LIGHT SOURCE FOR BALLISTIC PHOTOGRAPHY

As a light source for photographing artillery shells in flight, a condensed spark is usually used. The photography is carried out by the shadow method, and the photographs obtained make it possible to judge the position of the shell in space, as well as the nature of the shock wave excited by it. However, for shells of large caliber such a method becomes technically difficult. In addition, it gives no information about the condition of the surface of the flying shell or about its rotation. These shortcomings can be avoided if the shell is photographed with an ordinary camera in reflected light. In this case, of course, the possibility of observing the shock wave is lost.

For such photography it is necessary to have a light source giving an instantaneous, sufficiently intense flash. The duration of the flash, more precisely the time during which it remains actinic, must not exceed 1–2 microseconds at a shell velocity of the order of 1000 m/sec. Otherwise the image of the shell in the photograph proves blurred. Accordingly, the intensity of the light flux emitted during the flash must be approximately as many times greater than the intensity of a continuously operating light source as the duration of the flash is shorter than the exposure time required for photographing a stationary object illuminated by a continuously operating source, i.e., the entire second.

Fig. 1. Light source giving a short-duration flash.

The problem of creating such a light source was successfully solved by MacCormick, Madansky, and Ferbens*). The lamp they constructed is shown in Fig. 1. It is a Pyrex vessel filled to a pressure of 450–600 mm of mercury with a mixture of commercial krypton (90% krypton and 10% xenon) and hydrogen (approximately 25% of the total pressure). The flash occurs when a capacitor of capacity 0.5 μF (two 1 μF capacitors connected in series), charged to a potential difference of several (of the order of ten) thousand volts, is discharged through the lamp. The gas pressure and composition are selected so that, at the maximum potential difference between the electrodes, the discharge cannot arise spontaneously. To produce the discharge, a special automatic “igniting” device is used, set in action by a shot or by a shell and supplying at the required instant a high-voltage pulse to a coil of wire surrounding the central part of the lamp from outside. The circuit of the “igniting” device is given in the paper.

The main difficulty consisted in giving the light pulse emitted by the lamp the required form. Usually the light output of the discharge rises rapidly, reaches a maximum, and then slowly falls. As a result, the duration of the light pulse is determined primarily by the decay time, which may be comparatively large. Moreover, the oscillatory character of the discharge may lead to undesirable repeated

*) Journ. Appl. Phys., 19, 221 (1948).

flashes. These shortcomings can be eliminated by choosing a lamp design in which the discharge takes place in a relatively narrow space between two glass walls (see Fig. 1), as well as by selecting the parameters of the electrical circuit, the detailed schematic of which is given in the paper.

Measurements of the flash duration were made in two ways—photographically and with the aid of a photocell connected to an oscilloscope. In the first case, values of about 1–2 microseconds were obtained, whereas in the second, 4 microseconds. The discrepancy is explained by a possible lack of sensitivity of the photographic emulsion to the small intensities at the beginning and end of the flash. The maximum intensity was measured with a vacuum photoelement calibrated with the aid of a tungsten incandescent lamp (55 candles) with a color temperature of 2848° K, and proved to fluctuate for different lamps from \(4 \cdot 10^6\) to \(6 \cdot 10^6\) equivalent candles.

Fig. 2. Photograph of a 90-mm artillery shell in flight.

The entire apparatus, including the power-supply and igniting circuits, as well as a 35-centimeter metal reflector in which the lamp is installed, is mounted in a single compact box and weighs about 16 kg.

The authors present two photographs of flying shells obtained with the described light source and a camera with a lens diameter of 127 mm and relative aperture 1:4.7.

Figure 2 is a photograph of a 90-millimeter artillery shell flying at a speed of 960 m/sec.

Fig. 3. Photograph of a 200-mm armor-piercing artillery shell in flight.

Figure 3 gives a photograph of a 200-millimeter armor-piercing artillery shell flying at a speed of 840 m/sec. In this case, in order to increase reflectivity, the shell was coated with white paint, but when fired it was retained only on its front part. In the photograph one can see part of the solenoid in which the pulse is induced that actuates the lamp-ignition device.

Since all details of the surface are clearly visible in the photographs, it is not difficult, having a series of successive photographs, to determine the rate of rotation of the shell.

G. R.