This book really is a time-capsule. It will be most useful for historians of the 22nd century.

This book really is a time-capsule. It will be most useful for historians of the 22nd century.

— “Chemistry and Industry, 7 April 2008, 28 (Michael Gross)”

The Chemistry of Photography

From Classical to Digital Technologies

By David Rogers

The Royal Society of Chemistry

Copyright © 2007 Danercon Ltd.
All rights reserved.
ISBN: 978-0-85404-273-9

Contents

SECTION 1: CONVENTIONAL FILMS AND PAPERS,
Chapter 1 The Overall System (Capture and Output), 3,
Chapter 2 Gelatin, 16,
Chapter 3 Light Capture and Amplification, 24,
Chapter 4 Developers, 43,
Chapter 5 Processing Solutions, 54,
Chapter 6 Colour Forming Couplers, 66,
Chapter 7 Image Dye Formation and Stability, 94,
Chapter 8 The Chemistry of Colour, 109,
Chapter 9 Film Structures, 131,
Chapter 10 Paper Structures, 167,
Chapter 11 Kodachrome Films, 184,
Chapter 12 Motion Picture Films, 196,
Chapter 13 Instant Colour Photography, 202,
SECTION 2: THE CHEMISTRY OF DIGITAL PRODUCTS,
Chapter 14 Inkjet Paper, 216,
Bibliography, 228,
Subject Index, 230,

CHAPTER 1

The Overall System (Capture and Output)

The use of colour filters in the production of colour images pre-dates colour photography as we know it by almost 80 years. James Clerk Maxwell produced coloured images in 1861 by projecting the same image through three projectors, each of which had one of the three colour filters of red, green and blue. Maxwell produced silver positive images, which were created from emulsions that were not spectrally sensitised. These images were surprisingly good. They relied on the additive colour system, whereby the three primary colours (red, green and blue) produce the full gamut of colours on the composite final image, Figure 1.

1.1 The Additive Colour Process

Each filter was designed to transmit light of only one primary colour. This system might find limited application in transmission systems where three images can be carefully brought together in register. Unfortunately, this system is cumbersome. It is possible to produce a coloured image but it is very difficult to align the three colour images, certainly with any speed. The combination of the three filters into one pack, which would allow for image alignment, is not a practical option either as there would be no light transmitted through the pack under some circumstances, especially if the filters were to have much colour density to them, Figure 2.

A more practical alternative, at least in terms of image alignment, is to expose and view all the colour images at the same time. Under these circumstances, it is not possible to use the additive system as described above, for the obvious fact that there would be no light exiting the images.

The subtractive colour process, on the other hand, subtracts either red, green or blue light from the visible spectrum i.e. each dye subtracts one third of the visible spectrum and not two thirds, as is the case for the primary colours. Under these circumstances the original image produces a positive, through the intermediate step of a negative image, Figure 3.

1.2 The Subtractive Colour Process

The subtractive colour system uses the dyes cyan, magenta and yellow. The combination of cyan and magenta produces blue, yellow and cyan produces green and magenta and yellow produces red. A white positive image is achieved by producing a totally black negative, i.e. exposure of all three of the cyan, magenta and yellow records (in the negative), whereas a black positive image results in the absence of colour in the negative.

In practice, the situation is slightly more complex as there are some physical constraints. Silver halide crystals are inherently sensitive to blue light and so a yellow filter layer needs to be introduced between the yellow records and the other two records, making layer order important. Silver halide crystals need to be spectrally sensitised so that they capture as much of the available light as possible. The cyan, magenta and yellow dyes are not perfect dyes and therefore there is a need for colour correction. In addition, the initial material needs to contain, at least as far as possible, colourless components during the exposure of the negative. Additionally light may simply be reflected either between the silver halide grains or the various surfaces. Also the photographer may require or need exposure latitude, so that he/she can expose a scene under a variety of light levels. The exposed film may rest in a glove compartment of a car where the temperatures may rise somewhat. It may also be the case that the film is used to take pictures during Christmas holidays, and the film not used until the next year. Alternatively, the amateur photographer may try to use one film for both winter and summer shots, where the lighting conditions are completely different. The negative film therefore needs to be stable to humidity/light, etc. and have the capability of retaining the latent image for a long period of time.

These considerations aside, at least for the moment, the overall system of positive and negative, might be described in Figure 4.

The positive image displayed above may not, at first glance, look like a copy of the original scene. Perhaps it is easier to consider an actual scene. The picture below (Figure 5) is a yellow image of an actual scene. On close inspection, one can see that it is not entirely yellow, there is contamination from other colours. This is because the yellow dyes are not pure and have unwanted absorptions, which will be covered in detail in Chapter 8.

The corresponding cyan image is shown in Figure 6.

These images, when combined together, produce a green reproduction of the original scene, Figure 7.

Similarly, magenta (Figure 8) and cyan (Figure 9) images together form a blue image, Figure 10.

Finally the combination of yellow (Figure 11) and magenta (Figure 12) produces a red image, Figure 13.

