’ of paper.
The most critical use of dye sensitization occurs in tripack colour materials in which emulsions with distinct blue, green and red spectral sensitivity bands are required. This has required narrow sensitization peaks, precisely positioned in the spectrum. Spectral sensitivities of a colour negative film are shown in Figure 13.6.
A useful balance of sensitivity is provided by dye-sensitized, tabular crystals. These are used in ‘T-grain’ or ‘Delta’ emulsions. The crystals have large surface area, are very thin, contain little silver halide, do not absorb much blue light, and hence have low blue sensitivity. They have a large surface area and relatively large quantities of sensitizing dyes can be adsorbed onto them. Such emulsions can be very sensitive to spectral bands outside the blue region, and have little blue sensitivity. They are well suited to use in colour materials and may eliminate the need for a yellow filter layer, used to restrict the inherent blue sensitivity of red- and green-sensitive emulsions.
Figure 13.6 Spectral sensitivities of daylight-balanced colour negative film exposed to daylight.
STRUCTURE OF PHOTOGRAPHIC MATERIALS
Photographic materials are coated with suspensions of minute crystals (with diameters from 0.03 μm for high-resolution film to 1.5 μm for a fast medical X-ray film) of silver halide in a binding agent, almost invariably gelatin. These suspensions are called emulsions. The crystals are commonly termed grains. In materials designed for negative production, the halide is usually silver bromide, in which small quantities of iodide are also present. With papers and other positive materials, the halide may be silver bromide or silver chloride or a mixture of the two. The use of two silver halides in one emulsion results not in two kinds of crystal but in crystals in which both halides are present, although not necessarily in the same proportion in all crystals. Photographic materials containing both silver bromide and silver iodide are iodobromide materials, and those containing both silver chloride and silver bromide are chlorobromide materials.
Silver halides are uniquely useful in photography because they are developable, which means that the effect of light in producing an image can be amplified with a gain in image of about 1000 million times. The image produced when photographic materials are briefly exposed to light is invisible and termed the latent image.
Production of light-sensitive materials and sensors
The principal materials used in preparing a photographic emulsion, which is coated on a base to form the sensitive layer, are silver nitrate, alkali-metal halides and gelatin, and all must satisfy stringent purity tests. The gelatin is carefully chosen, and is a complex mixture of substances obtained from hides and bones of animals; although silver salts are the light-sensitive material, gelatin plays a very important part.
Essential steps involved in the preparation of photographic emulsions are:
1. Solutions of silver nitrate and halide salts are mixed in a gelatin solution under controlled conditions, where they react forming silver halide and a soluble nitrate. This is called emulsification.
2. This emulsion is heated under conditions in which silver halide is slightly soluble. The crystals of silver halide grow to sizes determining the characteristics of the emulsion: speed, contrast and graininess. This is the first, physical or Ostwald ripening.
3. The emulsion is washed to remove by-products of emulsification. Washing is achieved by causing the emulsion to precipitate by adding a coagulant to the warm solution. Unwanted liquid is then removed.
4. The emulsion is given a second heat treatment in the presence of a sulphur sensitizer. No grain growth should occur during this stage, but sensitivity specks of silver sulphide are formed on the grain surface and maximum speed is reached. This stage is second ripening, digestion or chemical sensitization.
5. Sensitizing dyes, stabilizing reagents, hardeners, wetting agents, etc. are now added.
Photographic manufacturers control all these stages and many other factors. They tailor-make emulsions, optimize them with respect to speed and granularity for specific applications, as well as maintaining appropriate tone and colour reproduction characteristics. Research and development work has led to colour negative films and papers of high sensitivity, low granularity and high image quality.
