Photographic hypersensitization

Last updated April 30, 2023

Photographic hypersensitization refers to a set of processes that can be applied to photographic film or plates before exposing. One or more of these processes is often needed to make photographic materials work better in long exposures.

Most photographic materials are designed for snapshot exposure of much less than one second. In longer exposures, such as those used in astrophotography, many such materials lose sensitivity. This phenomenon is known as low-intensity reciprocity failure (LIRF) or the Schwarzschild effect.^[1]^[2]^[3]^[4] The reciprocal relationship between flux and exposure time for photographic film implies that at a given light flux, doubling the exposure time would double the photographic effect. This holds with exposures up to a second or so, but in general does not hold over exposure times of minutes or hours. Several hypersensitization or "hypering" techniques have been developed to overcome this failure of the reciprocity law, and what follows refers mainly to work in astronomy.

Causes of reciprocity failure

A developable photographic latent image forms when crystals of silver halide in an emulsion layer are exposed to light. The initial nucleation phase is chemically and thermodynamically unstable; it is thus temperature sensitive, and involves the production of one, or very few silver atoms as sub-latent image specks in each silver halide crystal. Once a clump of a few silver atoms has formed at one site within a crystal it is capable of triggering the development of the whole crystal. This greatly amplifies the effect of relatively few photons to produce a metallic silver image "grain". With low-intensity light, the sub-latent image speck may rapidly revert to silver halide before sufficient photons have been absorbed to make it stable. Hypersensitization techniques are intended to lengthen the lifetime of the unstable sub-latent image, to increase the chances of the silver halide crystal receiving enough light to form an image that will catalyze the action of the developer.^[5]

Techniques of hypersensitization

Practical, user-applied hypersensitizing techniques have evolved over most of the last century and fall mostly into four types of treatments. Broadly, these involve liquid phase (washing), gas phase (out-gassing and baking and hydrogenation), exposure at lowered temperature, and pre-flashing. Some of these can be used in combination, but many severely shorten the shelf-life of a product and so can not be applied by the manufacturer.

Liquid-phase hypersensitization

Washing plates in water, dilute ammonia, triethanolamine or (more recently) silver nitrate solutions^[6] was found to be very effective, especially for red- and infrared-sensitive materials. Later types of fine grain, near-IR-sensitive plates were unusable without such hypersensitizing. However, much skill and persistence was required to obtain consistent and uniform results, especially with large plates, which were often treated at unsocial hours in observatory darkrooms on remote mountain tops.

The liquid-phase plate washing techniques operate by removing residual soluble bromides or iodides from the emulsion, thereby increasing the silver ion concentration in the vicinity of the photosensitive grain. However, this greatly reduced the shelf-life, and was usually done just before exposure and the plates were either developed immediately or stored at low temperatures before processing.

Gas-phase hypersensitization

Gas hypersensitization is the process of soaking or flushing the photographic film or plate for an extended period of time in nitrogen, hydrogen, or a hydrogen/nitrogen mixture called forming gas, sometimes with heating.

Some of the earliest gas-phase hypersensitization methods involved exposing the plates to mercury vapor^[7] before exposure to light. This was beneficial but was also hazardous and unreliable. More amenable was baking the plates in air^[8] in a moderate oven, usually in a light-tight metal box. Used from about 1940, this produced modest speed gains in the then-current coarse-grained emulsions. From about 1970,^[9] baking (about 65 °C for several hours) or prolonged soaking (20 °C for weeks) in an intermittent flow of nitrogen was used and could achieve a factor of 10 gain in speed for a one-hour exposure. In general this was used with the special "spectroscopic plates" made by the Eastman Kodak Company. These products were intended for long exposures, however it also worked to some extent with more conventional materials, including color film.^[10]

This process became especially important for the new generation of high detective quantum efficiency, fine-grained (but slow) plates Eastman Kodak had developed in the late 1960s. In 1974, researchers at Eastman Kodak announced that plates treated in pure hydrogen after nitrogen treatment were more sensitive at all exposure times than untreated plates,^[11] and this was quickly adopted by many observatories, some of whom used non-explosive forming gas (a 4–8% mixture of hydrogen in nitrogen) for reasons of safety. The optimum gas-phase processes combine the effects of heating and de-gassing with reduction sensitization by pure hydrogen to give a sensitivity gain of about 30 times for an hour-long exposure. This worked very well with fine-grain, high resolution emulsions on film, typified by Eastman Kodak's Tech Pan Film. They were also effective with negative and reversal color film, but were unpredictable and could produce difficult-to-correct shifts in color balance.

