That old razzle dazzle

IN THE second world war, many Allied ships were painted with dark and light stripes, and other contrasting shapes, making them look a bit like zebra. The idea was to distort an enemy submarine commander’s perception of the ship’s size, shape, range, heading and speed, so as to make it harder to hit with the non-homing torpedoes of the period. These had to be pointed not at the target directly but, rather, at the place where the commander thought the target would be when the torpedo arrived. At the time, it was only an educated guess that this so-called dazzle camouflage would work. But now someone has actually tested it and the short answer is that it does work—but not in the way that Allied navies thought it did. Ships move too slowly. Dazzle camouflage might well, however, have a role in protecting faster-moving vehicles, such as military Land Rovers.
Nicholas Scott-Samuel, of the University of Bristol, and his colleagues came to this conclusion by asking volunteers to watch patterned rectangles cross a computer screen. Some of the rectangles had horizontal stripes inside them. Some had vertical stripes. Some had zig-zags. And some had checks. Some, acting as controls, had no internal patterns. Each test involved a jazzy rectangle crossing the screen either before or after a plain one. Volunteers had to estimate which of the two was travelling faster.
In fact, in all cases, the two rectangles travelled at the same speed. But the researchers varied the conditions in other ways, without telling the participants. Sometimes both rectangles travelled slowly, at just over 3º of arc a second from the observer’s point of view, mimicking a ship. Sometimes they travelled much faster, at 20º a second, mimicking a land vehicle. The jazzy rectangles also differed. Some were low contrast (ie, not very jazzy) and some high contrast.
The upshot, as Dr Scott-Samuel reports in the Public Library of Science, was that participants were not fooled by slowly moving rectangles, nor by low-contrast ones. But fast-moving, high contrast ones did fool them. On average, an observer reckoned such a fast, jazzy rectangle was going 7% slower than was actually the case.
Ships, then, travel too slowly for dazzle camouflage to have an effect. In any case, modern torpedoes and missiles are guided to their target electronically. But 20º a second corresponds to the perceived movement of a vehicle 70 metres away that is travelling at 90km an hour. That is precisely the sort of distance from which an unguided rocket-propelled grenade might be fired at a lightly armoured military vehicle, and precisely the sort of speed such a vehicle might be travelling at. Perhaps the answer, then, is for modern armies to buy their Land Rovers second-hand from game parks, which often paint them in zebra stripes for effect. Whether real zebras are striped that way to confuse predators, has yet to be determined.