Low Intensity Reciprocity Failure Research Articles

A probabilistic model for reciprocity failure at an n-atom developable silver speck in a photographic emulsion

In this paper, we study probabilistic models for high and low intensity reciprocity failure in the photographic emulsion, in which a latent image silver speck must consist of a minimum of n s silver atoms in order to be stable, and n d silver atoms in order to be developable. Previous models for reciprocity failure assumed that n s = nd = 2, mainly because of the large increase in complexity (especially in those models which combined high and low intensity failure) in dealing with the situation in which n s and/or n d are larger than 2. In this paper, models for high and low intensity failure are separately studied, and results are obtained for general n d

Kinetic models for the development of density in radiographic film. Visible-light exposure

It has previously been shown that the traditional ‘characteristic curve’ is spurious and that simple kinetic models provide a satisfactory fit to data under some conditions. Further observations now show that low-intensity reciprocity failure in radiographic films can be explained by an additional parameter, and that an order of 2 can be used uniformly for such visible-light exposures.

A queueing model for reciprocity failure in the photographic grain

Under normal conditions, the developed density in a photographic emulsion depends on the incident light only through the product of the intensity and the duration of the exposure. This is called the Law of Reciprocity. However, under extreme conditions such a very low or high intensities, this law of reciprocity breaks down, with a resulting loss in efficiency of the photographic process. This paper presents a probabilistic model for reciprocity failure in photographic materials. The model is based on the Gurney-Mott hypothesis for the photographic process, and in addition to combining both low and high intensity reciprocity failure, allows for the formation of electron pools and recombination with holes. The analysis is similar to, but not derived from, the corresponding transient analysis for an enhanced M/G/1 queueing system.

A New Development in the Method of Measurement of Reciprocity-Law Failure and Its Application to Screen/Green-Sensitive X-Ray Film Systems

Since it has been confirmed by experiment that the intensity of X-rays varies approximately as the focus-film distance (FFD) to the minus 2.12th power, the X-ray intensity can be changed by varying the FED. It is shown in this paper that two types of reciprocity failure curve, density vs. exposure time for constant exposure and relative exposure vs. exposure time for constant density, can easily be obtained from several time-scale characteristic curves taken experimentally for several FED's in the rare-earth screen-film systems used. Only low-intensity reciprocity failure is present for exposure times of more than about 0.1 sec for one film, but both low-intensity and high-intensity reciprocity failures occur in the other one. The effects of reciprocity failure on the H & D curves can be seen in the shape of the curves and the relative speed.

Reciprocity-Law Failure in Radiographic Image-Forming Systems

The Reciprocity-law failure characteristics of three brands of medical blue-sensitive X-ray films exposed with intensifying screens are studied. The variation in X-ray intensity for different points on a reciprocity failure curve is obtained by varying the focus-film distance (FFD), since it is confirmed that the X-ray intensity varies approximately as FFD to the minus 2.15th power, for FFDs from 0.40 to 5.68 meters. All of the films investigated show no high-intensity reciprocity failure effects for exposure times of 1/120 to 5 seconds. However, they show remarkably low-intensity reciprocity failure effects for exposure time longer than about 0.3 sec. It is considered that the existence of this low-intensity reciprocity failure must be taken into consideration, since it would affect the patient dose significantly according to the selection of the exposure conditions.

The elimination of low intensity reciprocity failure on hypersensitized Kodak IIIaJ plates

The residual low intensity reciprocity failure, which is a property even for hypersensitized IIIaJ plates, for exposures between one and eight hours has been successfully eliminated by flushing the plates with a stream of dry nitrogen during exposure.

Chemical Hypersensitization of Infra-Red Emulsions

AbstractIt is shown that hypersensitization of an emulsion spectrally sensitized to the infra-red is a consequence of an increase in the silver-ion concentration in the grain environment. Hypersensitization leads to an increase in the overall efficiency of silver production, reduces low-intensity reciprocity failure, and increases the efficiency of spectral sensitization. Possible mechanisms of the effect are discussed.

Hypersensitization of spectroscopic films and plates using hydrogen gas

view Abstract Citations (12) References (8) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Hypersensitization of spectroscopic films and plates using hydrogen gas Babcock, T. A. ; Sewell, M. H. ; Lewis, W. C. ; James, T. H. Abstract A novel technique of treating spectroscopic plates and fflms with hydrogen gas appears to be an efficient means of achieving hypersensitization. Contact with the hydrogen gas at the time of exposure is not required. Hydrogen sensitization, especially when coupled with exposure under vacuum or in a dry, inert environment, will substan- tially reduce or eliminate low-intensity reciprocity failure. Comparisons of hydrogen sensitization with other methods of hypersensitizing spectroscopic plates, i.e., baking in a dry, inert gas and water bathing, have been made. Publication: The Astronomical Journal Pub Date: December 1974 DOI: 10.1086/111704 Bibcode: 1974AJ.....79.1479B full text sources ADS |

