Certain impurity centers that appear on chemical sensitization of photographic emulsions are shown to induce new photoluminescence (PL) bands of emulsion microcrystals at low temperature, as well as to control photographic properties at room temperature. The nature and function of these centers has been investigated using PL. We found that temperature quenching of some PL bands occurs by an ionic mechanism, specifically neutralization of electrons localized on the recombination center by mobile silver ions in competition with radiative recombination of holes and electrons. Thus, PL and latent image formation appear to be competing processes.We have also observed the appearance of new PL bands in near infrared (IR) spectral regions as a result of sulfur sensitization and have shown that (Ag₂S)_n clusters of different sizes determine this PL. Further, it has been shown that small clusters are hole traps and that large, mixed (Ag₂S)_p Ag⁺_k clusters are the centers of photosensitivity. Formation of (Ag₂S)_n and mixed (Ag₂S)_p Ag_k⁺ clusters during sulfur sensitization and Ag_m⁰ clusters during reduction sensitization involves silver ions from surface layers of the emulsion microcrystals. This process is accompanied by an increase in structural defects, so that the concentration of uncompensated surface Br_s⁻ and I_s⁻ anions increases. These anions are hole traps. Thus, we conclude that during chemical sensitization, hole trap centers are formed but their origin is not necessarily silver or silver sulfide.

Interfacial electron transfer is the most fundamental process driving all imaging technologies. The advent of ultrafast lasers has led to the development of novel experimental techniques to probe such dynamics. This article presents five time domain spectroscopies that allow direct measurement of different segments of the electron trajectory across a heterogeneous interface. Electro-optic sampling measures field-assisted transport of carriers to the surface. Time-resolved two-photon photoemission enables measurement of electron relaxation at surfaces. Timecorrelated single-photon counting, transient grating, and transient absorption techniques are implemented to determine electron transfer rates at interfaces. With these real-time approaches, the primary photophysical and photochemical processes at semiconductor/liquid interfaces and dye-sensitized semiconductors can be studied directly. The new information forthcoming from such studies is that electron transfer processes can be extremely fast at surfaces, in a range approaching adiabatic coupling conditions between the delocalized bond states and discrete molecular donor or acceptor states. This observation leads to a new conceptual framework for understanding photoinduced interfacial charge transfer processes.

Pulse radiolysis has been utilized over the last three decades to study a variety of physical and chemical systems, including those relevant to imaging processes. In this overview, we outline the similarities between photolysis and radiolysis and highlight the differences. In particular, we focus on the time-resolved variants of the two disciplines: pulse radiolysis versus flash photolysis. The strength (and weakness) of the radiolytic techniques is their nonspecificity; the energy is always absorbed by the majority medium, the solvent and not the solute. Therefore, once thermalization occurs (≪ 1 ps), the primary reactive intermediates are the same regardless of the solute. From this time on, the chemistry that follows is the chemistry of radicals, radical ions, excited states, metal ions at unstable oxidation state, and other reactive molecular products such as metallic and semiconductor clusters. Thus, radiation chemistry principles that were developed for one discipline are easily transportable to another. The pulse radiolysis technique with a wide arsenal of detection methodologies is currently used to identify short-lived intermediates and to determine their kinetic and thermodynamic properties. Together these studies provide mechanistic insight into the behavior of many chemical and physical systems. We demonstrate the utility of the approach in several areas of interest to imaging sciences, in particular, clustering of silver atoms, growth of silver halides, and medium effects on these systems. Other systems of relevance to imaging sciences include reactivity and redox potentials of quinone and one-electron reduced/oxidized dyes.

Microwave absorption (MWA) enables contactless measurements of electronic transport properties in solids. MWA is the only method for studying the lifetimes and mobilities of the photogenerated charge carriers that depend on the recombination process of photoelectrons. Therefore MWA is a powerful tool to understand the influence of phase structures of point defects like Y⁻, Ir³⁺, Pb⁺⁺, Cu⁺⁺, etc., dislocations; and chemical and dye sensitization on the formation of latent image specks.