T-grains (flat tabular crystals) allow maximum sensitizing dye adsorption to the grain surface, encouraging efficient absorption of light, and possess other useful properties giving higher resolution and lower graininess than expected from large crystals. Double-structure crystals optimize both absorption of light and graininess. Monodisperse crystals can minimize light scatter, improving image resolution. Sophisticated technology, devised for colour materials, has been applied to modern black-and-white materials. Examples include Ilford XP-2, a monochrome film yielding a dye image developed in colour-processing chemicals, Kodak ‘Tmax’ films using Kodak ‘T-grain’ technology, and Fuji Neopan films using ‘high-efficiency’ light-absorption grain technology.
The support
The finished emulsion is coated on a support, commonly film or paper base. Film base is usually a cellulose ester, triacetate or acetate-butyrate, although manufacturers also use newer polymers, such as polyethylene terephthalate, offering advantages, particularly dimensional stability. This makes them of special value in graphic arts and aerial survey. Polyethylene terephthalate has exceptionally high strength, much less moisture sensitivity than cellulose-derived films and an unusually small variation in size with temperature (Table 13.1), and is then stable throughout the temperature range encountered by photographic film.
Film base differs in thickness according to the product and type of base, most bases in general coming within the range of 0.08–0.25 mm. Roll films generally have 0.08 mm base, miniature films 0.13 mm base and sheet films 0.10–0.25 mm base. Polyethylene terephthalate can be somewhat thinner than cellulosic base because of its higher strength.
Glass plates have now been almost entirely replaced by films. They are still used occasionally in specialized fields, because of their dimensional stability and rigidity. An additional advantage of plates is that they lie flat in use. The advantages are, however, outweighed by high cost, fragility, weight, storage space requirements and loading difficulties.
Paper base used for photographic and digital hard copy must be particularly pure. Photographic base paper is therefore manufactured with the greatest care taken to ensure purity. Before photographic paper is coated with emulsion, it is usually coated with a paste of gelatin and a white pigment, baryta (barium sulphate), to provide a pure white foundation giving maximum reflection. Most modern paper bases are, however, not coated with baryta but coated on both sides with a layer of polyethylene and are known as PE or RC – polyethylene or resin-coated papers (Figure 13.7). The upper polyethylene layer contains titanium dioxide, as the white pigment, and optical whiteners. They are impermeable to water, which avoids paper base absorbing water and processing chemicals. This results in substantially shorter washing and drying times than are required by baryta-coated fibre-based papers.
Coating the photographic emulsion
Coating modern materials is a complex task and current coating methods are the subject of commercial secrecy. One of the earliest forms of coating flexible supports was ‘dip’ or ‘trough’ coating. Dip coating is slow because faster coating results in thicker emulsion layers which are difficult to dry, and may have undesirable photographic properties. In order to increase the coating speed this method was modified by the use of an air-knife, an accurately machined slot directing air downwards on to the coated layer, increasing the amount of emulsion running back into the coating trough (see Figure 13.8). This method enables the use of more concentrated emulsions, higher coating speeds and thinner coatings.
More modern coating employs accurately machined slots through which emulsion is pumped directly on to the support or to flow down a slab or over a weir on to the support (cascade coating). Such methods enable coating speeds to be far higher than were possible with traditional methods. This allows very high speeds, coating base material typically being 1.4 metres wide. Monochrome materials have more than one layer coated; colour materials may have as many as 14, while instant self-developing colour-print films have an even more complex structure. In modern technology, many layers are coated in a single pass through the coating machine, either by using multiple slots or a number of coating stations, or both. A non-stress super-coat is finally applied to reduce the effects of abrasion. The coating is then set by chilling, and dried under controlled conditions.
Figure 13.7 Construction of photographic papers: (a) baryta paper; (b) polyethylene- or resin-coated paper.
Figure 13.8 Schematic diagram of a coating machine.
With negative materials it is usually impossible to obtain the desired properties in any single emulsion. Two emulsions may then be prepared which, together, exhibit the desired characteristics. These may be mixed and coated as a single emulsion, or applied as two separate layers, an undercoat and a top coat. A supercoat is then added. For maximum speeds the top coat consists of a fast component while the undercoat is slower, allowing the control of speed, contrast and exposure latitude (see Chapter 8).
