An Introduction to Photothermography

M. R. V. Sahyun

Introduction

Photothermographic imaging materials comprise a class of media on which imagery or data are recorded optically to form an invisible pattern, i.e., latent image, which is rendered visible by application of heat. While exposure techniques vary, depending on application, the heating is usually accomplished by passing the exposed media over a heated roller. Alternative heat sources, such as microwave heating and thermostatted fluorochemical baths, have been discussed in the patent literature.

All the chemical components required to confer light sensitivity and enable development are contained in the photothermographic composition. Photothermographic materials are therefore thermodynamically unstable and, as a result, shelf stability of photothermographic products and stability of the images produced in these media have required major product development efforts by the various manufacturers. Most modern photothermographic compositions involve at least three components:

1. a light sensitive compound, usually silver halide, although the patent literature discloses utility of inorganic photoconductors, e.g., titanium dioxide, and molecular species including certain dyes and tetraarylborate salts;
2. a reducible silver salt, often the silver salt of a long-chain carboxylic acid which; and
3. reducing agents capable of reducing the second component on application of heat to form the metallic silver image.
4. This tutorial review will provide a brief history of photothermographic technology, discuss some applications of current and historical importance, and provide a taxonomy of contemporary photothermographic products.

History

The first reported photothermographic composition was disclosed by Fox Talbot in 1847; in this case thermal development was achieved over "…a gentle fire"! In the intervening years, a number of other proposals for photothermographic materials have been offered, many of them employing silver oxalate, as both the light sensitive and the image forming component.

Practical photothermography as practiced today arrived on the market in 1964 with introduction of 3M Dry Silver™ for microfilm application. The inventors of record were David Morgan, Ben Shely, Joseph Shepard, and David Sorenson. Accounts of the early history of this family of products have been provided by both Shepard¹ and Morgan². In some ways their technology, which, as initially described, utilized silver chloride as the light sensitive component and silver saccharin as the thermally developable, image forming component, was an extension of a more primitive photothermographic technology, the 3M Dual Spectrum™ process, inveneted by Wesley Workman, which grew out of the 3M ThermoFax™ products, thermographic imaging media invented by Carl Miller, and still used to make, e.g., overhead transparencies using thermal printheads.

In the Dual Spectrum™ process, originally developed as a coated-paper office copying product, an image is recorded by intense light exposure on a "pink" sheet comprising a rhodamine dye and a phenolic reducing agent. Photoexcitation of the rhodamine dye produces singlet oxygen, the same intermediate exploited in current photodynamic cancer therapies and also thought to be involved in biological aging processes, which destroys the phenolic compound. The exposed pink sheet is then laminated with a paper receptor, coated with a silver salt; on application of heat the phenol transfers to the receptor from areas of the pink sheet where it hasn't been destroyed, and reduces the silver salt to form a black image. This product was the first commercial application of singlet oxygen technology, but is no longer on the market. All major photographic manufacturers now offer photothermographic products for a variety of applications, although 3M spun off its entire Dry Silver™ product line to Imation Corp. in 1996, which, in turn, sold the product line to Eastman Kodak Co., which already had its own line of photothermographic products. Aspects of the history of photothermographic technology are also recounted in the article by Klosterboer³.

Applications

As mentioned above, silver-based photothermographic materials were initially introduced to the market for microfilm and microfiche applications. Subsequently developed applications included enlargements from microforms (now largely supplanted by electrophotographic technology), oil well logging (a form of strip-chart recording which tracks the progress of the drill bit in real time during well drilling), and military reconnaissance. The oil drilling application resulted in photothermography playing a role in detective fiction, Tony Hillerman's People of Darkness, in this case. The military applications enabled generation of hard copy from reconnaissance photography, both film and digital, under field conditions where water, chemicals, etc., used in regular photofinishing were not readily available. For the US, photothermographic image output was used extensively in the First Gulf War. Military applications also justified the funding of extensive R&D efforts on photothermographic materials in Russia in the Soviet era. Much of this work is only now reaching the open scientific literature, and remains a valuable resource of fundamental scientific understanding for ongoing commercial development efforts.

The principal surviving micrographic application for photothermography is in computer generated microforms (COM). In COM, digital information is composed into pages and written onto microfilm using a CRT, LED array or scanned diode laser. This operation provides an optically accessible, eye-readable record with good archival quality. The eye-readable feature of the record provides protection against information loss owing to data format obsolescence.

In 1996 3M introduced the Dry View™ laser imager which outputs digital medical diagnostic imagery onto photothermographic film. The product writes the film with a modulated near-infrared (NIR) diode laser; the specially formulated film is not only NIR sensitive, but its characteristic curve is designed to provide a tonal scale matching that of traditional medical radiographs and, like x-ray emulsions, it is coated on a blue base. An important feature of the Dry View™ technology is that output from various imaging modalities, e.g., ultrasound, magnetic resonance, digital radiography, computer-aided tomography, etc., can be combined and displayed in the same format, namely on a film that can be viewed by the radiologist on a light box like a conventional film x-ray. In the development of the imager-film system photothermography, rather than conventional silver halide film, was chosen for the output media so that the imager could be used at remote locations and independent of the availability of a photographic darkroom and ancillary utilities. The Dry View™ imager-film system is now a product of the Eastman Kodak Company and other photographic manufacturers now offer similar products. It is highly successful; over 300,000 imagers have been placed to date, and the annual photothermographic film market worldwide for this application alone is over $1 billion.

