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by Boris SHAPIRO, Dr. Sc. (Chemistry), President of the Russian Union of Scientific and Applied Photography
For all the complexity and orderliness of the human organism, its natural capabilities are rather limited. Take vision, the eyesight, a major source of our information. But as it is, human eyes perceive a narrow spectral band of wavelengths, just between 400 and 720 nm. This is the visible light. But the actual spectrum is much wider than that-it lies in the ultraviolet (400 -10 nm) and in the X-ray band (10 -5 - 10 2 nm), and in the long-wave, infrared range (from 1-2 mm to 0.74 m) as well.
This is what my teacher, Anatoly Heinemann, said:
"An ordinary human being sees with his eyes, but a clever one - with his head." He meant the art of photography above all, its materials and accessories. Speaking of the photomaterials, we should mention above all the classical silver halide (AgHal)*, now considered to be the best. Under the action of light its micron-size microcrystals form silver atoms which concentrate in one or in several centers of impurities.
The mechanism of this phenomenon that proceeds in two steps was discovered back in 1938 by two British physicists, Ronald Harney and Nevil Mott. First, under the effect of light a photoelectron is knocked out in an AgHal crystal; this photoelectron is "trapped" (say, by some impurity like 3 or 4 molecules of silver sulfide). Next, a silver ion is attracted by an electrostatic force to give rise to an atom of this element, silver. This process, if repeated many times, will produce a latent image.
Silver halides are also capable of enhancing the primary effect of light in the process of development (by a factor of 10 9 to 10 10 ), a phenomenon that has made it possible to obtain high-sensitivity photomaterials. The silver parti-
* See: B. Shapiro, "Next Century Photography", Science in Russia, No. 1, 2000. - Ed.
cles of a latent image (one that has not been developed yet) operate as catalytic centers stimulating the reduction of silver ions from AgHal microcrystals to metal (silver); and thus the latent, invisible image becomes visible. A quantum of light "forms" something like nine or ten billion atoms of silver.
However, silver halide is light-sensitive only in the blue and shorter regions of the spectrum (wavelength, <500 nm). To make AgHal sensitive to the visible and infrared region of the spectrum, one resorts to spectral sensitization by organic dyes adsorbed on the surface of microcrystals. These substances absorb light in a respective region of the spectrum and transfer photoexcitation energy to AgHal. More than 125 years have passed since Hermann Vogel of Germany discovered this phenomenon. Spectral sensitization has made staggering progress ever since. And yet... We cannot say that its mechanism is quite clear to us.
The first eosine*-sensitized photographic gelatine plates of silver bromide (AgBr isochromatic plates) came to be known in France in 1883. They were sensitive to yellow. Next, materials sensitive to the yellow-green region of the spectrum were developed. The now current term "orthochromatic" was intro-
* Eosine (C 20 H 5 Br 4 O 5 Na 2 ) - a scarlet dye of the triarylmethanes group. - Ed.
duced by Joseph Eder of Austria way back in 1884 - from the Greek orthos ("exact") and chromos ("color"). Then the sensitivity region was extended to the orange and red zones of the spectrum in panchromatic layers ( pan is the Greek for "universal").
In 1919 British photochemists synthesized infrachromatic dyes, an achievement that ushered in another stage in the development of innovative photomaterials. At first the sensitization zone reached 800-900 nm, and then it was upped to 1,100 nm. But here the makers of photomaterials came up against a formidable problem. It was a hard nut to crack.
You see, of all the known dyes only one class - the polymethine or cyanine dyes - are still in use. The longer their molecule (rather, a polymethine chain), the farther they go into the long-wave region of the spectrum to absorb light there; which means they increase the sensitivity of silver halide (today it is up to 1,500 nm).
But now we know it by experience: the extension of spectral sensitization into the infrared radiation zone results in a decrease in sensitization efficiency - it drops to 10 percent per 100 nm. This decrease is particularly pronounced at wavelength over 1,000 nm - photomaterials within this region have 0.001 sensitivity compared with that in the visible region of the spectrum.
It looks like the energy of a photoelectron formed as a dye absorbs
infrared radiation is not sufficient for the reduction of silver ions. How come? Has Dame Nature imposed an energy constraint on high-sensitivity infrared materials?
The real situation, thank God, has not proved as hopeless as that. Studies carried out by our research team have shown it abundantly clear: the matter is in secondary, light-potentiated chemical reactions, not in deficient energy.
