Full Text
INSTANT PHOTOGRAPHY
Recently a number of works have been published devoted to methods of photographing rapidly occurring processes with exposure times of \(10^{-6}\)—\(10^{-9}\) sec.
Photography by means of optico-mechanical shutters, based on the use of rotating mirrors and narrow slits or gratings[^1], has a number of substantial shortcomings, since obtaining short exposures is in this case connected with a considerable decrease in the aperture of the system. In addition, the long optical levers necessary for achieving sufficient resolving power make the entire system very cumbersome.
Pulse lamps, widely used for high-speed photography, in a number of cases do not provide a sufficiently short exposure time. Despite the development of special lamps with a reduced duration of glow, the use of barium titanate capacitors and the replacement of capacitors by a long line that provides a rectangular front of the discharge current through the lamp, the duration of the glow of pulse lamps still remains on the order of one microsecond[^2]. Moreover, the simplest way of using pulse lamps—the “open shutter” method—is associated with working at night or in a darkened room, and in a number of cases cannot be used at all. Thus, if the object being photographed is surrounded by a luminous medium, then with the shutter of the camera open the photographic film will be illuminated by constant light. To avoid this, the camera has to be equipped with a shutter that opens only for the duration of the flash of the pulse lamp. In this case the shutter cuts off the relatively weak, but prolonged, glow ...
of the surrounding medium, capable, under more prolonged action, of fogging the photographic film; while the flash lamp creates the illumination necessary for photographing the object. In those cases where the response time of ordinary mechanical shutters is too long to protect the photographic film from the action of extraneous light, and also for photographing with an exposure time shorter than the duration of the flash of the flash lamp, electro-optical shutters are used.
The combined use of a flash lamp and a Kerr shutter, described in one of the works, made it possible to photograph a projectile at the moment it emerged from the barrel of a gun, when the projectile was surrounded by a cloud of luminous gases. In the same way a projectile penetrating an armor plate and surrounded by a cloud of incandescent metal particles was photographed. The exposure time was equal to \(2 \cdot 10^{-6}\) sec, whereas when photographing with the aid of a Kerr shutter sufficiently bright self-luminous objects it can be reduced by a factor of 1000. The shutter was opened by a pulse of 36 kv, the application of which was synchronized with the moment
Fig. 1.
of the flash of the flash lamp by a special electrical circuit. The circuit was triggered when the projectile fired from the gun closed a contact located at the muzzle of the gun.
A major disadvantage of the Kerr shutter is that, in operation with it, in principle no more than 50% of the light can be used; in practice the losses are of much greater magnitude.
From this point of view it is much better to use an electron-optical converter as an inertia-free shutter.[^4] In Fig. 1 a block diagram of such a shutter is given. The image of the object being photographed, by means of an ordinary lens or objective, is focused on the photocathode of the electron-optical converter, to whose electrodes a high-voltage pulse is applied. Under the action of the applied voltage, the electrons liberated from the photocathode by light are accelerated, focused and, striking a fluorescent screen, create on it an image of the object, photographed with the aid of an ordinary camera. Since the image remains on the screen only during the time when the high-voltage pulse is applied (if the afterglow of the screen is not counted), the duration of the pulse determines the exposure time. To eliminate fogging of the photographic film by light that has passed through the converter, a filter was placed before the latter, transmitting only infrared rays, to which the photographic film is insensitive. Since the photocathode of the converter has a maximum of sensitivity in the infrared region of the spectrum, the presence of the filter had little effect on the brightness of the fluorescent image.
The application of the high-voltage pulse to the electrodes of the converter was carried out by means of a capacitive voltage divider, provid-
providing, for short pulses, the potential distribution necessary for correct focusing of the electrons.
Such a shutter, as the author notes, differs fundamentally from Kerr’s shutter, since light at its entrance and exit is produced by different sources. In this connection it can provide a light “transmission” exceeding 100%.
The shutter described was used for photography with exposure times of \(0.5—2 \cdot 10^{-6}\) sec. Further reduction of the exposure time, in the author’s opinion, was limited only by insufficient illumination of the object.
If a multivibrator with adjustable frequency is used as the synchronization circuit, the shutter described can be applied for stroboscopic purposes (Fig. 2). The frequency at which the shutter opens can be brought up to several thousand hertz with an opening time of the order of \(10^{-6}\) sec, which advantageously distinguishes such a device from ordinary stroboscopes.
Fig. 2.
