The darkening and subsequent fading of silver‐halide photochromic glass upon irradiation with a Q‐switched laser pulse has been investigated. Together with new steady‐state optical measurements and ESR observations of photolytic Cu++, the data reveal three major components of the darkening at room temperature: prompt (τ<20 nsec) visible absorption due to Cu++ ions in the silver‐halide crystallites, slower‐rising (τ?7 μsec) absorption due to silver colloids in the crystallites, and a prompt uv band tail due to ions in the matrix glass. The Cu++ component makes a significant contribution to the photochromic darkening at all excitation intensities, whereas that due to ions in the matrix glass becomes significant only for I≳105 W/cm2. Dependence of fading characteristics on intensity of the excitation pulse is shown to be due in part to variation in the relative magnitudes of the three components. Observations on the growth kinetics and temperature dependence of silver colloid formation are found to be consis...

Darkening mechanisms in silver‐halide photochromic glasses: Flash photolysis and ESR studies

The behavior of photochromic glasses during activation and bleaching is investigated. A two-state phenomenological model describing light-induced activation (darkening) and thermal bleaching is presented. The proposed model is based on first-order kinetics. We demonstrate that the time behavior in the activation process (acting simultaneously with the thermal fading) can be characterized by two relaxation times that depend on the intensity of the activating light. These characteristic times are lower than the decay times of the pure thermal bleaching process. We study the temporal evolution of the glass optical density and its dependence on the activating intensity. We also present a series of activation and bleaching experiments that validate the proposed model. Our approach may be used to gain more insight into the transmittance behavior of photosensitive glasses, which could be potentially relevant in a broad range of applications, e.g., real-time holography and reconfigurable optical memories.

Two-state model of light induced activation and thermal bleaching of photochromic glasses: theory and experiments

We describe and characterize a straightforward test setup for characterizing temporal and spectral dynamics of photochromic spectacle materials. Three measurement examples of contemporary silicate and organic photochromic spectacles are provided. The setup showed a good absolute accuracy of ≤5% of the luminous transmittance (τ(v)) and repeatability of better than 3%. The samples showed different fading times. The sample with the highest dynamic range was the slowest and showed a noticeable change in the transmission spectra during deactivation. The silicate had the lowest dynamic range but also the most homogeneous transmission spectra throughout activation and deactivation. The proposed test device provided accurate results for spectral and temporal dynamics of photochromic materials under realistic conditions.

Photochromic dynamics of ophthalmic lenses

Optical image processing with dynamic spatial filters

The aim of the present work is to study the dependence of the light-induced optical density, i.e., the color-center concentration, as a function of the distance traveled inside a photochromic glass body in stationary state. We propose an experimental setup that allows the direct observation of that concentration from a transversal view. In particular, we will see that for activating intensities much higher than certain threshold, the photochromic material saturates between the surface and an inflection point that happens at a constant normalized concentration of 2/3. The width of this region depends logarithmically on the activating light intensity. This fact will allow us to determine one of the photochemical constants of the material. Validation experiments are presented.

Transversal observation of the light-induced color-center concentration in photochromic glasses in stationary state.

Photochromic glass has developed into an optical component which can be used to advantage in optical systems. After a brief description of the characteristic properties of the glass, some applications which illustrate its potential and its inherent limitations is practice are detailed. These include digital storage, use in a cathode ray tube, contrast enhancement, and hologram storage.

G.K. Megla

Papers

Exploitation of photochromic glass