The purpose of this tutorial review is to illustrate the use of time-resolved dielectric loss (TRDL) and analysis of TRDL data on photoconductive materials for electrophotographic applications, as well as on photocatalytic imaging processes. Systems studied in our laboratories include particulate CdS for Canongraphic imaging and CdS powder dispersed in a xylene solution of phenylhydrazine, as a model for a photorecptor with separated charge generation and charge transport function. A study on prototypical molecular charge generation media is also reviewed, as is recent work on the characterization of TiO₂, a prototypical photocatalyst of demonstrated imaging utility.

Arsenic is added to amorphous selenium to retard crystallization and to improve its mechanical properties. Arsenic in sufficient quantities acts as a weak hole trap that results in buildup of residual potential when subjected to charge–expose–erase cyclic conditions employed in electrophotography. A halogen, chiefly chlorine, that by itself acts as an electron trap in amorphous selenium is added to arsenic containing selenium to eliminate residual potential when operated in a positive charging mode (hole transport). Some selenium alloys contain 0.33-atom % arsenic and 30-ppm chlorine. Chlorine in excess of that required to compensate arsenic in bulk does not affect hole transport. However, excess chlorine decreases the electron range. When these alloys are employed as photosensitive elements for xeroradiography purposes in which x-rays are bulk absorbed and both hole and electron transports contribute to the sensitivity, the excess chlorine will have the effect of reducing the sensitivity. In this article, compensation of arsenic by chlorine has been explained in terms of structural considerations. These considerations and heat of formation data suggest that approximately 30-ppm chlorine is required to compensate 0.33-atom % arsenic. A technique has been developed to map the excess chlorine profiles. This is based on the principle that if a small electron current is injected into the film from a biased substrate in a time-of-flight setup, the electrons are selectively trapped at these excess chlorine sites. The resulting electric field profiles have been measured and related to the excess chlorine profiles. Experimentally, a film containing 0.33-atom % arsenic and approximately 40-ppm chlorine is ideally compensated, and excess chlorine is observed in films containing more than 40-ppm chlorine.

Imaging characteristics are covered by the image transfer theory. But up to now, image transfer theory dealt mainly with observation of Lambertian (diffuse reflecting) objects on a Lambertian background. This model of reflection is quite a reasonable one for many natural and artificial objects to describe vision quality. We present here the mathematical description for images of non-Lambertian objects to permit their angular reflection patterns to be dealt with under unfavorable viewing conditions through a light-scattering medium. Retroreflectors are chosen as an convenient example of these objects. The small-angle diffusion approximation of the radiative transfer theory is used for the calculations of light characteristics under illumination by some source of an active vision system. The case studies consider, in particular: (1) Imaging of large-area objects where some of their parts would be seen as dark and others as bright, and (2) the interesting effect of enhancing the contrast of a retroreflector image with increasing optical thickness of a scattering medium. This is related to increasing the “effective” albedo of an “equivalent” Lambertian object, by which the retroreflector can be replaced. The results on imaging characteristics of retroreflective objects are compared with those for Lambertian. The corresponding differences are discussed.

Previous studies in ink-jet printing have considered ink penetration into paper or ink evaporation to be effective in the drying process. In the present study, a unified approach that allows for simultaneous evaporation and penetration during the drying process was applied. To this end, penetration rates were measured experimentally using a Bristow tester. In addition, evaporation rates were determined from a theoretical evaporation model that contained no adjustable parameters. It was found that for plain paper used in the office environment, the rate of penetration was at least 20 × higher than the rate of evaporation. Accordingly, ink drying is determined mainly by penetration, and penetration curves alone are sufficient to predict the dry time or ink disappearance on a plain paper surface. In practice, this is illustrated by ink-jet print quality on plain papers of various sizing treatments. The sizing level needs to be adjusted according to the desired ink-jet image. That is, full-color printing requires penetration rates sufficient to accommodate each of the process inks applied without color bleed or other ink-to-ink interaction processes. This may be at expense of character and line print deterioration due to fiber swelling in mono-color printing.

An efficient coding scheme for biomedical x-ray images based on the DCT basis decomposition is presented. The algorithm is adaptive to the local activities by changing the block sizes. A hierarchical binary tree structure is constructed. The residual sub-images (errors) of the original and the estimation achieved from an interpolation method are decomposed based on the DCT basis functions. Each subimage, regardless of its size, is reconstructed by only three low-frequency DCT coefficients. The computation is rather simple because only three components are manipulated. The experimental results are provided to evaluate the performance of the proposed algorithm.