Detecting Aircraft at Night
February 1942 Radio News

February 1942 Radio News

[Table of Contents] Wax nostalgic about and learn from the history of early electronics. See articles from Radio & Television News, published 1919-1959. All copyrights hereby acknowledged.

Most(?) RF Cafe visitors are probably familiar with British engineer John Logie Baird as being considered "the father of television." His work in the 1920s produced both live and recorded motion pictures transmitted and received electronically. What most visitors (including me) probably did not know is that he also developed television apparatus using infrared imaging; he called it "Noctovision." Noctovision (noct or nox meaning night) was a moving image form of the still imaging "Noctovisor," which was an early night vision system that converted an infrared image into an optical image. Radar was still in its infancy in 1942 when this article appeared in Radio News magazine, and had not been implemented widely enough to provided needed surveillance against nighttime bombing raids from Germany's Luftwaffe (air force) flying across the English Channel. According to author Rosen, infrared wavelengths have 16x the fog penetrating power of optical wavelengths, so it fit the need well. Big plans were being made for deployment on land, sea, and air. Oh, another new term I learned here is camera obscura.

Detecting Aircraft at Night

By Herbert Rosen

Combining both Noctovision and Television - the Noctovisor detects planes at night and thru fog.

The great steps of progress which have been made in the past few months in the anti-aircraft field in Great Britain are indeed amazing. The latest inventions in this field have made it difficult, in these recent months, for the Germans to bomb England as fiercely as they did at the beginning of the Blitzkrieg, and, on the other hand, has minimized the amount of damage done to England in general.

Improvements are being made from day to day, and eventually the mechanisms will reach such a high degree of perfection, that the Germans will confront great difficulty in even reaching the coast of England. In this respect we can say that we are all looking forward eagerly to the day, which, is very near, when the Allies will "dominate the Air." Quietly and without ostentation progress has been made within the past few months towards solving the critical problem of national defense. The experts are confident that the solution which has been found is without precedent, and a new phase of high efficiency in national defense has definitely begun. Without giving away any military secrets, we can tell you the basic idea of this invention.

This amazing new discovery is composed of a combination of the new twin-sciences - Television and Noctovision. Television, today is as familiar to everybody as the electric bulb and needs no explanation, but - what is Noctovision? This is, no doubt, not only a new science, but also a new word to many people.

The Invisible Ray

Noctovision is vision in darkness brought about by the invisible infra red ray which is transformed by means of a modified television apparatus into a visible ray. Simply explained, the apparatus itself is essentially a television transmitter and receiver mechanically coupled together, and constructed to be sensitive to invisible infra-red rays in place of light. The lens of the "Noctovisor" casts a little invisible image formed by the infra red rays upon the exploring disc of a transmitting "Televisor," similar to a cinematograph picture, and this mechanically-coupled receiving apparatus simultaneously reproduces and makes visible this image, which is then transmitted back to the screen at defense headquarters.

One may be led to believe that this invention is a recent one, but it is not. Actually, it is about fifteen years old, and the inventor is John L. Baird, the great British television inventor. Mr. Baird demonstrated Noctovision for the first time to the Members of the British Royal Institution in December, 1926. Since then it has been developed in secret. At one of the next experiments, one year later, for the "British Association for the Advancement of Science," the gentlemen were sitting in a pitch dark room in Leeds before the Noctovision transmitter. Much to their sincere amazement, they were one by one recognized by their colleagues who were present before the receiver at the Baird Laboratories in London. And from then on Mr. Baird has improved this device until it reached the high level of perfection at which it is today.

The Functions of the Apparatus

The apparatus today resembles a gigantic camera made of aluminum. A lens of about eight inches in diameter is attached to the end of a cone protruding from the front of the aluminum box. This box with its lens swings on universal pivots so that it can be directed to any degree. It is, in fact, a kind of camera obscura, differing from an ordinary camera obscura only in that this one renders visible not ordinary rays of light, but the infra red rays, those invisible rays which occupy that part of the spectrum beneath the visible red. Infra red rays have the power of penetrating fog and a number of substances which are opaque to ordinary light. Among them is ebonite, which, while opaque to ordinary light, is semi-transparent to these invisible red rays.

The Noctovisor shows on a little screen, resembling the ground glass screen of an ordinary camera, whatever object is in the focus of the lens. This image, it must be remembered, is made entirely by the infra red rays and not by visible light.

The essential part of the apparatus as already explained, consists of a television transmitter and a television receiver coupled together. The television transmitter operates only by the invisible infra red rays, so that objects invisible to the eye become visible on the screen of the television receiver.

