Recent Improvements in Iconoscopes

N. S. Khlebnikov, Moscow

I. Iconoscopes of the Ordinary Type

1. By iconoscopes of the ordinary type we mean devices whose operation does not differ from that of the original instrument, described at one time in the pages of this journal¹. In recent years the quality of instruments of this type has been considerably improved, chiefly as a result of systematic work on improvement—design, technological, and operational—carried out continuously in the RCA laboratories under the direction of Zworykin. These improvements have affected many parts of the instrument².

Fig. 1

Fig. 2

2. The construction of the electron gun has undergone substantial changes. Fig. 1 shows the new (a) and old (b) constructions. It is easy to see that the new gun has one additional electrode \(A'_2\), the presence of which makes it possible to change the focusing of the beam without changing the current strength in it, as was the case previously.

3. The shape of the bulb and the method of fastening the mosaic have also changed. Fig. 2 shows, in two projections, diagrams of the old (a) and new (b) iconoscopes. The new one has not a spherical bulb, as before, but a cylindrical bulb with a neck for the electron gun welded on at an angle of \(30^\circ\) to the axis. This shape of bulb makes it possible to have a large window size for projecting the optical image onto the mosaic. The bottom of the bulb, toward which the photosensitive surface of the mosaic is turned, is a slightly convex glass surface—a segment of a sphere of large radius—cut from glass flawless in respect to homogeneity and subjected to special grinding and polishing.

which is welded to the cylindrical part of the bulb. The other end of the bulb is sealed in an analogous manner (grinding and polishing are not needed here). Such a method of manufacturing bulbs became possible as a result of improvements in the technique of glass-blowing work. The mosaic in the iconoscope of the new type is attached by direct holders to the walls of the bulb, which gives a much greater rigidity of mounting than the usual method of mounting on a stem.

1. Of special interest is the improvement of the mosaic, expressed, on the one hand, in increasing its sensitivity and, on the other, in improving its spectral characteristic. We believe that these improvements were introduced on the basis of the results of a work by Soviet authors, since, judging from the literature, the corresponding data were published only in our press. This method is a modification of the Asao and Suzuki method and likewise consists in depositing (evaporating) silver onto a finished photocathode, but differs in the temperature regime of treatment and in the dosing of the additional silver. When properly carried out, an ordinary oxygen–cesium cathode can obtain a sensitivity of up to 90 μA/lm, with the long-wave maximum located at about 6000 Å. This means not only an increase in the overall sensitivity, but also a significant increase in the quantum yield at the maximum. The position of the maximum at 6000 Å greatly lowers the relative sensitivity in the near infrared region, which is very important for correct color rendering.

Fig. 3

As applied to mosaics, the deposition of additional silver increased the sensitivity from 3–6 μA/lm to 9–15 μA/lm. The change in spectral sensitivity may be roughly characterized by the following table:

In Fig. 3 are given the spectral characteristics of an ordinary oxygen–silver–cesium cathode and of a cathode subjected to treatment with silver. These curves give the limits within which the position of the selective maximum can be varied by this method.

1. An interesting improvement in the operating regime of the iconoscope is the use of illumination of the walls of the instrument bulb. The advantage of this method, discovered empirically, is that the walls of the bulb possess a certain photosensitivity, and the photoemission they produce improves the distribution of the field drawing electrons away from the mosaic. The increase in the output signal due to illumination may amount to from 100 to 120%.

II. ICONOSCOPES WITH IMAGE TRANSFER

1. One of the most important tasks in improving the iconoscope is to increase the output signal at an unchanged magnitude of illumination of the photosensitive surface, i.e., to increase its sensitivity. Every such improvement makes it possible to lower the illumination level in studio transmission, which means a reduction in lighting costs

tion and facilitates the working conditions in the studio (air temperature), and also broadens the possibilities of using the device for transmission under natural illumination. Along with increasing the sensitivity of the mosaic, other methods are also possible here, one of which is the transfer of the electronic image obtained on an undivided photocathode under the action of the optical image onto the mosaic, which here performs the function of a secondary-electron emitter.^4 This method, proposed independently by a number of authors as early as 1934, gives advantages, first, with respect to better utilization of the sensitivity of the photocathode, and, second, as a result of increasing the signal by amplification through secondary emission at $\sigma > 1$.

