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Diffused lighting

Naval Museum of Québec

Diffused lighting and its use in the Chaleur Bay

Diffused lighting on flickr


One little-known aspect of the Baie des Chaleurs operation has to do with a special technique that was used in the hope of tricking the submarine U-536. This technique, devised by the Canadian Navy, was diffused-lighting ship camouflage (also referred to as "D/L camouflage").

A Canadian invention, diffused lighting is a method that the naval forces of Canada, Great Britain and the United States studied between 1940 and 1945. This system of concealment at night made it difficult to identify a ship at short range and rendered it almost invisible to the eyes of a distant enemy observer.

In the following text, we shall see how the principle of diffused lighting was discovered, the role that the Allied Naval Forces planned for this technique in the Battle of the Atlantic and the results of the application of this system as tested by Canadian, British and American researchers during World War II.

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Origins of the system

The principle of diffused lighting was accidentally discovered in 1940 by Professor Edmund Godfrey Burr, of McGill University, while he was conducting work for the National Research Council of Canada in a field having no direct relationship with the concealment of ships in high seas.

Born in London in 1886, Burr came to Montreal in 1906 to study electrical engineering at McGill University. After obtaining his degree he was hired by the university first as a laboratory technician, then as a part-time lecturer and finally as an assistant professor of electrical engineering. He soon found himself involved in the scientific research efforts undertaken by the Allied countries during World War I. Under the direction of Professor Louis Anthyme Herdt, his department head, he invented a revolutionary method of producing a special copper and cadmium alloy urgently required by the government of France to manufacture military telephone cables. For Burr, it was the start of a research career that touched on domains as varied as audiometry, optics and the evaluation of human visual acuity under conditions of darkness.

In the fall of 1940 Burr had to evaluate military optical instruments designed for night observation for the National Research Council. While observing aircraft flying at night without navigation lights, he noted that their shapes were easily visible using special binoculars. He attributed this visibility to the contrast that exists between a completely blacked-out plane and the slight luminosity of the sky, which is never completely black even when the darkness appears total. Consequently, he wondered if it would be possible to reduce the visibility of planes by reducing this luminosity contrast in some manner.

On the night of December 4, 1940, a sudden and unexpected demonstration of this idea occurred by chance. A plane that Burr was observing in flight through his binoculars disappeared suddenly as it approached for landing. Burr deduced that this phenomenon could be explained by the fact that the area surrounding the airport was covered in freshly fallen snow, and that the light from the moon reflecting off this white surface had illuminated the underside of the plane's fuselage while it was flying at low altitude. This diffused lighting had eliminated the contrast in luminosity between the plane and the sky, rendering the aircraft invisible to Burr's eyes.

Professor Burr immediately informed the National Research Council of his discovery, who in turn conveyed it to the Canadian Naval Services Headquarters. The Canadian Navy realized that this technique could be used to counter one of the dangers facing Allied convoys at this time: the visual disadvantage under which the convoys operated in comparison with German submarines.

Being smaller than the ships of a convoy, with a compact superstructure and a low profile, U-boats sailing on the surface at night could, in effect, see their targets from further away than the escort ships protecting convoys could see the U-boats. Equipped with excellent night vision binoculars, German seamen could clearly make out against the horizon the shape of ships sailing at night.

The invention of high-precision centimetric radar eventually enabled the Allies to detect submarines on the surface using a method other than visual observation, but this device was not commonly used during the first years of the war. In 1940 diffused lighting seemed to offer an alternative solution to the problem of visual disadvantage. It could make a convoy as difficult to see from a U-boat as it was to see a U-boat from a convoy.

Scarcely a few days after having reported his discovery, Professor Burr was invited to Naval Services Headquarters to discuss the possible applications of this technique. Laboratory tests quickly demonstrated that the basic principle was valid. The decision was then taken to proceed, in January 1941, with a first trial on a Canadian Navy ship, the corvette HMCS Cobalt.

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Initial development (1941-1942)

The experiment undertaken on the HMCS Cobalt, which took place on January 22, 1941, off the Halifax shore, was conducted using rudimentary materials. Ordinary commercial-type light projectors were installed on one side of the ship with temporary supports and were attached to a manually operated intensity control system. Observers, located a certain distance from HMCS Cobalt, communicated by radio with the ship in order to inform it of the camouflage effect produced by each adjustment to the light intensity.