The combination of cyan, magenta and yellow returns the original scene, Figure 14.

The two-stage process of negative and positive provides the photographic manufacturer with the means to convert a positive image to an intermediate negative, and then back to the positive image again in the photographic paper. The use of the subtractive colour process with transparency film is somewhat more complex, as there is no physical intermediate. Transparency films are designed to be projected onto a screen. So how can a transparency film use the subtractive process and, without the use of a physical intermediate, afford a positive image capable of being viewed by projection?

The method used is to generate an intermediate negative within the photographic layers, during the processing stage, and to subsequently reverse the image during the processing stages.

The schematic diagram below shows the process, Figure 15.

This reversal process is unusual from many perspectives, see Chapter 11. While it is possible to evaluate pictures against criteria such as:

• How sharp is the film?

• How much photographic latitude does it have (the ability for under and over-exposure)?

• How grainy is the scene?

The more rigorous approach is to expose the film or paper to various test objects under standard lighting conditions, so that objective and quantitative photographic parameters can be measured, and several competitor’s products or experimental films and papers compared objectively.

On first inspection, one might assume that there is a simple relationship between the exposure given to a photographic film or paper and the subsequent density produced. The relationship between exposure and density is, however, more complex. In practice, density is plotted against the log of the exposure. A simplified schematic of a test object, which is commonly used to generate a density vs. log exposure curve is given below, Figure 16.

A white light exposure of a colour film exposes all three colour records, producing this graded image, more commonly known as a ‘grey scale’. In this case each distinct density area will produce a density reading in each colour record, as it needs three density records to produce a black negative image. The step wedge densities are known to a high accuracy, and so the exposure of the negative for each step on the wedge can be plotted. The density steps on the colour film or paper will, when plotted against the exposure, produce three curves. One example of the type of curves produced from a colour negative film is given below as Figure 17.

The straight-line portion of the curve is known as the contrast or gamma. It is different for different applications. In this particular case the contrast may be in the region of 0.7, in graphic arts films the contrast might be as high as 5.0-6.0, because that application requires the formation of dots and not a continuous image. The lower part of the curve is known as the ‘toe’, and the upper part of the curve is known as the ‘shoulder’ or ‘Dmax region’. This is the maximum density that is possible to create with the combination of exposure/dye and silver laydown levels used for that formulation. The relative separation of the dye curves with exposure provides the filmbuilder with the challenge of creating consistent colours in a scene, if photographed under different lighting conditions. This issue is extremely important and will be covered in the discussion concerning the chemistry of colour, see Chapter 8. The density/log exposure or sensitometric curves for colour films and papers appear in many of the standard texts and are discussed in various subsequent chapters.

Graphic arts films require high contrast as the method of generating the image with graphic arts films is to generate a halftone image, which is made up of dots, the size of which determines the level of light and dark in the picture. For example, Figure 18 shows a uniform halftone dot.

For demonstration purposes, Figure 19 shows our standard scene as a halftone image.

A close up of the image shows the halftone dots, see Figure 20.

Photographic components can be application-specific, for example black and white films do not use colour couplers. Chapter 6 discusses these compounds. Similarly photographic paper products do not use clear plastic base as films do. The most complex photographic product is colour film. Prior to a discussion of the chemistry of this or indeed any other product, it is worth considering the type of chemicals that are coated in an average colour negative film, Figure 21.

1.3 Cross-Section of a Typical Colour Film Layer

The size and number of shapes have been used for demonstration purposes only. They do not represent the relative amounts of the various chemicals coated during the manufacturing process.

The light sensitive silver halide crystals capture the light when film is exposed. During processing the latent image is magnified, oxidising the colour developer. This then reacts with the couplers to form dye in the image areas, or reacts with image correction chemicals. Gelatin is the coating medium for all of these chemicals.

The types of chemicals listed above are not the only ones coated. There is a range of coating aids, usually coated in one layer (but capable of diffusion to all of the layers) and hardener to cross-link the gelatin, etc. During the next few chapters, each of the various components will be described in more detail.

CHAPTER 2

Gelatin

Gelatin has been the medium of choice in which to disperse and coat photographic materials for well over 100 years. Over that time there have been many research projects that have looked at potential alternatives. For one reason or another, all of the synthetic alternatives have properties or costs, which prohibited their use as a photographic medium. While the properties of gelatin will be covered in more detail later in this chapter, it is worth recording here that gelatin is a naturally occurring substance and is a bi-product of other industries.

Gelatin is extracted from collagen, which is the most abundant protein of the higher mammals, being present in connective tissues such as bone, cartilage, ligaments, skin and tendons. Gelatin can be extracted from collagen using either basic or acidic conditions, the residue being lipids, mucopolysaccharides, non-collagen protein and polynucleic acids.

The Gelatin Manufacturer’s Institute of America (established 1956) was formed to carry on research in the manufacture and usage of gelatin and to carry on promotional work in its uses. Representing the interests of gelatin manufacturers across North America they published the protein quality of gelatin on their website (http://www.gelatin-mia.com/html/gelatine_health.html), some data from which is reproduced below (Figure 1). It is unclear if these results are from the analysis of acid or alkali washed gelatin. This data, however, records the types and amounts of amino acids present in gelatin – at least in the higher animals.