SIZES AND FORMATS OF PHOTOGRAPHIC MATERIALS
Photographic materials are supplied in many formats, some of which are shown in Table 13.2. Film sizes have to correspond to standard dimensions if they are to fit cameras in use but printing papers, except in automatic printers, are often trimmed after printing to suit the aesthetics of images, and standardized sizes are less vital. In practice, printing paper is available in a wide range of sizes, including the familiar metric dimensions A3, A4, A5 and A6, as well as Imperial sizes: 4 × 5, 6.5 × 8.5, 8 × 10, 10 × 12 and 16 × 20 inches.
SPEED OF PHOTOGRAPHIC MATERIALS
Two films differ in speed if the exposure required to produce a negative on one differs from the exposure required to produce a negative of similar quality on the other. The material requiring lower exposure is said to have the higher speed. The speed of a material varies inversely with the exposure required, and we can express speed numerically by selecting a number related to exposure. The response of photographic emulsions is complex, and speed depends on many variables, the following being most important:
• Exposure
The colour of the illuminant, e.g. daylight or tungsten light.
Intensity of the exposing light. This is because of reciprocity law failure (see Chapter 8).
• Development
Composition of the developer, e.g. high or low activity.
The degree of development, indicated by the contrast achieved. This depends principally upon the development time, temperature and degree of agitation.
Strictly it is not possible to define the speed of a material completely by a single value. A speed number can, nevertheless, provide a good guide to the performance that may be expected from a material and can be determined for photographic materials provided all the variables are defined.
Speed systems and standards
The various standards adopted by the American, British and German national standards organizations were brought into line in all respects, except for the type of speed rating employed, in 1960–62, the American and British Standards specifying arithmetic numbers and the German Standard logarithmic ones. This followed work showing good correlation between speeds based on a fixed density of 0.1 above Dmin (see Chapter 8) and the fractional gradient criterion, for a wide variety of materials when developed to normal contrast. Current ISO practice specifies a speed point at Dmin + 0.1 and development conditions such that density increases by 0.80 over an exposure increment of 1.30 from the speed point (see Figure 13.9). Arithmetic and logarithmic speeds are calculated from the following formulae where exposure (H) values are in lux seconds:
The quantity Hm is the exposure in lux seconds at the speed point specified above. Table 13.3 shows equivalents between arithmetic and logarithmic speeds. Doubling the arithmetic speed gives a logarithmic speed increment of 3 and a multiplication of the arithmetic speed by 10 gives an increment of 10 in logarithmic speed. With these notes, and the table, the corresponding equivalence for any standard logarithmic or arithmetic speed value may be calculated.
Table 13.3 Arithmetic and logarithmic ISO speed ratings compared
ARITHMETIC (S) |
LOGARITHMIC (S) |
10 |
11 |
20 |
14 |
25 |
15 |
32 |
16 |
40 |
17 |
50 |
18 |
64 |
19 |
80 |
20 |
100 |
21 |
1000 |
31 |
ISO speed ratings shown on boxes of films are daylight ratings, currently shown in both the following ways:
• ISO 200, arithmetic speed.
• ISO 64/19°, both arithmetic and logarithmic speeds.
These speed ratings are not necessarily true ISO speeds, but each is the manufacturer’s recommendation indicating an appropriate setting for a camera, or exposure meter, in order to achieve optimal results under normal conditions of use.
ISO film speeds are not published for materials which are not commonly used for pictorial photography. In such cases useful speed values have to be found by ‘trial and error’ methods.
CHARACTERISTICS OF PHOTOGRAPHIC MATERIALS
Graininess and granularity
A grainy pattern may be detected in photographic negatives even though the individual crystals of a photographic emulsion cannot be seen by the unaided eye. These patterns may be detected at much lower magnifications in prints. Enlargement of the negative, subject matter, development and viewing conditions all contribute to the impression of graininess and these aspects are detailed more extensively in Chapter 24.