This first trial produced results of sufficient promise to encourage the Canadian Navy to develop a more sophisticated version of the equipment that was used on board the HMCS Cobalt. Developed over the next three months, the new equipment was tested on the corvette HMCS Chambly in May 1941.

The new results were superior to the ones obtained during the test on HMCS Cobalt. The improved projectors were equipped with blue-green coloured filters, which eliminated the reddish tinge which the lights emitted when operating at low intensity. In addition, they were now mounted on retractable supports, allowing the projectors to be deployed over the side at night to illuminate the ship's hull. They were then stowed inboard during the day to protect them from the impact of the waves.

Even though the equipment was still too heavy and too fragile to survive in high seas, it allowed a ship's visibility to be reduced by 50 per cent in most conditions, and by as much as 75 per cent in very favourable conditions. The decision was therefore taken to manufacture a more solid version that could be used with success in the harsh environment of the North Atlantic. In the meantime, the experimental equipment was transferred to the corvette HMCS Kamloops in order to conduct additional tests.

These tests, held in September 1941, were intended to test a prototype of an automatic light intensity control system. This system used a photoelectric cell to measure the luminosity of the sky and compare it to the luminosity of the ship; if a difference was detected between the two, an automatic adjustment to the projectors was made to correct the difference.

In parallel to these early Canadian experiments, tests on the principle of diffused lighting camouflage were held in Great Britain and the United States as naval authorities in these countries were informed of Professor Burr's discovery.

In Great Britain, the equipment used in the first Canadian trials was tested on the corvette HMS Trillium in March 1941. Later on, towards the end of 1941, the Royal Navy installed a manually operated camouflage system developed by the British General Electric company on HMS Largs, a support ship. The British judged the obtained results to be inadequate and decided not to pursue further research into this technique.

In the United States, the U.S. Navy's interest was equally brief. The General Electric Company of New York developed an automatically controlled camouflage system that was installed on the military transport ship USS Hamul in January 1942. Part of this equipment was then transferred to patrol boat PC 464 so that additional tests could be undertaken in June 1942. The American Navy then discontinued further research, since the country's recent entry into the war had caused its priorities to change radically. The Americans did, however, send the control mechanism and the projectors used in their tests to the National Research Council so that Canada could pursue work in this domain.

The National Research Council evaluated the American equipment and decided to use it in combination with the improved supports on which Professor Burr was working. The decision was made to install this overhauled equipment on two Canadian ships that had entered shipyards for refitting: the corvettes HMCS Edmundston and HMCS Rimouski.

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Deployment on corvettes HMCS EDMUNDSTON AND HMCS RIMOUSKI (1943)

HMCS Edmundston's stay in shipyard lasted from January to June 1943; HMCS Rimouski's stay was from March to August of the same year. The camouflage device installed on each of these ships at this time consisted of sixty or so projectors with their supports and control mechanisms. The projectors that illuminated the hull were mounted on retractable supports, while those that illuminated the superstructure were mounted on fixed supports.

The three following pictures have been modified in order to point out the precise location of the equipment needed to apply the diffused lighting technique.

During the summer and fall of 1943 two types of tests were conducted on the camouflage system of each ship. First, tests conducted under experimental conditions took place in the St. Margaret's Bay region during the training period following the end of the refits. Then the ships were deployed on escort duty to North Atlantic convoys, which enabled the evaluation of the efficiency of their camouflage under operational conditions.

This evaluation period demonstrated both the strengths and weaknesses of the equipment used on corvettes HMCS Edmundston and HMCS Rimouski. Under experimental conditions and depending on meteorological conditions and the distance of the observers, the system effectively reduced the visibility of the corvettes by up to 70 per cent. However, on the high seas, it was obvious that the electrical equipment was not able to withstand the pounding it received from the waves and storms. Malfunctions were numerous and a lot of time was spent on repairs. In addition, the control system's response time was not fast enough and the tint of the coloured filters was thought to be too green.

The conclusion drawn from these tests was that the equipment still needed to be improved before it could be used on a larger scale. However, at the end of 1943, the requirement for this type of equipment was less pressing than before. Since the spring of that year, the various measures taken by the Allies to fight off U-boats had finally reached a satisfactory level of efficiency and convoys were now able to make the crossing in relative safety. In this context, refining the diffused lighting system was not a high priority for the Canadian Navy.