The gas-phase methods, especially nitrogen baking, involve the removal of traces of oxygen and water from the gelatin matrix, which increases the efficiency of the first stages of latent-image formation. Finally, hydrogen is a chemical reducing agent, which 'seeds' the dry, de-oxygenated silver halide crystal with a few atoms of silver. These are stable, sub-latent image clusters that subsequent photoelectrons from exposure to light can build into a several-atom latent image speck that catalyzes the development of the whole silver halide crystal. Photographic gelatin soaks up ambient moisture rapidly, so in humid climates, "hypered" plates were usually exposed at the telescope in an atmosphere of nitrogen.

In the AAS Photo Bulletin^[12], Jack Marling describes the process of gas hypersensitizing Kodak Technical Pan Film. This was an extremely fine-grained, high-contrast, extended-red-sensitivity panchromatic film that benefited dramatically from hypersensitization. It has sadly been discontinued. Hypersensitization was also used, and can still be used, with other black-and-white materials and with color films, especially the Kodak Ektachrome line.

Hypersensitization with forming gas or hydrogen was widely used by professional astronomers on plates and by amateur astronomers on film up until the wide adoption of CCD astronomical cameras relieved them of the tedium. Amateurs were able to buy hypersensitizing equipment and gas from Lumicon or build their own hypering chambers. Details of the process can be found in books by Wallis and Provin^[13], and Reeves,^[14] among others. Note that digital cameras of all sorts, including the DSLRs now widely used by amateur astronomers, have zero reciprocity failure and outperform even the best hypersensitized film.

Cold camera

It had been known since the 1930s that LIRF was less severe during low-temperature exposures.^[15] Cooling the emulsion during the exposure reduces reciprocity failure by extending the lifetime of the unstable single-atom stage of latent image formation. Accordingly, many experimenters built film cameras with 'cold backs', metal plates in contact with the film, often cooled with solid carbon dioxide. These were awkward to use because of film embrittlement and condensation, but some good results were obtained with color film,^[16] and cooling seemed to affect all the sensitive layers of color film equally, so shifts in color balance were small.

Preflashing and latensification

Preflashing is not strictly a hypersensitizing technique but it was often used in conjunction with Kodak's spectroscopic emulsions, sometimes together with hypering. It involves a brief, uniform, low-intensity flash of light sufficient to produce a small increase in the unexposed fog level. This was usually done just before a long exposure and gave modest increases in effective speed. Latensification works similarly but is applied after the exposure.

The techniques^[17]^[18] are useful when the main exposure was filtered or otherwise arranged so that the image being recorded was completely free from contamination by sky background or scattered light, as in narrow spectral band imaging. The main effect was to change the shape of the toe of the characteristic curve. In photographic terms, preflashing lowered contrast and improved the shadow detail without significantly affecting the highlights of the image.

Related Research Articles

Film stock is an analog medium that is used for recording motion pictures or animation. It is recorded on by a movie camera, developed, edited, and projected onto a screen using a movie projector. It is a strip or sheet of transparent plastic film base coated on one side with a gelatin emulsion containing microscopically small light-sensitive silver halide crystals. The sizes and other characteristics of the crystals determine the sensitivity, contrast and resolution of the film. The emulsion will gradually darken if left exposed to light, but the process is too slow and incomplete to be of any practical use. Instead, a very short exposure to the image formed by a camera lens is used to produce only a very slight chemical change, proportional to the amount of light absorbed by each crystal. This creates an invisible latent image in the emulsion, which can be chemically developed into a visible photograph. In addition to visible light, all films are sensitive to X-rays and high-energy particles. Most are at least slightly sensitive to invisible ultraviolet (UV) light. Some special-purpose films are sensitive into the infrared (IR) region of the spectrum.

<span class="mw-page-title-main">Holography</span> Recording to reproduce a three-dimensional light field

Holography is a technique that enables a wavefront to be recorded and later re-constructed. Holography is best known as a method of generating real three-dimensional images, but it also has a wide range of other applications. In principle, it is possible to make a hologram for any type of wave.

Photographic processing or photographic development is the chemical means by which photographic film or paper is treated after photographic exposure to produce a negative or positive image. Photographic processing transforms the latent image into a visible image, makes this permanent and renders it insensitive to light.