Granularity is an objective measure of the inhomogeneity of a photographic image and is determined with the aid of a microdensitometer making traces of the density fluctuations of uniformly exposed areas of emulsions. Interestingly, there is a simple relationship between the area of the scanning aperture used in the microdensitometer and the standard deviation measured from the noise trace that yields a constant value. Known as Selwyn granularity, G, this single number is a useful indication of the granularity of an emulsion or print material. Selwyn granularity may be affected by emulsion parameters, development and exposure. The relationship to these factors is also explored in Chapter 24.
Contrast
Contrast has two related but distinct applications. It is widely used to describe the appearance of a scene: a cross-lit bright sunny urban landscape, without clouds, may have a large difference between the luminance of shadow and highlight areas. It is a high-contrast subject, possessing a high subject luminance range. The same scene on a foggy day will be a low-contrast subject, with a low subject luminance range. An average outdoor subject, lit by the sun, and with some clouds, gives a subject luminance range of 160:1, a log subject luminance range of 2.2.
Figure 13.9 Principles of the method adopted for determining speed in the current ISO standard.
Figure 13.10 Microdensitometer traces of edge images for a medium-speed film developed in: (a) an ‘acutance developer’, giving edge effects; (b) a standard general-purpose developer, without edge effects.
The second use describes how photographic materials reproduce the log exposure range presented to an emulsion in the camera, or at the printing stage. This is sensitometric contrast and its significance has been discussed in Chapter 8. The relationship between subject and image scales forms the study of tone reproduction described in Chapter 21.
Sharpness and acutance
Sharpness is a subjective term describing how sharply focused images appear. If an image looks out of focus, it will not be described as sharp. It also carries implications about the subject. An image can only appear sharp if the subject contains sharply defined material, such as an edge. Our judgement of sharpness depends heavily on edge detail. The objective correlate with sharpness is termed acutance and can be assessed. Certain photographic procedures that improve sharpness acquire a description using the term ‘acutance’ – acutance developers give high edge gradients and may show significant ‘edge effects’ – shown in Figure 13.10; acutance dyes added to emulsions absorb scattered light at exposure. Both give sharper results as described by observers. The reader should refer to Chapters 19 and 24 for extended reading on sharpness and its measurement.
CHEMISTRY OF THE PHOTOGRAPHIC PROCESS
Developers and development
Development forms a visible image corresponding to the invisible latent image. It continues the effect of light on the silver halide grain by converting exposed grains to black metallic silver (see Figure 13.1). The reaction may be described by the chemical equation:
In chemical development, each emulsion crystal acts as a unit: it is either developable as a whole, or not. A second type, physical development, derives the image from silver salts contained in the developer. This is not normally used but some physical development occurs in most developers, because they contain complexing agents, which dissolve some silver halide giving silver compounds in the developer. The latent image exists mainly on the surfaces of emulsion crystals. With sufficient exposure, latent images of a developable size are formed and become development centres. On development, the crystals are attacked, at development centres, by the developing agent and those having received more than some minimum exposure are reduced to metallic silver.
Developing agents are classed chemically as reducing agents. Very few reducing agents are selective enough to distinguish between emulsion crystals with a latent image and those without. For those that are sufficiently selective, the action on exposed and unexposed (or insufficiently exposed) crystals is distinguished by its rate. It is not that unexposed silver halide crystals do not develop, but that exposed crystals develop very much more quickly. Latent image accelerates development, but cannot initiate a reaction that would not otherwise occur. Normally the proportion of unexposed crystals developed is small, but with very prolonged development practically all the emulsion crystals will develop. Density resulting from development of unexposed crystals is called chemical fog.