In fact, there was only one occasion during the war where the diffused lighting technique was used during a direct attack on a U-boat, and this event took place even before the corvette HMCS Rimouski was deployed on convoy escort duty after her stay in the shipyard. This event was the Baie des Chaleurs operation, in September 1943, whose aim was to spring a trap on submarine U-536. In this operation, HMCS Rimouski was to discreetly approach the submarine once it was detected by radar to be on the surface, in the hope of springing a surprise attack on the U-boat.

Accounts of this operation indicate that the HMCS Rimouski was using both its diffused lighting and a few navigation lights upon its approach to the submarine. The corvette was therefore not trying to be completely invisible, since the invisibility created by the diffused lighting was only fully effective at a distance, but more to give the impression that she was a small and inoffensive ship. The camouflage system appears to have hidden the HMCS Rimouski efficiently on this occasion, as Kapitänleutnant Schauenburg did not detect her approach while his submarine was on the surface. It is because the Canadians ashore sent the wrong signals to the submarine that Schauenburg discovered the trap, therefore allowing him to dive in time and avoid the attack intended for him.

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Final developments (1943-1945)

Following the tests on corvettes HMCS Edmundston and HMCS Rimouski, the development work on diffused lighting entered its last phase. The objective was to correct the three main weaknesses identified during the trials: the low reliability of the automatic light intensity control system, the inadequate solidity and impermeability of the projectors and the incorrect proportion of blue and green used in the coloured filters.

This work proceeded without apparent urgency because of the turnaround of Allied fortunes in the Battle of the Atlantic that took place in the spring of 1943. At certain times after this date, technological innovations such as the acoustically guided torpedo gave the U-boats a temporary advantage, but on the whole, the Allied convoys were sufficiently protected until the end of the war. The decryption of German codes, increased use of high-precision centimetric radar, improved tactics used by escort ships, better training for escort crews and the use of long-range aircraft to protect the convoys from one side of the Atlantic to the other were some of the decisive factors that enabled the Allies to achieve victory at sea.

In connection with one of these factors, the aerial protection of convoys, it must be pointed out that the diffused lighting technique briefly held the attention of experts looking to improve the performance of aircraft assigned to the hunt for submarines. It was, in fact, possible for a vigilant U-boat to visually detect the approach of a distant aircraft during the day, as its darker profile was easily distinguished against the ambient luminosity of the sky. Thus alerted, the U-boat could dive before the aircraft was close enough to launch an attack against it.

Hoping to reduce this visual contrast, the British adapted Professor Burr's diffused lighting system in order to use it in the air. The first experiments were not successful, since a great quantity of electricity was required to sufficiently illuminate the underside of an aircraft's fuselage in broad daylight. In addition, the presence of external projectors mounted on supports was not desirable because of the aerodynamic disturbances they caused.

Nonetheless, the British conveyed the results of their tests to the Americans, who developed a more efficient version of the technique. This system used a series of lamps installed on the nose and on the front of the wings of the aircraft. The light from these lamps was directed forward, in the direction of a possible observer on the plane's approach path. When the intensity of these lamps was properly adjusted, their light considerably reduced the visibility of the frontal profile of the plane on which the equipment was mounted. Designated by the code name ?Yehudi? as well as by the spelling variation ?Yahoody,? this aerial diffused lighting system was used between 1943 and 1945 on long-range B-24 Liberator bombers, on TBF Avenger-type torpedo bombers and on an American Navy experimental remote-controlled glide bomb.

Even though the results of the American tests were positive, ?Yehudi? never progressed beyond the experimental stage. It appears that the installation of centimetric radars on board aircraft, which took place around the same time, was judged preferable to visual camouflage. In conjunction with a powerful searchlight called the Leigh Light, radar-equiped aircraft could successfully hunt U-boats at night, when the latter surfaced to recharge their batteries under the cover of darkness. As for daylight attacks, it was recognized that the visibility of the aircraft was not necessarily a disadvantage in the fight against submarines. By making a U-boat dive, the aircraft succeeded, by their presence alone, in reducing the submarine's speed and operational radius, which considerably lessened the threat they represented.

At sea, the final tests of the diffused lighting technique were conducted on the corvette HMCS Rimouski, the equipment installed on the HMCS Edmundston having been removed in 1944. Plans and drawings were prepared for improved supports and more watertight projectors with improved filters; however, these were never built. On the other hand, an improved control system, fitted with a gyroscopic stabilization mechanism, was installed on the HMCS Rimouski towards the end of 1944. Testing took place over the following months, first in Bermuda and afterwards in Scotland's Clyde River region.

