A New Photographic Process

In a communication by Sheffert and Oughton[^1] a description is given of a new photographic process based on the phenomena of photoconductivity and triboelectricity. The essence of this process consists in forming an electrostatic image on a layer of a photoconducting substance, which is “developed” by charged powder adhering to the charged areas.

plates. The powder is then transferred to another surface, giving a print. This process is based on the work of P. Selenyi[^2], who described various processes for forming an electrostatic image on plates made of nonconducting material (“electrography”) and tested various methods of “developing” such an image with the aid of powders (“xerography”[^3]). A characteristic feature of this process is that, by the dry method, a positive image is obtained at once; moreover, the plates are not irreversibly altered during exposure and can be used many times. For obtaining prints, any materials having a sufficiently smooth surface are used.

“Xerographic” plates are prepared by depositing (evaporation in vacuum) an even layer of photoconducting material (selenium, sulfur, anthracene) on a backing plate (paper, glass, plastic). The dark resistance of the photoconductor must be very high (\(10^{15}\ \text{ohm}\cdot\text{cm}\)), so that the charge remains for a long time on the charged surface. The resistance of the backing must be many times lower (for example \(10^{10}\ \text{ohm}\cdot\text{cm}\)).

Fig. 1. Formation of an electrostatic image on a xerographic plate.

The first operation in “xerography” consists in “sensitizing” the plate, i.e., in applying an electric charge to its surface, for example by rubbing it with fur (or cloth), or with the aid of a corona discharge from a bundle of wires charged by an electrostatic generator (or by more powerful modern means). To apply charges to the plate, 10–12 seconds are sufficient. These operations must be carried out in darkness, or under inactive light.

The second stage is exposure, performed under the conditions usual for photography. The plate is placed in a camera (or printing frame, cassette, etc.), and the image to be printed is projected onto it. In the illuminated areas the resistance of the photolayer falls and the charges flow off to the backing (Fig. 1). In the unilluminated areas the charges remain, thereby forming a lat—

tic electrostatic image, so stable that it can be preserved on the plate for several days.

The process of “development” consists in the fact that the electrostatic image on the plate becomes visible when a charged fine powder is applied. Its charge is opposite in sign to the charge of the plate, and therefore the powder particles adhere firmly to its charged places. The appearance of charges on the powder particles is connected with the fact that this fine powder ($d = 0.1$—20 μ) is mixed (before “development”) with a coarser carrier powder ($d' \sim 300$ μ), and as a result of friction charges arise on both substances; moreover, the finer powder is retained on the surface of the “carrier” particles (Fig. 2). The selection of substances for the developer is determined by their triboelectric properties. Thus, for example, in the list below one can find information as to what charge a given substance acquires upon contact with other substances of the same series:

1. Flour (+ end of the series).
2. Cork.
3. Calcium carbonate.
4. Dyed lycopodium.
5. Rosin.
6. Sand.
7. Copper vitriol.
8. Tartaric acid (− end of the series).

Fig. 2. Diagram of a developer particle

The magnitude of the electric charge arising on each of two substances depends on their mutual position in the list: if they stand close to one another in the series, the charge is small, and vice versa.

By combining different substances—one in the form of small particles, the other in the form of coarse particles—one can set the magnitude and sign of the charge arising on the powder. The authors used, for example, the following developers: 1) fine tartaric-acid powder and flour as carrier, 2) dyed lycopodium and sand. The first mixture was used for positive plates, the second for negative plates. The development process is carried out in a cuvette by rocking it from side to side and sprinkling the “developer” over the surface of the plate.

Transfer of the image is carried out as follows: the plate is covered with a sheet of paper (cloth, glass, plastic, etc.) and is charged from the other side with a charge opposite in sign to the charge of the powder. The powder adheres to the paper and forms an impression on it.

If the powder giving the image is a fusible resin*), then the impression, when heated, the resin melts and gives a permanent image; this is the fixing process.

The resolving power of this photographic process depends only on the sizes of the powder forming the image. The authors estimate it at 7.5–10 lines per mm. With respect to the spectral sensitivity of the “xerographic” plates, the authors say little. They indicate only that plates with a selenium coating have a spectral sensitivity distribution similar to that of orthochromatic photographic plates.

*) The authors do not give precise indications as to what resinous substance they used, and there is some lack of clarity in their description.

The sensitivity of the plates is low*) and, apparently, does not exceed the sensitivity of good silver-bromide papers.

The “xerographic” copies of line drawings and diagrams presented by the authors are quite satisfactory. Reproduction of photographs by means of “xerography” is not entirely successful: the copy comes out too harsh, since halftones are poorly rendered.

The authors point to the possibility of multiple copying by a simple contact method: if a large number of copies is required, the developed image is fixed on the “xerographic” plate itself by heating or by spraying with a solvent. After this, the charged plate is illuminated, powder is applied again, and the image is transferred to a new sheet, etc. Thus, the process of multiple copying does not require the preparation of a cliché.

Another application of this method in printing is connected with the use of plates with an insulating layer on a conducting substrate. For example, a drawing or diagram may be applied to the surface of a metal plate coated with an insulating varnish. However, a more practical method of obtaining an insulating image on such a plate is the use of ordinary photomechanical methods (lithography and photoengraving). After exposure in contact with a positive or negative image of the object, the plate is treated by development or etching in order to obtain a sharp, durable image on the metal surface. Printing copies from such a plate may be obtained by the following operations: 1) applying a charge to the plate (the charge will remain only on the insulating, unetched parts of the plate surface), 2) dusting with powder, 3) electrostatic transfer. This method requires neither pressure nor liquid inks. The authors believe that printing by this method can proceed at speeds of up to 360 linear meters per minute. The process is also suitable for obtaining color images by the usual methods of multicolor printing.

As regards the applicability of the method to scientific photography, there are as yet no reports; however, as the sensitivity of the method is increased, such applications will undoubtedly find a place.

A. Ilyina

CITED LITERATURE

1. R. M. Schaffert and C. D. Ouchton, Xerography: A New Principle of Photography and Graphic Reproduction, J.D.S.A. 38, No. 12, 991, 1948.
2. P. Selenyi, J. Appl. Phys. 9, 637, 1938, Zeits. techn. Physik 16, 607, 1935, “Photography on Selenium,” Nature 161, 522, 1948.
3. N. Langer, “Electrography,” Radio News, Eng. Dept. 32, 22, 1944.
4. L. B. Loeb, Science 102, 573, 1945.
5. See, in more detail, Yu. N. Gorokhovskii, Methods of Photographic Sensitometry. Moscow, Goskinoizdat, 1948, p. 73.

*) On the ASA scale the authors estimate it at 0.3 (ASA is the new American system of sensitometry⁵).