Composition of a developing solution
Developing agents are not used alone; a developer solution almost always contains other essential constituents. A developing solution usually comprises:
• Developing agent (or agents) – to reduce exposed silver halide emulsion crystals to silver metal.
• Preservative – to (a) prevent wasteful oxidation of developing agent(s), to (b) prevent discoloration of used developing solution and consequent staining of negatives or prints, and to (c) act as a silver halide solvent in certain fine-grain developers.
• Alkali (or accelerator) – to activate developing agent(s) and buffer the pH at a constant value.
• Restrainer – to increase selectivity of development and minimize fog by decreasing the development rate of unexposed grains compared to that of exposed grains.
• Miscellaneous additions – include wetting agents, water-softening agents, solvents for silver halides, anti-swelling agents for tropical processing, development accelerators, etc. In addition, there must be a solvent, nearly always water.
Developing agents
Almost all developing agents in use are organic compounds. Not all behave in exactly the same way and, for certain purposes, one agent may be preferred to another. Consequently, a number of different agents are in use, the characteristics of important examples being described below. Names of organic chemicals follow international rules but common usage means that there can be different names used for the same compound.
L-Ascorbic acid (vitamin C)
This is a reducing agent. Like hydroquinone, it is more active at higher alkalinities. It forms very active super-additive developers with phenidone or Metol.
Glycin (4-hydroxyphenylaminoacetic acid)
This is a white crystalline powder. Slightly soluble in water, it is freely soluble in alkaline solutions. Glycin developers are non-staining and have exceptionally good keeping properties, but are too slow in action for general use. It is used, with other developing agents, in some fine-grain developers.
Hydroquinone (quinol; 1,4-dihydroxybenzene)
This forms fine white crystals, fairly soluble in water. The dry substance is kept well stoppered, otherwise it tends to become discoloured. Hydroquinone is mainly used together with Metol or phenidone, forming superadditive mixtures.
Metol (4-methylaminophenol sulphate)
This is readily soluble in water. Metol developers yield high emulsion speed, low contrast and fine grain, valuable when maximum shadow detail is required. Low-contrast developers may be made up using simply Metol and sulphite. In general, Metol is used with a second developing agent, hydroquinone, in a superadditive combination. This is illustrated in Figure 13.11. Metol–hydroquinone formulations may be referred to as MQ developers, the letter Q representing quinol.
Paraphenylenediamine
Many fine-grain developers are based on para-phenylenediamine (p-phenylenediamine, 4-aminoaniline, 1,4-diaminobenzene). When used alone it requires very long development times. To shorten development, most paraphenylenediamine developers contain either glycin or Metol. Derivatives of 4-aminoaniline are present in modern colour developers.
Figure 13.11 Superadditivity in a Metole–hydroquinone developer.
Phenidone (1-phenyl-3-pyrazolidone)
This possesses most of the photographic properties of metol together with important advantages. It activates hydroquinone, so that a phenidone–hydroquinone (PQ) mixture forms a useful, very active developer. Used alone, phenidone gives high emulsion speed but low contrast, and has a tendency to fog. Mixed with hydroquinone, with various alkali concentrations, phenidone features in a wide range of developers. Much lower concentrations of pheni-done than Metol are required.
More stable derivatives of phenidone have been proposed, and used in concentrated liquid developers. These include phenidone Z (1-phenyl-4-methyl-3-pyr-azolidone) and dimezone (1-phenyl-4,4-dimethyl-3-pyrazolidone).
Of the developing agents above, only phenidone, Metol and hydroquinone are widely used today for monochrome development, while analogues of p-phenylenediamine (4-aminoaniline) are essential in colour developers. All developing agents should be regarded as hazardous by skin absorption, and rubber gloves should always be worn when preparing or using developers.