Effects of Moisture and Oxygen on Inherent and Spectral Sensitivity

Oxygen and moisture promote low intensity reciprocity failure in many silver halide emulsions. This failure is reduced and in some cases eliminated when the emulsions are outgassed in vacuum and exposed in either vacuum or inert gases. Dry or moist oxygen causes a decrease in sensitivity.Several lines of evidence suggest that oxygen captures trapped photoelectrons to form O2", which is destroyed in secondary reactions, e.g., reaction with holes, water, or organic material. Water vapour added after vacuum outgassing decreases sensitivity, primarily in the low intensity exposure region, and may act independently or may facilitate desensitization by residual oxygen.Some sensitizing dyes are without significant effect on the desensitization caused by oxygen and moisture; others markedly increase the desensitizing effect. A correlation has been obtained between the electron trapping levels of the dyes and the increased desensitization by oxygen-moisture. It is postulated that the dye acts as the initial electron trap. This trapping is potentially reversible with respect to the conduction band of the silver halide, but is rendered irreversible by loss of the electron to oxygen or water. As a rough correlation, the lower the trapping level below the bottom of the conduction band, the greater is the probability that the electron will be lost to oxygen or water rather than escaping back into the conduction band.

Radiography with low intensity neutron beams: some reciprocity failure characteristics of films used with light-emitting intensifying screens

The reciprocity law failure characteristics of photographic films used with light emitting screens (NE421 and NE905) have been studied to facilitate the estimation of exposure times at low neutron beam intensities. Observations were made in the intensity range from 102 to 105 neutrons per cm2 s - the limits of practical interest for light emitting screens. In this region it was expected that only low intensity reciprocity failure would be observed and this was confirmed. The reciprocity characteristics observed are shown as plots of exposure time t as a function of beam intensity , which is thought to be the most useful presentation for radiographic use and enabled the degree of applicability of the Schwarzschild relation (tp = constant) to be ascertained. The relation was found to fit the observations well with a constant value for the index p which is characteristic of the film and close to the values found in photographic work.

Surface Conduction Theory and the Farnell-Chanter Counts

AbstractThis quantitative theory of the emulsion and the mechanisms of exposure and development is based on Tamm's quantum-mechanical theory of surface conduction. Photo-electrons fall from the conduction band into one or other of two conducting surfaces—that of the silver halide crystal and that of an adsorbed layer of silver iodide—giving rise respectively to the internal and external images. Directly two electrons are present in a surface a “sub-centre” is produced which can be augmented by further electrons acting singly. In the case of the external image one electron suffices to“build-up” the sub-centre to a readily developed “centre”: but access to the sub-centre is not immediate. Several sub-centres, slow to develop, may be formed before a centre appears, causing high intensity reciprocity failure. Low intensity reciprocity failure is attributed to the presence in the colloid of sub-microscopic crystals, the surface of each acting as a single electron trap. The loss in this manner of a final electr...

Low-intensity reciprocity failure of a AgBr emulsion: effect of halogen acceptors.

Low-intensity reciprocity failure (LIRF) of a silver bromide emulsion was drastically reduced, but not eliminated, by the addition of a halogen acceptor, such as acetone semicarbazone, before coating. The remaining failure has a slope of approximately minus unity on an isodensity plot of log intensity vs log intensity × time. This is interpreted as exemplifying two types of LIRF, an intergranular type stopped by a sufficient amount of halogen acceptor, and an intragranular type caused by the thermal instability of a photolytically produced silver atom. A LIRF curve was also obtained in terms of the number of absorbed photons necessary to make a grain developable. These data suggest that considerable recombination occurs before nucleation of the latent image. The quantum efficiency of formation of print-out silver is 0.2 silver atom per absorbed photon. This value indicates that growth of the latent image after nucleation is efficient.

The Tamm Level and Reciprocity Failure

Early grain counts imply that the entry of two photo-electrons into a unique feature of the grain makes certain the formation of a development centre either internal or external, the site being decided afterwards. The unique feature is identified with the Tamm conducting surface level, lying between the crystal and a colloidal coating formed during digestion. The Tamm level separates order from disorder and is undisturbed by the external centre but is obstructed by the internal which interferes with the ordered crystal surface. The commercial emulsion favours the external image, the undigested emulsion the internal. Both are affected by recombination of the first electron, and the properties of the colloidal coating, which alone is sensitive to weak light, are deduced. The external centre is a new source of low intensity reciprocity failure for the internal image, and the internal centre a new source of high intensity reciprocity failure for the external image. The early grain counts are identified with those for an undigested emulsion developed “to completion”.

The Dual Mechanism of Latent-Image Formation In Photographic Emulsions*

AbstractUnsensitized sliver-bromide emulsions are shown to have greater low-intensity reciprocity failure for exposures to blue light than to minus-blue light. This difference of reciprocity-failure behaviour Increases with groIn size but decreases or vanishes on spectral sensitization of the emulsions. It is shown that it is necessary to distinguish the (unction of chemical sensitization at high intensities from that at law intensities of exposure, The results are Interpreted on the basis of two different modes of formation of latent Image.