An electron-optical converter can be used not only as an inertia-free shutter, but also as an instrument that makes it possible to transform the temporal sequence of phenomena into a spatial one.[^5] For this purpose the same shutter circuit was used (Fig. 1), but the converter was provided with a magnetic deflection system, similar to that used for sweep in oscillographs. When the magnetic field was varied, the image of the object moved over the screen of the converter, making it possible to obtain both a continuous record of the phenomenon and a series of photographs separated by a definite time interval.
The electron-optical converter used by the author had a fluorescent screen that made it possible to resolve 20 lines/mm. Thus, at a sweep speed of \(10^5\) m/sec, the resolving power of the instrument was such as to make it possible to distinguish phenomena separated by a time interval of \(10^{-9}\) sec.
An experimental test of the resolving power of the instrument was carried out with the aid of the setup schematically shown in Fig. 3. The light arising upon detonation of lead azide \(A\) was focused onto the slit \(S\). Part of the light flux from the slit was focused by lenses \(L_1\) and \(L_2\) directly onto the cathode of the converter \(T\), while another part, after reflection from prisms \(P_1\) and \(P_2\), was also directed there. During the lag of the light along the path between the prisms (the additional path was \(2.62\) m), the electron image of the slit on the fluorescent screen \(F\) of the converter, produced
by the magnetic lens \(M\), and deflected by the second lens \(D\), had time to shift noticeably, which was recorded by the camera \(C\).
The apparatus described was used to study the explosion of lead azide, and, as the author notes, may be used to study a spark discharge, shock waves, the flight of rockets and projectiles, and also to study an object under illumination by X-rays.
In addition to an electron-optical converter, one can use, as an inertialess shutter for high-speed photography, an iconoscope employed for ordinary television. The proposal of this, published in 1949[^6], was recently described in greater detail[^7]. The principle of operation of such a shutter is as follows.
If, in darkness, an electron beam is made to pass over the mosaic of an iconoscope and is then locked, the mosaic acquires a certain negative potential. If an optical image is then focused on the mosaic, then each element of the mosaic, owing to photoemission, acquires an electric charge whose magnitude depends on the brightness of the image at that point. Owing to this charge distribution on the mosaic, an electronic image is formed that corresponds to the optical image previously created. Usually such an image, owing to good insulation of the mosaic, is preserved without distortion for 10–20 sec. By causing the electron beam to run once over the entire mosaic before the expiration of this interval, the image can be reproduced in the usual way on the fluorescent screen of a receiving tube, photographed with the aid of a simple camera.
Fig. 3.
The difference between such a shutter and a television camera consists in the fact that the process of formation of the electronic image and its transmission take place at different times. Since the exposure time is determined by the first process, it is necessary to be able to control the photoelectron emission from the mosaic, allowing it only during a definite interval of time equal to the specified exposure time. For this purpose, instead of the normal ground potential, a negative potential of 100 V is applied to the anode of the iconoscope, preventing electron emission. At the desired moment a positive pulse of about 150 V is applied to the anode, the duration of which determines the exposure time. Then ground potential is again applied to the anode, and the image is reproduced on the screen of the receiving tube.
Theoretically, the exposure time when working with such a shutter can be brought down to \(10^{-8}\) sec. and is limited only by the brightness of the object being photographed. The authors, however, investigated the operation of the shutter in the region of exposure times from several seconds down to \(2 \cdot 10^{-6}\) sec.
The authors come to the conclusion that, despite the high sensitivity and a number of other advantages of the shutter described, because of the complexity of the circuits and apparatus it cannot compete with the Kerr shutter, unless this is dictated by additional considerations (for example, if the image must be transmitted over a distance). In any case, as the authors themselves note, such a shutter is inferior to a shutter with an electro-optical converter.
L. I.
REFERENCES
- M. Sultanoff, Rev. Sci. Inst. 21, 653 (1950).
- J. A. Fitzpatrick, J. C. Hubbard a. W. J. Thaler, J. App. Phys. 21, 1269 (1950).
- H. F. Quinn, W. B. McKay, O. J. Bourque, J. App. Phys. 21, 995 (1950).
- A. W. Hogan, Proc. I. R. E. 39, 268 (1951).
- J. S. Courtney-Pratt, Proc. Roy. Soc. 204, 27 (1950).
- H. A. Prime a. R. C. Turnock, Rev. Sci. Inst. 20, 830 (1949).
- H. A. Prime a. R. C. Turnock, Proc. Inst. Elec. Eng. 97, Part II, 793 (1950).