Preservatives
Sodium sulphite is commonly used as preservative in developing solutions, although potassium (or sodium) metabisulphite is sometimes used, either by itself or with sulphite. It is available in crystalline and anhydrous forms. Anhydrous sodium sulphite is a white powder which dissolves readily in water. The crystals dissolve most easily in water at about 40°C, giving a weakly alkaline solution (pH approximately 8.5). The pH of a solution is an internationally agreed measure of its acidity or alkalinity. It is defined as:
On this scale, pure water (a neutral solution) has a pH of 7. An acid solution has a pH < 7 and an alkaline solution a pH > 7. The greater the amount by which the pH of a solution differs from 7, the greater is its acidity or alkalinity. Since the pH scale is logarithmic, quite small changes in pH may indicate significant changes in the activity of a solution.
One advantage of using potassium metabisulphite rather than sodium sulphite is that it forms a slightly acidic solution (pH 4–5), which decreases aerial oxidation in concentrated two-solution developers, i.e. one solution containing the developing agent and preservative and the other containing the alkali. Alternatively sodium meta-bisulphite may be used.
A preservative is present to prevent aerial oxidation of the developing agent(s). Sulphite can be regarded as removing oxygen from air in the solution, or at the surface, before it can oxidize the developing agent. This is an over-simplification. The action of preservatives is not simply a preferential reaction between sulphite and oxygen; the rate of uptake of oxygen by a solution of sulphite and hydroquinone is many times smaller than that by either the sulphite or hydroquinone alone. Sulphite also reacts with developer oxidation products and prevents staining of the image by forming soluble sulphonates:
In addition to being a preservative and stain preventer, sulphite has a third function. It is a silver halide solvent and promotes some physical development, leading to finer-grained images, if its concentration is sufficient.
Alkalis
By suitable choice of alkali, the pH of a developer can be adjusted to a required level and a range of varying activities can be achieved. One alkali commonly used is sodium carbonate. In some developer formulae, potassium carbonate is used as the alkali. It is supplied in anhydrous form and should be kept tightly sealed. Potassium salts have increased solubility, allowing them to be used at a higher concentration. In high-sulphite, low-energy fine-grain developers, borax (sodium tetraborate) is a common alkali.
Certain alkaline salts also act as buffers to maintain the pH value constant during development, on storage, or standing of the developer in a processing machine. A solution is buffered when it shows little or no change in pH on the addition of acid or alkali. Water is not buffered and its pH is greatly affected when only a little acid or alkali is added. Buffering of photographic solutions is commonly achieved by using relatively large amounts of a weak acid, e.g. boric acid, and the sodium salt of that acid, e.g. borax.
Restrainers (anti-foggants)
Two main types of restrainer are employed: inorganic and organic. The function of a restrainer is to reduce the development of unexposed silver halide crystals, preventing fog. Restrainers also reduce the development of minimally exposed crystals, i.e. at the foot of the characteristic curve, to some extent, and thus affect film speed. The effectiveness of a restrainer in minimizing fog, and its effect on film speed, vary from one developing agent to another and also depend on the pH of the solution.
Potassium bromide is the most widely used restrainer. Soluble bromide is produced as a by-product of the development process and affects the activity of the developer. Inclusion of bromide in the original developer helps to minimize the effect of this released bromide. Most developer formulae therefore use bromide as a restrainer, including those that contain organic restrainers. Developers for papers always include bromide because the smallest amount of fog is objectionable.
Organic restrainers, such as benzotriazole, are especially valuable in phenidone developers. The activity of most phenidone formulae is such that, to prevent fog with high-speed materials, the amount of bromide required as restrainer would be so great that there would be a risk of stain, bromide in excess being a mild silver halide solvent. Use of an organic restrainer avoids this risk. Some bromide is, however, usually included in phenidone formulae to keep developer activity constant with use.
Miscellaneous additions to developers
Besides the main ingredients – developing agent(s), alkali, preservative and restrainer – a developing solution sometimes contains other components for specific purposes. These may include wetting agents, silver halide solvents, anti-swelling agents for high-temperature processing, calcium-sequestering agents (water-softening agents) and development accelerators.
