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Silver Halide Materials: General Emulsion Properties - Why Use Silver Halides?, Silver Halide Emulsions, The Silver Halide Grains, The Gelatin Medium, Emulsion Coating

photographic process light characteristics

ALAN HODGSON, PH.D.
Alan Hodgson Consulting

Why Use Silver Halides?

Many light-sensitive substances are known to exist with a wide variation in their sensitivity characteristics. More than 2000 years ago the Phoenicians of the city of Tyre found that the pale yellow slime of the Purpura snail turned purple when exposed to air and light. Although not used in any photographic sense, this process was used to produce the rich Tyrian Purple cloth.

Around 1826 Joseph Nicéphore Niépce created the earliest camera photograph still in existence, utilizing bitumen as the light-sensitive substance. An exposure time of around 8 hours was used and the resulting image, View from the Window at Le Gras, is currently in the Gernsheim Collection at the University of Texas at Austin. However, only a few substances have ever made their way into commercial systems; for example, some of these photopolymers have found use in applications such as holography and photoresists in electronics manufacture and printing. Coming closer to photography, the diazo process has been used for document reproduction. However, the most successful systems for photography are based on the chemistry of silver.

In the 1720s a German professor, Johann Heinrich Schulze, observed that silver compounds darkened when exposed to light. Although he recorded some shadows in his work he did not produce actual photographs. By 1800 an English chemist, Thomas Wedgwood, had produced images on leather treated with silver compounds.

Commercial photographic materials now rely almost exclusively on the light sensitivity of silver halides. These are compounds formed by the combination of silver with elements known as halogens—in this case bromine, chlorine, and iodine. In the 1830s Louis Jacques Mandé Daguerre found that silver iodide exhibited much greater light sensitivity than Niépce’s bitumen. Independent of Niépce, William Henry Fox Talbot was using a similar system based on silver chloride.

The reason that silver halide systems are so attractive for photography is that the system can be made to be extremely sensitive to light. During the chemical development process large visible changes can be produced in the recording material from only a minimal amount of light exposure. This amplification step in development is either missing in some other processes such as diazo or much smaller in others, which leaves silver halide a big sensitivity advantage. Unlike some other processes, the unexposed silver salts can be easily removed, fixing the image and markedly increasing permanence.

Photographic products containing silver halide emulsions have been produced where the action of light alone produces a printed out image. Because the wet chemical development step and the associated gain in sensitivity are missing, they were much slower in sensitometric terms than conventionally processed materials. However, they did find use in contact printing and oscillograph recording papers.

Mention should also be made of photothermographic technology using compounds such as silver behenate. These are “dry silver” processes where the silver compounds are reduced to metallic silver to form the image using heat. A number of commercial products have appeared using this process.

Silver Halide Emulsions

The use of the word emulsion in this context is actually a misnomer. Photographic emulsions are in reality suspensions of minute crystals of silver halide in a polymer matrix. Although this polymer is now often gelatin, other chemicals, notably latex polymers, are now added to the gelatin to modify its characteristics.

The relative proportions of silver halide and gelatin vary with the type of product required. In general, photographic films and papers tend to have much more gelatin by volume than silver halide. This gelatin excess allows the product to swell when wet, facilitating effective chemical processing (see below). However, some products are much more concentrated in terms of volume fraction of silver halide. In practice, the upper limit is around equal volume fractions of silver halide and gelatin. Nuclear emulsions and plates are silver-rich products that are manufactured at these concentrations. They are designed to produce continuous lines of grains recording the passage of energetic particles through the concentrated emulsions.

The silver halide crystals in emulsions are sometimes referred to as grains. Unfortunately the same word is also used to describe the developed grains of metallic silver that make up the developed image.

The Silver Halide Grains

As described above, silver halides are compounds made from silver and chlorine, bromine, or iodine. In commercial emulsions it is not normal to produce a pure silver chloride, bromide, or iodide. Instead, a mixed halide is produced where two halides are present in each individual crystal, although not necessarily in the same proportion throughout each grain. For example, in emulsions designed for the production of negatives, the halide employed is often silver bromide with a small quantity of iodide, known as an iodobromide emulsion. In emulsions for photographic papers the halide mix may be silver bromide, silver chloride, or a chlorobromide.