The multi-stage process of converting bones into gelatin is detailed on the websites of various suppliers of photographic gelatin, see for example (http://www.eastmangel.com/ (suppliers to Eastman Kodak Co. since 1930) and http://www.rousselot.com/index.html). Gelatin produced from Rousselot is used in the United States and Europe. The incoming raw materials can be from a number of sources, the chemical content of which varies.

2.1 The Gelatin Manufacturing Process

Figure 2 outlines the key process variables and steps in the process deemed important in photographic quality gelatin. Eastman Gel reports that it takes about 6 kg of cattle bone to produce each kilogramme of gelatin, which serves to show the level of waste. It is also not a quick process. Even though there are checks throughout the process, some trace element concentrations vary from batch to batch. A few of these trace elements have photographic effects, as they react with or affect the silver halide crystals. These trace elements cannot be removed but their concentration levels can be tolerated if the levels of the impurities are kept constant. In some cases this can only be achieved by blending different gelatin batches.

Other non-photographic industries use gelatin, including baked goods, icing and gelatin desserts. Gelatin is also used as a base for cosmetic and pharmaceutical products, for example the coating on a gel cap style pill. The gelatin produced for these food applications is actually produced to a level of impurities that is higher than for photographic quality gelatin.

Gelatin is a mixture of natural polymers, and the medium into which photographic components are mixed, prior to coating. Once coated into products the layers are hardened by cross-linking the gelatin polymers. The photographic industry uses the following properties of gelatin:

• vertical swell

• optical rotation of light

• hardening/cross-linking capabilities

• impurities

• available viscosity range

• a range or molecular weights

• ‘crackability’/the ability to bend.

These have been known for many years since the broad application, i.e. a medium in which photographically active materials are dispersed, has not changed from the early days of photography. Indeed, Sheppard and his co-workers at Eastman Kodak Co. published some of the seminal work over 70 years ago.

There are several analytical tests to which photographic products might be subjected, which are designed to test/understand coated and dried gelatin layers. They include

• bloom

• gel strength

• melting point

• scratch resistance

• swelling

• wet abrasion, sometimes known as mushiness.

These tests are designed to simulate the uses (and abuses) of commercial products. Samples are also incubated in ovens of varying temperature and relative humidity (perhaps 70°C with a relative humidity of 50%), again designed to test the hardened/cross-linked gelatin layers.

Melting point is the term that is used to describe the temperature at which a gelatin layer will separate from the base upon which it is coated. Gelatin is known to change its properties above 40°C, as it becomes a mix of extremely polydispersed molecules. Below 40°C, however, the coiled gelatin molecules undergo a transition into a helical conformation and resemble stiff rods. The melting point test may reach or exceed this temperature, but as the melting point test is a destructive test, the gelatin structure is not an issue.

These and other properties of gelatin are often reported at international conferences – a series of which ran in the 1970s. The Imaging Science Group of the Royal Photographic Society sponsored a gelatin conference in 2005 – see http://www.rps-isg.org/gelatin2005.php.

Gelatin coatings need care when they are dried as it is possible to affect the coating, particularly if low temperature chilling is followed by high temperature drying. The defect created by such thermal treatment is known as ‘reticulation’, see the example in Figure 3. The picture below is part of the standard picture, magnified to demonstrate reticulation. It is produced in black and white so that the reader is not distracted by any dye cloud issues.

Repeating the drying cycle at more appropriate temperatures with a fresh sample of photographic material should resolve the issue, as would an increase in the hardener level. Hardener levels are strictly controlled during the manufacturing process, and so should not be the cause of reticulation as seen by a customer. Nevertheless, abnormally low hardener levels will affect the film properties. It would also affect the swelling characteristics of the photographic product.

Hardeners may be derived from a number of different chemical families, for example

• Aldehydes, e.g., formaldehyde – higher alkyl homologs of formal dehyde have no effect on gelatin

• Aldehyde acids, e.g., 2,3-dichloro-4-oxo-2-butenoic acid

• Bisaziridines

• Bisepoxides

• Carbodiimides

• Compounds with activated double bonds, e.g., divinylsulfones. NI, NII-trisacryloylper-hydro-s-triazine

• Dichlorotriazine derivatives, such as 4,6-dichloro-2-hydroxy-5-triazine

• Diketones, e.g., 2,3-butanedione, 1,2-cyclopentanedione

• Dihalides, e.g., 1,3-dichloropropanol

• Diisocyanate bisulfite adducts

• Epoxides

• Isocyanates

• Polybasic acids – specifically anhydrides and acid chlorides

• Sulfonate esters

• Sulfonyl halides e.g., bis(sulfonyl chlorides).

(Continues…)Excerpted from The Chemistry of Photography by David Rogers. Copyright © 2007 Danercon Ltd.. Excerpted by permission of The Royal Society of Chemistry.
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