As far back as the 1920s Samuel Sheppard of the United States hypothesized that the initial act of spectral sensitization was a chemical reaction whereby a dye reduced silver ions on the surface of AgHal microcrystals, not within. I used this long-forgotten idea as a basis of my research. Atoms of silver are formed indeed on the surface of microcrystals under the effect of a dye; these atoms, however, are quite unstable, for they are immediately oxidized by atmospheric oxygen and by water molecules of the air. This process is catalyzed by an infrared dye which acts as a catalyst here. Its molecules, persisting in a non- excited state, transfer electrons from the silver atoms to the oxygen and water. The longer the wavelength of infrared radiation, the more vigorous the "destructive" process is.
Accordingly, we have developed techniques of protecting the atoms of silver. As a first step, we designed a "buffer" between the dye and AgHal to protect the newly formed particles of reduced silver against destruction. Such kind of buffer does not bar the transport of a photoexcited electron of the dye to a silver ion, but it obstructs the backward movement.
Our next step was to protect silver from water. To begin with, we kind of "bundled up" the AgHal crystals in a hydrophobic "cost" made of water-repellant substances. Then we created a reducing medium that prohibited silver oxidation in the photosensitive layer. This way we managed to arrest the "destructive" reaction.
Our new methods of supersensitization have enabled us to boost the sensitivity of infrared film, especially for wavelengths above 1,000 nm. Such film has become compatible in its characteristics with photomaterials for visible light. In the long run we have developed brands of long-wave infrared film without peer elsewhere. One brand, I-1060B-2 with a sensitization maximum of 1,060 nm, is tenfold as sensitive as the best one available to date abroad - IZ, manufactured by the Eastman Kodak of the United States.
It's a fact: we were the first in the world to develop and produce infrared films in the longest wavelength band. For example, the 1-3 band is sensitive enough to register light waves of up to 1,300 nm. We have manufactured prototypes of 1- 1200 and 1-1370 having sensitization maxima of 1,200 and 1,370 nm, respectively. The latter brand (1-1370) could even register a mercury spectral line of 1,529.5 nm. Quite an achievement!
And now, in a nutshell, why on earth we have taken so much trouble.
As a matter of fact, infrachromatic films are now indispensable in studies of the sun and bright stars. Using such films, astrophysicists of the Crimea observatory have photographed stellar spectra in the 1,200 nm band. On the film 1-1060 B-2 it has become possible to register a unique line of the helium spectrum (10830 A) shedding light on the nature of thermonuclear reactions on the sun and on hot stars.* High-sensitivity infrared films are quite good for the direct photography of extended objects, such as gaseous nebulae, the maser effect**, cosmic (extragalactic) clusters, and the like.
* See: A. Marakushev, "The Solar System and Its Stellar Analogs", Science in Russia, No. 5, 2001. - Ed .
** Maser - microwave amplification by stimulated emission of radiation. The maser effect - here, stimulated (induced) emission of interstellar molecules concentrated in gas-and-dust clouds and gaining excitation energy from nearby stars. - Ed.
Our new photomaterials are a welcome aid in studies of laser-assisted thermonuclear fusion at some research centers of this country.
Owing to the high penetrating power of long-wave infrared radiation in a hazy atmosphere, the I-1060B-2 film has good prospects for photography in poor visibility. This film and its analogs are also used for the restoration of art masterpieces. Infrared rays, you see, can pass through opaque varnishes and thin layers of paint without damaging them. Infrared techniques allow to authenticate a work of art-identify its author, time and palimpsests, if any; and these are rapid techniques, mind you, with the exposure taking the shortest time possible. For instance, the I-1060B-2 film was used with much success in the restoration of the iconostasis of the St. Cyril Monastery on Belozero*, in restoring the Dionisiy frescoes in the Ferapontov Monastery** (both at Vologda in Russia's north), and in studying the altars of Tallinn's cathedrals in Estonia.
And last, infrared film is precious to artistic landscape photography. Otherwise you will get green plants of light, pallid hues, and the blue sky and water-pitch- black.
Henry Vogel's dream has come true - now we can photograph in ultrared light just as we can do in visible and ultraviolet light. Scientists of our country have made a significant contribution toward the realization of this dream. Their work has merited six certificates of authorship and patents of the Russian Federation.
* See: V. Darkevich, "Northern Thebes" Science in Russia, No. 3, 2000. - Ed.
** See: V. Darkevich, "Frescoes of Dionisiy" Science in Russia. No. 4, 2000. - Ed.
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