The photographic recording action of silver halide can be described as follows. A silver halide crystal consists of an ordered matrix of positively charged silver ions (denoted as Ag + ) and an equal quantity of negatively charged halide ions (such as Br – ). When a photon of light is absorbed by the crystal an electron is liberated that combines with a silver ion to produce a silver atom. After this particular crystal has absorbed a small number (around 3 +) of photons, a small, stable cluster of silver atoms is formed. This small cluster is known as a latent image as it is still too small to be seen, even with an electron microscope. When the crystal is processed in a photographic developer, the latent image promotes the reduction of the whole crystal to a visible particle of silver. The silver from all of the crystals that have recorded a latent image in this manner make up the processed recorded image.

The Gelatin Medium

The polymer matrix used as a binder for the grains must be more than just a passive entity. It has to have some very important characteristics. It must be chemically compatible with the emulsion-making process. In addition to the requirement to work in aqueous solutions, the polymer must ensure that the resultant silver halide grains remain separate and stable. It must be optically clear so as to allow light to reach the silver halide grains during exposure. This means that it must be transparent at the relevant wavelengths. Its low refractive index ensures that light interacts effectively with the high refractive index silver halide crystals.

When dried, the polymer must bind the grains to the base and give the photographic layer the requisite physical characteristics to work in a photographic process. For example, roll film must resist abrasion, scratching, and sticking in cameras and photographic papers must be able to be packed in stacks in a box without sustaining damage. During photographic processing the polymer must allow the solutions to penetrate and then be adequately washed clean for archival permanence. Gelatin also has the advantage that the swelling characteristics with pH are well suited to the pH at which typical developers are active.

After processing the polymer must again impart suitable physical characteristics, quite often different to those pre-process. For example, in photographic papers the surface should now have the right optical gloss characteristics and resist finger marking. One of the first binders used was collodion. The major disadvantage of this chemical was that it needed to be exposed while still wet or the ability to effectively process the emulsion was lost. This requirement made the use of collodion plates difficult but gave rise to the name “wet collodion.”

Collodion was rapidly replaced by the use of gelatin, which had four major advantages.

  1. The emulsions could be exposed while dry. Not only was this a major convenience with glass plates, but it paved the way for film-based products that could be stacked or rolled, before and after exposure, with processing delayed until convenient.
  2. Only water was needed as a solvent in manufacture. This not only made the emulsions cheaper to produce due to the absence of organic solvents but also reduced fire and explosion hazards in manufacture.
  3. Plates coated with a gelatin-based emulsion were found to be a number of stops higher in sensitivity to light than the corresponding collodion preparation. These early photographic chemists had happened upon the science of chemical sensitization. Long exposures became possible, extending the sensitivity beyond that of the eye and allowing faint objects such as unseen stars and spectra to be recorded.
  4. The use of gelatin to bind photographic emulsions stems from the 1870s and since then it has become the preferred binder for emulsions. Chemically modified gelatins and mixtures with other polymers such as polyvinyl alcohol (PVA) are used, but gelatin is still essential in the photographic process.

Gelatin is manufactured from collagen, a natural product extracted from the hide and bones of animals. As it is produced from a natural product it can be of variable composition and in an unpurified form can contain many additional compounds that are photographically active. As a result there is a considerable art and science in purifying and blending gelatins to ensure an end product of uniform characteristics.

Gelatin has survived as the preferred medium for silver halide technology over the last century. This is not only because of many useful properties in a photographic context but also because the photographic process has evolved around gelatin-based emulsions. The useful characteristics of gelatin in a photographic context can be described as follows.

Gelatin is hydrophilic and readily disperses in water. The resultant solution makes a convenient medium for the chemical reactions that produce silver halide. Once the silver halide crystals have been formed the gelatin coats the surface of each crystal, keeping them individual and dispersed in the liquid. From a manufacturing standpoint the fact that gelatin works in aqueous solution without the need of high temperatures or expensive, flammable solvents has done much to keep production costs low.

In warm aqueous solution a gelatin-based emulsion has relatively low viscosity, which allows it to be pumped and coated onto base material. However, when chilled it sets rapidly to a jelly giving the coating some physical stability during the drying process. Chemicals can be added during the coating processes that cross-link the gelatin molecules. These can promote adhesion of the emulsion to the base material and improve the characteristics of the layer by making it more physically robust.

Gelatin has very good optical properties from a photographic point of view. It is transparent throughout the range of visible wavelengths and is not optically active. This transparency extends out into the near infrared for scientific, creative, and surveillance applications and out into the near ultraviolet too. Only in a limited number of scientific or technical applications requiring far ultraviolet or charged particle detection are special coatings needed where the bulk and/or the surface of the emulsion layer has a high silver halide:gelatin ratio.

