PHOTOSENSITIVE GLASS

The graininess and turbidity of ordinary silver-halide gelatin photographic emulsions often hinder the production of sufficiently distinct images. Attempts to overcome these shortcomings either by changing the technology for preparing and processing photosensitive layers (gelatin-free emulsions, an adsorbed photosensitive layer with subsequent “physical” development, etc.), or by seeking new photochemical reactions, have so far met with little success. Although in a number of cases considerable progress has been achieved in increasing the resolving power of the photosensitive layer, successes in this direction have been bought at the price of such a loss of photosensitivity and such a complication of the photographic process that they have deprived these methods of serious practical significance.

Among such unsuccessful substitutes for the photographic emulsion belongs the photosensitive glass recently issued by the Corning firm, some of whose properties were investigated by Riss, Bohm, and Rebaum*) However, in a number of respects it is of undoubted interest.

In outward appearance photosensitive glass does not differ from ordinary glass and can be worked like any glass, by hot methods, including glass-blowing operations. Under the microscope it appears as a homogeneous mass containing no inclusions. Both before and after irradiation it is completely transparent and darkens only as a result of a process of “development,” consisting in prolonged heating of the glass at high temperature. The latent image can be preserved in the glass for a long time, just as in an ordinary emulsion.

The chemical composition of the glass is unknown. The mechanism of image formation is also unknown. The authors suppose that submicroscopic metallic particles are dissolved in the glass; under the action of radiation and subsequent heating they form colloidal particles of larger size, which color the glass. If exposed and developed glass is examined from the edge, then at small exposures and weak development a comparatively thin layer is observed at the surface, scattering blue light. If the glass has been strongly exposed and strongly developed, the bluish layer moves into the depth of the glass without changing its thickness, while the whole space from the surface to this layer is colored a dense red. The color of the glass when viewed by transmitted light changes correspondingly. The figure gives the curves of transparency of photosensitive glass before development (curve a), after 10-minute development (curve b), and after 150-minute development (curve c). In the region 200–310 mμ the glass is opaque. Well-exposed glass is opaque up to 340 mμ. The region of photosensitivity of the glass is very narrow and extends approximately from 310 to 340 mμ, with a maximum near 320 mμ, i.e., lies in the region of strong absorption at the boundary of the region of transparency (see figure). The photosensitivity is very small. It can be characterized by the following figures. The normal density of darkening

*) Amer. Journ. of Phys. 16, 398 (1948).

is obtained when exposed to a mercury lamp from a distance of 45 cm for 40 minutes. The single actinic spectral line of mercury (312–313 mµ) is obtained on the spectrograph after 8 hours, and normal blackening from it after 24 hours.

For “development” the glass must be heated at a temperature of 565°C. Below 540°C the development process does not proceed. The authors do not recommend using a higher heating temperature, though they do not give reasons for this indication. The paper gives characteristic curves (the density of the darkening of the glass \(D = \lg I_0/I\) as a function of the logarithm of the exposure time) for different development times. For \(t_{\text{expos.}} < 10\) minutes (irradiation by a mercury lamp at a distance of 45 cm) the darkening is very slight (region of underexposures); then, approximately up to \(t_{\text{expos.}} = 15\) minutes, it increases rapidly (region of normal exposures); after that the growth of \(D\) slows sharply, continuing up to \(t_{\text{expos.}} \simeq 100\) minutes (region of overexposures), and finally it decreases slightly (region of image reversal). As a function of development time, \(D\) at first increases rapidly (approximately up to \(t_{\text{develop.}} \simeq 1000\) seconds), after which the growth slows. At the same time the steepness of the section corresponding to normal exposures increases. The factor

\[ \gamma = \frac{dD}{d \lg t_{\text{expos.}}} \]

varies from 0.44 for \(t_{\text{develop.}} = 200\) sec. to 2.40 for \(t_{\text{develop.}} = 2600\) sec. The values of \(D\) lie within the limits 0.1–0.3. Under the microscope the darkened regions are completely homogeneous and have no granular structure. Noteworthy is the complete absence of fog after development of unexposed glass.

The comparatively narrow region of normal exposures and the extension of the overexposure region, in comparison with bromosilver emulsions, are explained by the authors as the superposition of two photochemical reactions: a “blue” and a “red” one.

In conclusion the authors present a reproduction of a positive—or rather a diapositive—portrait obtained by printing from an ordinary negative made on a photographic plate with ultraviolet glass. As can be judged from the reproduction, the image is very distinct, but excessively contrasty. As already indicated, it has a red tint.

It may be expected that further work on the creation of photosensitive glasses will lead to a considerable improvement of their properties and may yield results of practical significance. In any case, even this first attempt has given us a method that makes it possible to obtain photographic images guaranteed against destruction by time to an incomparably greater degree than all those available up to now.

G. Rozenberg