<span class="mw-page-title-main">Photographic paper</span> Light-sensitive paper used to make photographic prints

Photographic paper is a paper coated with a light-sensitive chemical formula, like photographic film, used for making photographic prints. When photographic paper is exposed to light, it captures a latent image that is then developed to form a visible image; with most papers the image density from exposure can be sufficient to not require further development, aside from fixing and clearing, though latent exposure is also usually present. The light-sensitive layer of the paper is called the emulsion. The most common chemistry was based on silver halide but other alternatives have also been used.

<span class="mw-page-title-main">Lippmann plate</span>

Gabriel Lippmann conceived a two-step method to record and reproduce colours, variously known as direct photochromes, interference photochromes, Lippmann photochromes, Photography in natural colours by direct exposure in the camera or the Lippmann process of colour photography. Lippmann won the Nobel Prize in Physics for this work in 1908.

In the processing of photographic films, plates or papers, the photographic developer is one or more chemicals that convert the latent image to a visible image. Developing agents achieve this conversion by reducing the silver halides, which are pale-colored, into silver metal, which is black. The conversion occurs within the gelatine matrix. The special feature of photography is that the developer acts more quickly on those particles of silver halides that have been exposed to light. Paper left in developer will eventually reduce all the silver halides and turn black. Generally, the longer a developer is allowed to work, the darker the image.

The gelatin silver process is the most commonly used chemical process in black-and-white photography, and is the fundamental chemical process for modern analog color photography. As such, films and printing papers available for analog photography rarely rely on any other chemical process to record an image. A suspension of silver salts in gelatin is coated onto a support such as glass, flexible plastic or film, baryta paper, or resin-coated paper. These light-sensitive materials are stable under normal keeping conditions and are able to be exposed and processed even many years after their manufacture. The "dry plate" gelatin process was an improvement on the collodion wet-plate process dominant from the 1850s–1880s, which had to be exposed and developed immediately after coating.

Photographic printing is the process of producing a final image on paper for viewing, using chemically sensitized paper. The paper is exposed to a photographic negative, a positive transparency , or a digital image file projected using an enlarger or digital exposure unit such as a LightJet or Minilab printer. Alternatively, the negative or transparency may be placed atop the paper and directly exposed, creating a contact print. Digital photographs are commonly printed on plain paper, for example by a color printer, but this is not considered "photographic printing".

Silver bromide (AgBr) is a soft, pale-yellow, water-insoluble salt well known for its unusual sensitivity to light. This property has allowed silver halides to become the basis of modern photographic materials. AgBr is widely used in photographic films and is believed by some to have been used for making the Shroud of Turin. The salt can be found naturally as the mineral bromargyrite.

A silver halide is one of the chemical compounds that can form between the element silver (Ag) and one of the halogens. In particular, bromine (Br), chlorine (Cl), iodine (I) and fluorine (F) may each combine with silver to produce silver bromide (AgBr), silver chloride (AgCl), silver iodide (AgI), and four forms of silver fluoride, respectively.

In photography, reciprocity is the inverse relationship between the intensity and duration of light that determines the reaction of light-sensitive material. Within a normal exposure range for film stock, for example, the reciprocity law states that the film response will be determined by the total exposure, defined as intensity × time. Therefore, the same response can result from reducing duration and increasing light intensity, and vice versa.

The science of photography is the use of chemistry and physics in all aspects of photography. This applies to the camera, its lenses, physical operation of the camera, electronic camera internals, and the process of developing film in order to take and develop pictures properly.

A latent image is an invisible image produced by the exposure to light of a photosensitive material such as photographic film. When photographic film is developed, the area that was exposed darkens and forms a visible image. In the early days of photography, the nature of the invisible change in the silver halide crystals of the film's emulsion coating was unknown, so the image was said to be "latent" until the film was treated with photographic developer.

Tabular-grain film is a type of photographic film that includes nearly all color films, and many black and white films like T-MAX films from Kodak, Delta films from Ilford Photo and the Fujifilm Neopan films. The silver halide crystals in the film emulsion are flatter and more tabular.

A chromogenic print, also known as a C-print or C-type print, a silver halide print, or a dye coupler print, is a photographic print made from a color negative, transparency or digital image, and developed using a chromogenic process. They are composed of three layers of gelatin, each containing an emulsion of silver halide, which is used as a light-sensitive material, and a different dye coupler of subtractive color which together, when developed, form a full-color image.

Forming gas is a mixture of hydrogen (mole fraction varies) and nitrogen. It is sometimes called a "dissociated ammonia atmosphere" due to the reaction which generates it:

A nuclear emulsion plate is a type of particle detector first used in nuclear and particle physics experiments in the early decades of the 20th century. It is a modified form of photographic plate that can be used to record and investigate fast charged particles like alpha-particles, nucleons, leptons or mesons. After exposing and developing the emulsion, single particle tracks can be observed and measured using a microscope.

