MULTI-SLIT SHUTTER

In the technique of the modern physical experiment, methods for obtaining and recording short-duration light pulses play an essential role. The problem of recording very short-duration pulses arises in the study of explosions, shock waves, detonation phenomena, combustion, and processes of relaxation of the most varied physical nature, and in the study of phosphorescence of short duration, fluorescence, etc. This problem is also very important in optical location.

In this connection, the idea of a multi-slit shutter, put forward by S. M. Raiskii and E. Ya. Pumper¹, which makes it possible

Fig. 1.

to obtain and record short-duration light pulses, is of considerable interest. At short exposure times, the use of an ordinary curtain shutter is difficult: the speed of motion of the shutter must be high and the slit of the shutter itself sufficiently narrow, thus transmitting little light. In a multi-slit shutter the curtain is replaced by a grating with a row of parallel-

of slits. Next, the motion of the shutter is replaced by the motion of its optical image; the use of a rotating mirror makes it possible to obtain considerable speeds in this way. The shutter acts as the next image. The condenser $L_1$ (Fig. 1) illuminates with a beam of light, close to parallel, the grating $P_1$, in which transparent and opaque bands alternate. The objective $L_2$ projects the source $O$ onto the surface of the plane mirror at the point $O_1$ and gives an image of the grating $P_1$ in the plane $P_2$. In this plane a second grating is placed, similar to the first. The objective $L_3$ gives at the point $O_2$ an image of the mirror surface and thereby an image of the light source. When the mirror rotates, the image of the grating $P_1$ moves over the grating $P_2$. The bright bands of the image fall on the transparent and opaque parts of $P_2$. As a result, at the point $O_2$ the image of the source alternately appears and disappears. A time sweep on the oscilloscope screen gives a group

Fig. 2.

of light pulses (Fig. 2). The shape of the pulses depends on the ratio of the parameters of the two gratings: the width of the slits and the width of the dark bands. In this variant the method of a multilayer shutter is used to obtain short-duration light pulses. With the width of one slit $d_1 = 0.02 \text{ cm}$, the focal length of the objective $L_2$ $R = 100 \text{ cm}$, and the number of revolutions of the mirror per second $N = 300$, the duration of the light pulse $\tau$ is

$$ \tau = \frac{d_1}{v} = \frac{d_1}{4\pi R N} = \frac{0.02}{4\pi \cdot 100 \cdot 300} = 5.3 \cdot 10^{-8}\ \text{sec}. $$

The oscillograms shown in Fig. 2 were taken by the authors under the following conditions: a) grating period 3 mm, slit width $d_1 = 1 \text{ mm}$, $N = 13$ revolutions/sec., $R = 30 \text{ cm}$, $\tau = 5 \cdot 10^{-5}$ sec.; b) the grating period and slit width are the same, $N = 50$ revolutions/sec., $R = 30 \text{ cm}$, $\tau = 1.4 \cdot 10^{-5}$ sec. The time marks on both oscillograms are 100 $\mu$sec. The light source is a 300 W cinema lamp; the photoelement is antimony–cesium.

A modification of this method is also possible for purposes of instantaneous cinematography. Then grating $P_2$ must obviously be replaced by a photographic plate, on which, as the shutter moves (more precisely, its image), a series of successive photographs of the object $O$ is obtained. This application of it was later carried out by Sultanov^2 and described by us^3.

However, in the cited reference 3 we did not note that priority in this question indisputably belongs to the named Soviet scientists.

M. Ginzburg

Cited Literature

1. S. M. Raiskii and E. Ya. Pumper, ZhTF 20, issue 7, 822 (1950).
2. M. Sultanoff, Rev. of Scien. Inst. 21, 653 (1950).
3. UFN 43, 140 (1950).