Without improved projectors, it was difficult for the assessors to judge the new control mechanism's efficiency during these tests; they noted, however, that the quality of the results was not consistent from one test to another. Without doubt, it would have been possible to correct the final deficiencies of the system if the work had continued, but this did not take place. Following the Allies? victory in Europe in May 1945, the research on diffused lighting came to an end and the HMCS Rimouski, as was the case earlier for the HMCS Edmundston, was stripped of her special concealment equipment.

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Even though the diffused lighting technique was ingenious, it proved to be too complex and inefficient a solution to the problem of convoy defence. It could indeed neutralize the U-boats' visual advantage at the start of the war, but considerable resources would have been required to perfect it and to manufacture equipment in sufficient quantity for large-scale use. Because of the competing interests between other scientific, technical and industrial needs during World War II, it did not appear justifiable to place such a priority on a system whose usefulness was relatively marginal.

Rather than seeking to neutralize the visual advantage which the U-boats held over the highly visible cargo and escort ships, it was more advantageous for the Allies to protect convoys by other means. For example, the invention of centimetric radar eventually made electronic detection more efficient than visual detection at night. When a submarine approched a convoy, it was now the submarine that risked being detected first despite its relatively small size. The assignment of long-range bombers to anti-submarine warfare likewise gave better protection to convoys, while the Allies' improved ability to decipher German communications sometimes helped them intercept a U-boat even before it was able to locate the convoys it was hunting.

Still, the incident at the Baie des Chaleurs demonstrates that, in the context of a special operation with a limited and precise objective, diffused lighting could be an asset. Using the system in this manner did not require that a large number of ships be equipped with it, nor even that the system be fully developed in order to accomplish the job. If the circumstances of that operation had been such that a signal sent to the submarine had kept it on the surface for a longer period of time, the stealthy approach of the corvette HMCS Rimouski, carried out under the cover of its diffused lighting, might well have resulted in a successful attack again U-536.

© 2001 Naval Museum of Québec. All rights reserved Marc Richard.

Special contribution by Marc Richard
Associate Librarian
McGill University
Montréal, Québec, Canada

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Selective bibliography

The following bibliography is a selective list of published works which make mention of diffuse lighting. The main unpublished documentation on the topic takes the form of archives stored at the National Archives of Canada, at the Public Record Office in Great-Britain and at the Archives II section of the National Archives and Records Administration of the United States. A more detailed bibliography on the topic will appear in two of the author's articles: a biography of Professor Burr and a study on diffuse lighting, currently in preparation.

Burr, E. Godfrey. "Illumination for Concealment of Ships at Night." Transactions of the Royal Society of Canada (Third series, volume XLI, May 1947, p. 45-54).

Burr, E. Godfrey. "Illumination for Concealment of Ships at Night: Some Technical Considerations." Transactions of the Royal Society of Canada (Third series, volume XLII, May 1948), p. 19-35.

Fetherstonhaugh, R.C. McGill University at War: 1914-1918, 1939-1945. (Montreal: McGill University, 1947), p. 337-341.

Hadley, Michael L. U-Boats Against Canada: German Submarines in Canadian Waters. (Montreal: McGill-Queen's University Press, 1985), p. 180-182.

No Day Long Enough: Canadian Science in World War II. Editor: George R. Lindsey. (Toronto: Canadian Institute of Strategic Studies, 1997), p. 172-173.

Pickford, R.J. "Sublieutenant 'Commando' and Young Corvette Skipper." Salty Dips, volume 1 (Ottawa: Naval Officers' Association of Canada, 1983), p. 4-5.

Schuthe, George M. "MLs and Mine Recovery." Salty Dips, volume 1 (Ottawa: Naval Officers' Association of Canada, 1983), p. 83.

Summary Technical Report of Division 16, NDRC. Volume 2: Visibility Studies and Some Applications in the Field of Camouflage. (Washington, D.C.: Office of Scientific Research and Development, National Defense Research Committee, 1946), p. 14-16 and 225-241. [Declassified August 2, 1960].

Waddington, C.H. O.R. in World War 2: Operational Research

Against the U-Boat. (London: Elek Science, 1973), p. 164-167.

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