During the last few years it has become commonplace to include an image capture capability in almost all cell phones. These capture devices have now reached megapixel capability, enabling them to be used photographically. The Fuji Photo Film Co. has recognized the need for a suitable output device for this imagery and introduced in 2004 a miniaturized, portable printer with IR communication capability, which outputs cell phone captured imagery onto a color photothermographic paper with photographic quality.

Taxonomy

In this section a categorization of photothermographic media according to composition as well as application is presented.

1. Contemporary grayscale photothermographic media comprise silver halide, usually silver bromide, as the light sensitive component and a silver salt of a carboxylic acid, most often silver behenate, as the source of developable silver. The first reported examples of the 3M Dry Silver™ technology employed the silver salt of saccharin as the source of image silver, however. This material continued to be used for a number of years in products of the Fuji Photo Film Co. The developing agent is usually a bisphenol compound, though non-carboxylate silver sources may require more reactive developers. The detailed chemistry of silver halide-silver carboxylate materials has recently been reviewed by Cowdery-Corvan and Whitcomb⁴.
   
   It is critical to the function of these materials that there exists a silver halide-silver carboxylate heterojunction. This "catalytic proximity", to use Morgan and Shely's phrase, of silver halide and silver carboxylate can be achieved in two ways.
   1. The silver halide can be prepared in situ, by addition of a halidizing reagent to a dispersion of the silver carboxylate. This method has been most important, historically, and is used, typically, in micrographics products.
   2. The silver halide can be preformed by conventional silver halide emulsion making techniques, and the silver carboxylate then synthesized in its presence. This method facilitates tighter control of silver halide grain size and morphology and enables the use of lower silver halide concentrations, which is important for post-processing image stability. This technique is used for the medical diagnostic films.
2. A silver source other than silver carboxylate may be employed in the case of color photothermographic materials.
   1. David Morgan2 led a long-term effort at 3M to develop a full color photothermographic material based on silver carboxylate, but this product was never commercialized.
   2. Another long-term goal for photothermographic technology is the achievement of sufficient photographic speed to allow exposure in a conventional camera under ordinary photographic conditions. This goal was realized in the context of a full color system by a team at Eastman Kodak Co. led by Gary House, and presented in 2004.
      
      Their product utilizes silver benzotriazole or the silver salt of phenylmercaptotetrazole as the image forming component, combined with novel blocked color developers. The product concept associated with this material is a scan-only color negative film, which would enable "available everywhere", i.e., inexpensive, consumer-operated, highly distributed, photofinishing for customers using film cameras.
3. Commercial color photothermography dates back nearly 20 years with the advent of Fujix Pictrography™, which may also be considered an "instant photography" system⁵. In this case only silver halide is used, and developed with dye releasing developers, which are activated by the physical and/or thermal release of a small amount of water as well as heat. The dye released during the development process thermally transfers to a receptor layer to form a negative image. Originally Pictrography™ media comprised two separable sheets (negative and receptor), but later integral constructions became available. It was originally envisioned as a medium for full color digital office copying, and thus Pictrography™ media are usually exposed using LED's. This technology is the basis for the miniaturized printer for cell phone imagery described above.

Future

Photothermographic imaging materials continue to be a growth area for the photochemical imaging industry, represented by Agfa-Gevaert and Konica-Minolta, along with the companies mentioned above. It should be realized that most of the important applications for these media comprise examples of hybrid imaging, i.e., they provide output for digitized image information (COM, medical diagnostics, cell phone printer), or, in the case of the Eastman Kodak concept for distributed photofinishing, image capture for subsequent digitization. This compatibility with digital technology, along with convenience of use, rapid access, and environmental friendliness of photothermographic technology, promise a continued strong future for it in such applications.

References

1. J. W. Shepard, Appl. Photogr. Eng. 8, 210 (1982).
2. D. A. Morgan, J. Imaging Technol. 13, 4 (1987).
3. D. H. Klosterboer, in Imaging Materials and Processes, Neblette's 8th Ed., J. M. Sturge, V. Walworth and A. Shepp, Eds., Van Nostrand-Reinhold, New York, 1989, chap. 9.
4. P. J. Cowdery-Corvan and D. R. Whitcomb, Handbook of Imaging Materials, A. S. Diamond and D. S. Weiss, Eds., Marcel Dekker, New York, 2002.
5. V. K. Walworth and S. H. Mervis, in Imaging Materials and Processes, Neblette's 8th Ed., J. M. Sturge, V. Walworth and A. Shepp, Eds., Van Nostrand-Reinhold, New York, 1989, chap. 6.