SUPERADDITIVITY (SYNERGESIS)
Many combinations of two developing agents have been found to have a far greater effect than the sum of their individual effects. This phenomenon is called superadditivity or synergesis. Figure 13.11 illustrates this phenomenon for Metol and hydroquinone developing agents.
If a film is developed in an MQ–carbonate developer for the recommended time, an image of normal contrast showing both highlight and shadow detail is obtained. If, however, the film is developed in the same basic developer formulation for the same time but omitting Metol, only the brightest highlights are recorded. If the film is developed for the same time in the developer, omitting hydroquinone, an image is obtained containing both highlight and shadow detail but somewhat lacking in contrast. These results show that a developer containing both Metol and hydroquinone produces a photographic effect greater than a simple addition of the effects of both.
Development time
Development is not usually instantaneous, and control of both development time and temperature is required for reproducible results. The effect of development time variation for monochrome film is illustrated in Figure 8.7 and for paper in Figure 8.11. Recommended development conditions for photographic materials are customarily specified at 20°C for monochrome materials and at higher temperatures for colour materials. While development times for monochrome materials may be adjusted to compensate for different developer temperatures, this is not recommended for colour processing where accurate time, temperature and agitation control is essential.
PRINTING
Sensitometric properties of printing paper have been described in Chapter 8 and the construction has been reviewed in this chapter. It remains to comment briefly on the features of print materials in practical use. Referring to Figure 8.8, we note, particularly, the high contrast, low Dmin and high Dmax. These qualities combine to give prints on glossy paper bright whites and rich blacks with good shadow detail.
It is generally agreed that, in a good print, there should appear:
• All the important subject tones, present in the negative.
• The full range of tones between black and white that the paper can produce. (Even in high-key and low-key photographs it is usually desirable that the print should show some white and some black, however small these areas may be.)
The first criterion indicates the log exposure range required of the paper (Figure 8.10), which must accept the density range of the negative, and that the negative must have received sufficient exposure to record all required shadow tones. The second requires a correct printing exposure and a suitable contrast grade of paper. It is clear that there is very little exposure latitude in printing, although there is some development latitude (Figure 8.11), usable for small adjustments by visual inspection during safe-lit dish development. Contrast grades are so designated that a negative of an average subject, correctly exposed and developed, will usually print well on grade 2 or 3 paper depending on the enlarger used.
Modern variable contrast papers comprise two emulsions of differing contrasts and spectral sensitivities. The contrast is varied by suitably filtering at exposure; typically the highest grades require a magenta grade 5 filter, and the lowest a yellow grade 0 filter. Intermediate contrasts require a set of appropriately designed colour filters. The characteristic curves corresponding to contrast grades approximate those found for traditionally graded papers.
The ranges of surfaces and paper grades (Figures 8.9 and 8.12) give photographers the opportunity to vary the appearance of prints and to cater for differing tastes – but any surface texture lowers the maximum black. Observer preferences tend to favour less than maximum black in deep shadow only if the subject was, itself, clearly of low contrast – misty landscapes and diffusely lit portraits being examples.
Meeting the criteria for a good print does not therefore necessarily imply that the resulting print will be aesthetically pleasing. In practice, the print will usually be satisfactory if the subject luminance range is not high, but with high-contrast subjects results may appear ‘flat’ and visually uninteresting, even though the criteria have been met. This is because the log luminance range of a high-contrast subject is far greater than the maximum density range that can be obtained on printing paper. Two remedies are possible. The simplest is to print the negative on a harder grade of paper and to sacrifice detail at either the shadow or the highlight end of the scale. This, in effect, abandons the first criterion but is often satisfactory. A preferred remedy is to use a ‘harder’ printing paper but to isolate the various tone bands within the picture, and treat them individually by additionally printing in lighter tones. In this way we can obtain the contrast we require within the various tone bands of the picture without sacrificing detail at either end of the scale. For aesthetic reasons it may thus be appropriate to introduce a measure of distortion in the tones of the final print.