Remarks on the Critical Time Period and the Schwarzschild Exponent in Photographic Processes

The formation of a stable sublatent image as a result of the action of pairs of light quanta has been investigated theoretically on the basis of transitions from a ground stage through an unstable level to a higher level. Expressions for the fraction of grains in the sublatent image stage for continuous as well as for intermittent exposures have been derived, the latter for the purpose of determining the critical time period. The possibility that the absorption of more light quanta is necessary for the formation of a developable image has been taken into account. A model including low and high intensity reciprocity failure is outlined briefly.

Experiments on Photographic Sensitivity

AbstractThe methods used for preparing thin sheets of silver chloride, silve r bromide, binary and ternary mixed crystals of silver halides, and mixed crystals of silver bromide with silver sulphide and other metallic bromides are described in detail. The results of expriments on the chemical sensitization of these materials are presented. The conclusion has been reached that, as far as the formation and development of the latent image is concerned, there is no significant increase in the sensitivity of the mixed crystals compared with that of strained crystals of pure silver bromide. Experiments with antifogging agents, antiplumming agents, development accelerators and dye sensitizers and supersensitizers are briefly described , The causes of low intensity reciprocity failure in the formation of the surface and internal latent images in the thin sheets are discussed in detail and methods which may be used to diminish low intensity failure in the formation of the surface latent image are outlined.

Reciprocity Law Failure in Desensitized Emulsions*

The reciprocity law failure of desensitized emulsions has been studied in the range of the moderate and extremely high intensity exposures. It is shown that in the desensitized emulsion some of the decreased efficiency associated with normal saturation effects is removed in the high intensity range relative to the nondesensitized basic emulsion. Two of the three dyes studies have shown two steps of low intensity reciprocity failure: a strong, primary low intensity failure and a weak, secondary failure. Speculations on the mechanisms of these two failures are given.

A Contribution to the Theory of Photographic Sensitivity

AbstractA model for a micro-crystal of silver bromide in a chemically non-sensitized emulsion is proposed. Each crystal has a sub-structure consisting of a number of polyhedral blocks, slightly misaligned, the boundaries of which form intense local dislocations. The network of dislocations so formed is responsible for the localized separation of silver, and a high density region of dislocations constitutes the sensitivity centre of the crystal. The mechanism of the formation of the latent image is described on this model. Chemical sensitization is then explained by the adsorption of the sensitizer to the more active regions formed by the intersection of the dislocations and the surface of the crystal. Subsequently the sensitizer may spread over the rest of the surface. Its function is to trap the positive holes formed during exposure. Surface and internal latent image, and both high-intensity and low-intensity reciprocity failure are accounted for by this model.

Photographic Sensitivity as a Function of Exposure Time and Temperature*

Reciprocity failure curves in the range between 1- and 256-seconds exposure time of three fine-grain experimental emulsions containing 2.4 mole percent iodide were determined in the temperature range between +50 and −195 C for radiation of 400-mµ wavelength with surface and internal development. The light absorption of the same emulsions in the temperature range between +25 and −137°C for radiation of 400-mµ wavelength was measured.With internal development, practically no low intensity reciprocity failure occurs at and below −50°C, while with surface development no failure occurs between −50 and −150°C. In no case was high intensity failure observed in this temperature range.With surface development, an extremely large decrease in sensitivity is observed between −150 and −195°C, together with the appearance of a new type of high intensity reciprocity failure. In the same temperature region, a strongly temperature-dependent fluorescence is observed. It is believed that these two effects are connected, and it appears from sensitometric results that (a) the effect of the fluorescence is restricted to the grain surface, and (b) the fluorescence increases with the intensity of the exciting light at a rate higher than the first power of intensity.

Low Intensity Reciprocity-Law Failure in Photographic Exposure II Multiplet-Quantum Hits in a Critical Time Period*†

In a previous paper, the author analyzed the low intensity reciprocity-failure curve by means of probability formulas applied to the exposure conditions required to establish the stable sub-latent-image speck. Experiments have shown that low intensity reciprocity failure results from instability of the latent image in its initial stages of formation due to thermal motion. The theory given is based on the postulate that a stable subspeck can be formed in a grain only when that grain receives a multiplet hit of size 2, 3, or, in general, s quanta within a small critical time period τ. By application of the probability theory, it is possible to calculate what fraction of grains will acquire a stable subspeck when a large number of identical grains are subjected to an exposure of total duration, t, in which the average number of effective quanta per grain in the whole exposure is y. By varying the time and intensity factors of exposure reciprocally, the reciprocity failure can be calculated and compared with experiment. If it is assumed that the critical τ-period is a constant invariable quantity for all grains, it can be shown that the reciprocity-failure curve has a slope at low intensities that is characteristic of the number of quanta required to establish the stable subspeck, the slope being zero for one quantum,−1 for two quanta,−2 for three quanta, etc. In the present paper, the theory is extended to the case of variable τ-period among the grains, and it is found that the same simple conclusions as to slope of the reciprocity-failure curve hold in this case as were found in the constant τ-period case.