Mathematically speaking, infra red rays have sixteen times the fog penetrating power of the ordinary rays of the spectrum, and their value for discovering the approach of hostile aircraft under cover of darkness or fog is obviously inestimable. The longer the wave of light, the greater is its penetrative power. For this reason the light at the blue end of the spectrum which contains the shortest rays can penetrate fog to a much less degree than the red rays. This fact explains how it is that the sun seen through a fog is red, for the fog absorbs all the rays except the red ones. it is for a similar reason that, when the sun sets, it turns red, for the dust and fog in the atmosphere have filtered out all the rays but the long red ones. The infra red rays are far longer than the red rays, and therefore possess an enormously greater fog penetrating power.

It will be interesting to learn that this instrument was used, not exactly in Baird's manner, but in a similar style, in peace times on ships. When the wireless equipment of the new S.S. Normandie was first described, mention was made at the time of a special, secret device which had been installed, and which enabled the vessel to discover the presence of an obstacle in its path, such as an iceberg or another vessel which, by reason of fog or other obstructions to vision, might otherwise not be detected. We are able to give an account of this "feeler" equipment which makes use of the properties of micro-waves in a novel manner, and is known as the "obstacle detector."

The device is dependent upon the property possessed by micro-waves that they are reflected from a solid object. The exact nature of the object does not matter. If, then, a micro-wave transmitter sends out a beam which strikes an obstacle, reflection of the beam takes place, so that it can be picked up by a receiver which may be located comparatively near the transmitter, provided that precautions have been taken to prevent any direct reception of the transmitter at the receiver. If there is no obstacle in the path of the transmitted beam no reception is obtained at the receiver, and it is only when some obstacle is encountered that the reflected energy is picked up at the receiver.

Method of Estimating Distance

Another important point to be considered is how it is possible to locate the obstacle and estimate its distance from a ship. A wavelength of the order of 16 centimetres is used, giving the advantage that it is possible to concentrate a very sharp beam of only a few degrees with small-sized reflectors. This makes it possible to tell the direction of the obstacle to within a very few degrees. The distance of the obstacle from the transmitter and receiver can be calculated by treating the line between the transmitter and receiver as a base and rotating the reflector at the receiver through a few degrees until maximum signals are obtained. Then, knowing the angle of the reflectors both of the transmitter and receiver to the base line, lines drawn through the angles to form a triangle with the base will intersect at a point corresponding to the distance of the obstacle from the ship.

In some of the first tests carried out, the transmitter and receiver were placed on the side of a ship about eight metres above the level of the sea, the distance between the transmitter and receiver being six metres. After this, the receiver was placed as far as was conveniently possible from the transmitter in order to have as long a base line as possible and to assist in preventing direct pick-up by the receiver. The receiver incorporates a similar valve to that of the transmitter; but in this case operating as a detector. The receiving antenna is placed as before, within the glass container of the valve, and the whole included in a similar parabolic reflector to that used in the case of the transmitter.

When a beam is reflected by an obstacle and reaches the receiver, the current detected is carried to an amplifier, and is received on ear-phones, or on a visual indicator. As soon as signals are detected, the reflectors, which are  normally revolving automatically through about forty degrees, are stopped and precisely directed for maximum received signals. It is then possible to make all the necessary calculations for obtaining bearings of the obstacle.

The significance of this apparatus can now be readily appreciated. Mounted on the bridge of a steamer, a mariner is enabled to detect an approaching ship or the harbor lights of a port, although the fog renders them invisible to the naked eye. However, it must be borne in mind that the perfection of this invention was not achieved immediately. Its development covered a period of more than a decade, during which time painstaking efforts were put forth to produce the perfected apparatus.

This is the history of the "Noctovisor" and obstacle detector which is the new weapon of England's war machine.

It is now possible to install along England's coast this apparatus or even to equip airplanes with Noctovision. In darkness or fog a fighting plane so equipped would have an enemy plane, not so equipped, at its mercy. It would direct its invisible rays on an enemy machine floundering in the dark quite unaware that it was under observation and harried by a foe which it could not see approaching in the darkness.

In a similar manner, a naval battle between a fleet equipped with Noctovision searchlights and one relying on ordinary means of observation would in the dark be a one-sided affair. In the majority of cases it is likely that the sweep of an infra red beam all difficulties of fog, poor visibility, and smoke screens disappear. Distance, too, is no drawback. The Televisor or the Noctovisor can see no further than the human eye; but the images themselves can be converted into visible rays and transmitted by wireless to a far distant receiving station. - Thus, the progress of science brings us new hope for the victory of the democracies. With these amazing new devices to assist us, we feel sure that the foes of freedom will be suppressed forever.

Posted January 21, 2022