Figure 13.12 Control of print contrast by pre-flashing the printing paper.
Some skilled monochrome printers use an additional method for controlling tone reproduction. A brief uniform pre-exposure, or pre-flash, is used to modify the characteristic curve of the printing paper. The higher the intensity of the uniform pre-flash the lower is the contrast obtained. There is usually some increase in the minimum density, although this may be acceptable if it is not too high. Results obtained with pre-flashes of increasing intensity are shown in Figure 13.12, which shows a large increase in useful exposure range. This method of controlling print contrast has, in fact, been successfully used in large-scale production using an automatic printer.
Darkroom work
Printing and development are carried out in a darkroom, using red or orange illumination to which black-and-white printing paper is not sensitive. Negative images are projected on to printing paper using an enlarger, which enables image magnification to be controlled, at a convenient lens aperture and exposure time. The former is usually set to give the highest sharpness in the image, at some aperture midway between minimum and maximum. The exposure time may be controlled manually, or by a suitable timer. Because a wide range of film and paper formats, as well as enlarging magnifications, may be encountered, the ISO speed of a printing paper is usually irrelevant. An initial stepped exposure is customarily used to determine printing exposures from first principles for a chosen negative. This guides both exposure time and choice of contrast grade, and the optimum print obtained is a guide for subsequent exposures from other negatives in the same printing session.
Development
Development may be manual using dishes or, for large-scale work, a processing machine can be used. In either case, reproducibility is vital. It is customarily achieved by a choice of development time sufficiently long for minor time variations to produce no perceptible image changes. Figure 8.11 illustrates the minimal effect of small development variations beyond 120 s for a particular set-up. Modern papers often require shorter development times to achieve a useful constancy of result. An acid fixer will both stop development immediately and fix the print after development. Complete fixing and washing are essential for print stability, and are followed by drying.
Critical inspection of the finished print under good lighting conditions is useful in deciding whether to improve the result by reprinting, with suitable modifications, and in developing the photographer’s critical faculties.
BIBLIOGRAPHY
Allbright, G.S., 1991. Emulsion speed rating systems. Journal of Photographic Science 39, 95–99.
Coote, J.H., 1982. Monochrome Darkroom Practice. Focal Press, London, UK.
Diamond, A.S. (Ed.), 1991. Handbook of Imaging Materials. Marcel Dekker, New York, USA.
Eggleston, J., 1984. Sensitometry for Photographers. Focal Press, London, UK.
ISO 6:1993, 1993. Photography – Black-and-White Pictorial Still Camera Camera Negative Films/Process Systems – Determination of ISO Speed.
ISO 6846:1992, 1992. Photography – Black-and-White Continuous Tone Papers – Determination of ISO Speed and ISO Range for Printing.
Jacobson, R.E., Jacobson, K.I., 1980. Developing, eighteenth ed. Focal Press, London, UK.
Jacobson, R.E., Ray, S.F., Attridge, G.G., Axford, N.R., 2000. The Manual of Photography, ninth ed. Focal Press, Oxford, UK.
James, T.H. (Ed.), 1977. The Theory of the Photographic Process, fourth ed. Macmillan, New York, USA.
Langford, M., Bilissi, E., 2008. Langford’s Advanced Photography, seventh ed. Focal Press, Oxford, UK.
Proudfoot, C.N. (Ed.), 1997. Handbook of Photographic Science and Engineering, second ed. IS&T, Springfield, VA, USA.
Tani, T., 1995. Photographic Sensitivity. Oxford University Press, Oxford, UK.
Todd, H.N., Zakia, R.D., 1974. Photographic Sensitometry. Morgan & Morgan, New York, USA.
Walls, H.J., Attridge, G.G., 1977. Basic Photo Science. Focal Press, London, UK.