Press release

A CNRS-Université Paris-Sud laboratory, working in collaboration with AGFA-Gevaert.N.G., succeeds in enhancing tenfold the sensitivity of photographic emulsions

Paris, December 23, 1999
 
Photographic emulsions are made up of a film of gelatin into which silver bromide crystals, which are the photosensitive elements, are incorporated. Working in collaboration with AGFA-Gevaert.N.G., a research team from the "Laboratoire Physico-Chimie des Rayonnements" (a joint CNRS Université Paris-Sud – Orsay laboratory performing research into the physical chemistry of radiation) has succeeded in enhancing the sensitivity of photographic emulsions tenfold. Their work was published in the December 23, 1999 issue of the journal Nature. These findings are of exceptional interest on two levels: for fundamental research because of optimum efficiency being obtained for light absorption in a photosensitive material; and for applied research because of the implied consequences on the sensitivity of photographic emulsions. This concerns all of the processes related to silver photography (black and white photography, color photography, radiography, offset printing, holography, etc.) and would suggest that we can expect a major increase in performance levels in this field.

The latent photographic image is formed by exposing silver bromide (AgBr) crystals to light very briefly. For each photon absorbed, an initial electron-hole pair is formed. The electrons are trapped by the Ag+ ions which are reduced. Depending on the light intensity at each point of the image, from zero to about ten silver atoms are formed in each crystal. During the subsequent chemical development, only those crystals which contain at least a minimum number of atoms can be completely reduced by the developer into dark particles that are visible to the eye (negative image). Under modern emulsion conditions, this critical number is 3 atoms per crystal. A properly exposed image is thus an image in which about one half of the crystals have more than 3 atoms each.

However, competitive processes work against each photon producing one atom (theoretical quantum
yield _theory = 1) (Figure 1). One of these processes is the extremely fast recombination of the initial electron-hole pair with no subsequent chemical effect. The other process is the destruction by the hole of the atom that was formed by the parent electron, since the hole has oxidizing properties. In spite of the strategies developed by photographers to minimize these effects, the effective yield in optimized emulsions was previously _eff = 0.20 atom per photon, which represents 80% of the photons being absorbed with no effect (15 photons are necessary to make the crystal developable).

Jacqueline Belloni, a research director at CNRS, "Laboratoire de Physico-Chimie des Rayonnements" and members of her team—Mona Treguer, a young researcher, and Hynd Remita, "chargée de recherche" at CNRS—suggested an original approach to AGFA-Gevaert.N.G. (René de Keyzer), with whom they have been collaborating for several years now. The aim of this approach is to increase the effective quantum yield by inhibiting these antagonistic processes; it consists of doping the AgBr crystal with silver formate, which, in solution, is known to trap oxidizing radicals effectively and then to give an additional electron.

Experiments confirmed that, in the doped emulsions, the holes were trapped by the formate prior to any recombination, thereby releasing one electron per photon (Figure 2). They also showed that an additional silver atom was formed for each trapped hole (Figure 3). The yield is then _eff = 2 silver atoms per photon, i.e. 10 times higher than prior yields. The mechanism is such that the number of atoms is proportional to the number of photons in a very wide range of light intensities, including zero or low exposure, and that the sensitivity gain does not suffer from any accompanying fogging effect.

The increase in the yield may be used to reduce the exposure duration (to catch a fast movement, or to reduce the dose of irradiation in radiography), or else, for an identical exposure time, to make use of a lower light flux (possibility of taking pictures without a flash), or to increase the definition by reducing the size of the crystals. Since such a reduction in size also increases the contrast, it is possible to reduce the silver content and the amount of additives in the emulsion.

This research work has led to the filing of several patents.

Figure 1
Of the 3 e--hole pairs formed for 3 absorbed photons, only the top pair leads to a silver atom. In the other pairs, the hole (h+) destroys the parent electron or else the atom formed by the electron.

Figure 2
In the presence of the formate dopant HCO2- the holes are trapped before any recombination with the electrons and every electron produces an atom Ag.

 

Figure 3
In a slower, second step, the CO2°- radicals resulting from the trapping of the holes are each capable of producing an additional silver atom.

 

Contacts :
Jacqueline BELLONI, Laboratoire de Physico-Chimie des Rayonnements (CNRS-Université Paris-Sud)
Tel : +33 1 69 15 55 50 and +33 1 69 15 75 71
E-mail : jacqueline.belloni@lpcr.u-psud.fr

Martine HASLER, medias relations, CNRS
Tel : + 33 1 44 96 46 35
E-mail : martine.hasler@cnrs-dir.fr

Laurence MORDENTI, département des sciences chimiques (chemical sciences dept.), CNRS
Tel : +33 1 44 96 41 09
E-mail : laurence.mordenti@cnrs-dir.fr