OPTICAL LOCATION

Despite the rapid development and successes of radiolocation, optical location (in particular, using infrared rays) has been the subject of a number of papers published in recent years in the foreign literature. This is explained by the better resolving power of optical methods, the absence of signals reflected from extraneous objects, and also the relative simplicity and small dimensions of the apparatus used.

In 1946 an optical locator was described, intended for the rapid determination of cloud heights. As in radiolocation, the distance is estimated from the time of passage of the direct and reflected signals; however, the signals used are not radio waves, but short light pulses of about one microsecond duration. The block diagram of the instruments is given in Fig. 1. Light pulses are produced 60 times per second by discharging, through an air gap, a capacitor charged to 10 kV between aluminum electrodes. The spark gap is placed at the focus of a parabolic reflector, which sends an intense beam of light in the direction of the object. Part of the light reflected from the object is caught by a second parabolic reflector, at whose focus a photocell is placed. The signal from the photocell, after preliminary

...of amplification is fed to a special oscilloscope, triggered at the moment the light pulse is produced by means of a separate synchronizing photocell. Since the sweep speed of the oscilloscope is known, the transit time of the light pulses is determined by the displacement of the reflected signal relative to the start of the sweep. The scale on the oscilloscope screen makes it possible to read off directly the distance to the object.

A device constructed on the same principle is also described, intended for surveying work in difficult-to-access terrain.

Figure labels: Receiver; Transmitter; Light-pulse generator; Synchronizing photocell; Amplifier; Oscilloscope; Signal; Amplifier; Trigger button; Ground; ~115 V.

Fig. 1.

The instrument includes a reflector with a special optical system; a pulsed lamp is used as the source of light pulses, and a photomultiplier serves as the receiver.

An entirely different principle underlies an optical locator intended for surveying terrain within a range of several hundred meters. Having obtained the image of an object with the aid of a lens and having determined the position of the image, it is not difficult to find the position of the object itself, provided only that the object is not too distant. In those cases where the distance from the object to the lens exceeds the focal length of the lens by more than 200 times, its image falls almost at the focus, and precise determination of the distance to the object becomes difficult.

Thus, for purposes of location it is necessary, first, to determine the position of an individual image and, second, to systematically examine the image space in search of all the images contained in it. In the instrument described, these tasks are solved by means of—

by the power of the modulating device and the photocell (Fig. 2). Modulation is effected by moving the grating across the plane of the image, as a result of which the light that has passed through the grating is modulated at a frequency equal to the number of grating slits crossing a given point of the image in one second. If the plane of the grating coincides with the plane of the image, the modulation becomes deepest (the curve in the lower part of Fig. 2 shows that, as these planes diverge, the modulation percentage rapidly falls). The light that has passed through the grating falls on the photocell, as a result of which a photocurrent arises whose variable component indicates the presence of an image at the given point. For a systematic investigation of the image space it is sufficient to shift the plane in which the grating moves relative to the focus of the lens.

Fig. 2.

Obviously, in cases where an object of uniform brightness is being considered, occupying the entire field of view, there is no modulation. Even if individual details of the image have nonuniform brightness, there still exists a certain probability of the absence of the alternating component of the photocurrent owing to mutual compensation of the light flux from different details of the image.

In addition to the purposes of location, the device described may be used for automatic focusing of motion-picture and television cameras. A device based on the same principle for guiding the blind is also described.

I. L.

Cited Literature

1. J. Frank, Moles General Electric Review 49, 46 (1946).
2. W. W. Hansen, Trans. Amer. Inst. Elect. Eng. 67, part I, 660 (1948).
3. Electronics No. 4, 102 (1950).