1. In Fig. 4 is shown the scheme of an iconoscope with image transfer. The photocathode $K$, having a concave surface (necessary for eliminating distortions^5), is a semitransparent layer of silver, treated, as usual, with oxygen and cesium and then subjected to additional treatment with silver in order to improve sensitivity. The integral sensitivity of such a cathode may amount to $20—50\ \mu\mathrm{A}/\mathrm{lm}$ under reverse illumination. The electron lens $L$ can be implemented both in the electrostatic and in the magnetic variant.

Fig. 4

1. As for the mosaic $M$, a number of materials were tested here. The first of these was an ordinary silver mosaic treated with oxygen and cesium. Considerably better results were obtained when a purely silver surface was treated by discharge in oxygen and then with cesium vapor. However, the best qualities were possessed by mosaics consisting of the finest particles of dielectric deposited directly on the metallic surface of the signal plate (thin dielectric layers give a very high secondary emission^8).

2. The electron gun used in iconoscopes of this type does not differ from ordinary ones. The resolving power also has the usual magnitude. At present work is being carried on toward a further increase in sensitivity by transferring the electron image, amplified by the first mosaic, to a second, and so on, and also by using electron multipliers to amplify the current of secondary emission from the mosaic.

III. SCANNING AN IMAGE BY MEANS OF SLOW ELECTRON BEAMS

1. By scanning with a slow electron beam (in contrast to the usual scanning with fast electrons) is meant a system in which the photosensitive mosaic is examined by an electron beam whose velocity is sufficiently small that the coefficient of secondary emission of the mosaic is less than unity. Thus, in scanning with a slow beam, there takes place only the removal of the positive charge accumulated under the action of light on the mosaic, and not the imparting to it of a positive potential, as occurs in ordinary systems.

2. The indicated basic feature (operation at $\sigma < 1$) makes it possible to count on obtaining a number of substantial advantages, namely:

a) elimination of the “dark spot,” which owes its origin to the redistribution over the surface of the mosaic of the secondary electrons knocked out by the beam;

b) complete utilization of the photosensitivity of the mosaic (instead of approximately 10% when scanning with a fast beam). This circumstance is due to the fact that the potential of a mosaic element after passage of the beam acquires the potential of the cathode of the gun (several tens of

volt), so that between it and the collector there exists a strong field, which sucks out all the emitted electrons (in the case of a fast beam, owing to secondary emission, the element is charged to a certain positive potential) until the potentials of the element and the collector become equal. From this follows the third advantage:

c) the possibility of obtaining large maximum signals (theoretically up to the potential difference between the cathode of the electron gun and the collector).

1. However, to realize these very substantial advantages in the form of obvious advantages proves to be rather difficult, primarily because of the difficulty of focusing the electron beam at low velocities, which is necessary for obtaining the required sharpness. Therefore the use of ordinary electron guns is not possible here, and work is proceeding chiefly along the line of creating a photoelectronic scanning system.

Fig. 5

One variant of this method is illustrated in Fig. 5. In this system the mosaic \(M\) is arranged at an angle of \(90^\circ\) to the flat photocathode \(P\), and both electrodes are placed in a magnetic field, so that the electrons released from \(P\) under the action of the positively charged electrode (not shown in the figure) and of the magnetic field fall on the mosaic. Electrons from \(P\) are selected by the image of a spot on the screen of the kinescope \(K\), which moves over the screen under the action of ordinary crossed fields.

When working with slow beams it is necessary to compensate the earth’s magnetic field.

LITERATURE

1. V. K. Zworykin, Advances in the Physical Sciences, 14, 778, 1934.
2. R. B. Janes and W. H. Hickok, Proc. IRE, 27, 535, 1939.
3. N. S. Zaitsev and N. S. Khlebnikov, Journal of Technical Physics, 8, 1023, 1938.
4. H. Janes, G. A. Morton and V. K. Zvorykin, Proc. IRE, 27, 541, 1939.
5. V. K. Zworykin, Advances in the Physical Sciences, 16, 814, 1936.
6. A. M. Gurevich and I. G. Kesaev, Communications Technology, 1938.
7. N. S. Khlebnikov and N. S. Zaitsev, Advances in the Physical Sciences, 19, 278, 1938; Journal of Technical Physics, 9, 44, 1939.
8. A. S. Korshunova and N. S. Khlebnikov, Journal of Technical Physics, 9, 860, 1939.
9. A. Rose and H. Janes, 27, 547, 1939.