The exposed photographic layer is processed using aqueous solutions. When the gelatin layer is placed in these solutions it takes up the solution and swells. This has the double benefit of bringing the processing solution quickly into contact with the silver halide crystals, which promotes rapid processing and the
swelled layer allows effective exchange of residual chemicals with washing solutions. This facilitates the efficient removal of chemicals in the final washing stages.

As mentioned above, emulsions made in gelatin are significantly faster than those made in collodion. This is because gelatin can also contain compounds that act as sensitizers to silver halide. Although this is no longer an issue now that the science of sensitization is better understood, it was a major advantage a century ago. Gelatin also plays an important role in determining the size, size distribution, shape, and sensitivity of the silver halide grains, as discussed below.

Emulsion Coating

In parallel with emulsions, coating technology has made many advances over the past century. Coating machines have evolved covering a wide spectrum of capabilities from small-scale systems for coating onto individual sheets of glass to huge web coaters that can simultaneously coat multiple wet layers on flexible substrates. As one of the major variables in this process is the emulsion support, this should be our first consideration.

Types of Emulsion Support

In the majority of cases the silver halide emulsions are coated onto some type of support, commonly referred to as the “base.” However, there are a few exceptions that find use in scientific applications. The first is the use of pellicles. Pellicles are sheets that are cast from molten emulsion to thickness of up to 1 mm. They are used in the detection of charged particles in particle physics experiments. Because of the laborious nature of the processing and the difficulties in handling this type of product, this is not a common application.

Much more usual is the use of liquid emulsions in autoradiography. In this technique, micro-autoradiographers use a bottle of emulsion and coat their own samples.

The most common base materials are now plastic film and paper. However, rigid substrates of which the most common is glass, are still important. Historically, glass was the base of choice.

The Coating Process

Originally, emulsions were applied to the base by pouring from a receptacle (for example, a teapot) and spreading the emulsion out with a tool. Modern coating machines, where the emulsion falls in a steady flow onto the moving base, are capable of much higher speeds and uniformity. However, the underlying science of the process remains the same.

The clean base must be prepared to ensure good adhesion and wetting of the emulsion. This is done by first coating the base with a thin layer of a substance such as hardened gelatin. The emulsion layers are then coated onto this base by passing them under a coating head. The coating head can be made to dispense several layers on top of each other as a stable curtain or cascade of liquid. In this way multiple layers can be coated in one pass, giving the assembly designer many options in terms of photographic and physical properties design. This design often incorporates a protective non-stress supercoat to reduce the effects of abrasion and to give photographic papers the requisite gloss characteristics. This type of coating process is often referred to as Slot Die.

The coating is then chilled to set the coated layers, which are then dried. The rate and conditions of drying are also a variable determining the characteristics of the final product.

Film and paper products are coated as large rolls, typically over a meter in width and hundreds of meters long. These are then slit, chopped, and perhaps reeled into the required sizes. Glass plates are coated on glass of various areas and thickness depending on the application. Small plates are prepared by coating larger sheets of glass and cutting them down to the required size.

Modulation Transfer Function

The MTF in photography is the optical equivalent of the frequency response plots commonly produced for audio systems. In the case of optical systems it is the spatial frequency response in lines and spaces (line pairs) per millimeter that is measured rather than sound waves in cycles per second (Hz). In both cases the normalized response is plotted as a function of the relevant frequency measurement.

MTF measurements have a better correlation to perceived sharpness and “sparkle” of an image than resolution. However, they are a good deal more difficult and expensive to perform and generate a curve rather than a single number. For scientific applications there is the advantage that the MTF curves for successive steps in an imaging process can be cascaded to produce an MTF of a complete system.

The Future of Silver Halide

For many years pundits have been forecasting the death of silver halide photography but it has stubbornly refused to disappear. Although there is a definite trend to digital photography for general consumer and professional photography, some key areas such as creative and scientific photography look set to retain an analog component in the longer term. There are also imaging applications of silver halide technologies that are not photographic. Research papers have been published citing liquid crystal displays, charge-coupled device (CCD) sensors, thin layer electronic devices, photo catalysts, and dye-sensitized solar cells as potential applications.

Silver halide technology looks set to be with us for some time yet!

Silvera, Frank (1914–1970) [next] [back] Silver Dye-Bleach Photography - Basic Color Photographic Principles, History of Silver-Dye Bleach

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