Photographic film is a strip or sheet of transparent film base coated on one side with a gelatin emulsion containing microscopically small light-sensitive silver halide crystals. The sizes and other characteristics of the crystals determine the sensitivity, contrast, and resolution of the film.

In photography, solarization is the effect of tone reversal observed in cases of extreme overexposure of the photographic film in the camera. Most likely, the effect was first observed in scenery photographs including the sun. The sun, instead of being the whitest spot in the image, turned black or grey. For instance, Minor White's photograph of a winter scene, The Black Sun 1955, was a result of the shutter of his camera freezing in the open position, producing severe overexposure. Ansel Adams had also earlier created a solarized sun image, titled Black Sun, Owens Valley, California, 1939, by overexposure.

Cold Camera Photography is a technique used by astrophotographers to reduce the electronic noise that accumulates during long exposures with the electronic sensors in DSLRs and dedicated CMOS or CCD astro-cameras. Cooling is usually accomplished with a Peltier thermo-electric cooler. By cooling the cameras sensor one can take longer shots without the worry of the chip heating up, thereby reducing thermal, shot and read noise.

References

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↑ Jones, L. A.; Huse, E. (1925). "On the Relation Between Time and Intensity in Photographic Exposure[Second Paper]". Journal of the Optical Society of America. 11 (4): 319. doi:10.1364/JOSA.11.000319.
↑ Jones, Loyd A.; Huse, Emery; Hall, Vincent C. (1926). "On the Relation Between Time and Intensity in Photographic Exposure". Journal of the Optical Society of America. 12 (4): 321. doi:10.1364/JOSA.12.000321.
↑ Jones, Loyd A.; Hall, Vincent C. (1926). "On the Relation between Time and Intensity in Photographic Exposure*Fourth Paper". Journal of the Optical Society of America. 13 (4): 443. doi:10.1364/JOSA.13.000443.
↑ Babcock, T. A. (1976). "A review of methods and mechanisms of hypersensitization". AAS Photo Bulletin. 13 (3): 3. Bibcode:1976AASPB..13....3B.
↑ Jenkins, R. L. & Farnell, G. C. (1976). "Chemical Hypersensitization of Infra-Red Emulsions". The Journal of Photographic Science. 24 (2): 41. doi:10.1080/00223638.1976.11737866.
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↑ Smith, A.G. Schrader, H. W. and Richardson, W. W. (1971). "Response of Type IIIa-J Kodak Spectroscopic Plates to Baking in Various Controlled Atmospheres". Applied Optics. 10 (7): 1597–9. Bibcode:1971ApOpt..10.1597S. doi:10.1364/AO.10.001597. PMID 20111169.{{cite journal}}: CS1 maint: multiple names: authors list (link)
↑ Jerry Lodriguss Film for Astrophotography. Astropix.com. Retrieved on 2011-11-07.
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↑ Marling, J. (1980). "Gas Hypersensitization of Kodak Technical Pan Film 2415". AAS Photo Bulletin. 24: 9. Bibcode:1980AASPB..24....9M.
↑ Wallis, B & Provin, R. W. (1988). A Manual of Advanced Celestial Photography. Cambridge University Press. ISBN 0-521-25553-8.
↑ Reeves, R. (2000). Wide-field astrophotography: Exposing the universe starting with a common camera. Richmond, VA: Willman-Bell Inc. ISBN 0-943396-64-6.
↑ Webb, J. H. (1935). "The Effect of Temperature upon Reciprocity Law Failure in Photographic Exposure". Journal of the Optical Society of America. 25 (1): 4. Bibcode:1935JOSA...25....4W. doi:10.1364/JOSA.25.000004.
↑ Hoag, A. A. (1961). "Cooled-Emulsion Experiments". Publications of the Astronomical Society of the Pacific. 73 (434): 301. Bibcode:1961PASP...73..301H. doi: 10.1086/127683 .
↑ Alter, G., Barber, D. R. and Edwards, D. L (1940). "The Fog Effect in Photographic Astro- and Spectro-Photometry". Monthly Notices of the Royal Astronomical Society. 100 (7): 529. Bibcode:1940MNRAS.100..529A. doi: 10.1093/mnras/100.7.529 .{{cite journal}}: CS1 maint: multiple names: authors list (link)
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