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Know It All: Hawk Eye -- Catching the Ball

Published in Geare magazine, Issue #36, 2006

Well, as the cricket season draws to a close for the summer, we have yet again seen the spectacle of umpires squirming. This has probably been happening since slow motion TV replays commenced. But what has really made things uncomfortable for cricket decision makers first appeared during the 2001 Ashes Test series.

That innovation was 'Hawk-eye', a system of new technology able to indicate whether a cricket ball, having hit the batsman's pads, would have gone on to strike the stumps. If you've watched international cricket at all in the past five years you would have seen it in operation. There's a close leg-before-wicket decision, and a few seconds later the TV shows a graphic indicating where the ball would have struck the stumps, if indeed Hawk-eye thinks it would have. The system is also used to generate pitch maps, showing where the ball struck the pitch (or where all a particular bowler's balls pitched), and the position at which each ball passed the batsman.

How does it do this?

It uses technology similar to that used in medicine. Computer-aided imaging devices aren't really like X-Rays. They can use various sensor means -- ultrasound, magnetic resonance, radioactive dyes -- to find where things are in the human body. What makes them so impressive is the software which, having received the co-ordinates of the various points identified in the heart or the brain, can render a three dimensional image of them which is displayed on a computer screen. The points are simply triangulated. If you have the precise directions from three known points to a fourth, unknown, one, you can exactly determine its location.

Hawk-eye doesn't use magnetic imaging, or a transmitter in the ball or anything like that. It simply uses six more-or-less conventional video cameras to gather its data. These are spread around the circumference of the cricket ground, pretty much at the points of a hexagon, with two roughly level with mid-wicket and two more at thirty degrees off leg and on side at each end. With their locations firmly established, and all six of them trained at the wicket, the ball can be tracked very precisely along the whole length of the wicket.

How accurate? Hawk-eye claims to be 100 per cent accurate to within five millimetres.

The six cameras themselves, remember, are dedicated to this task. They aren't the same cameras used for broadcast, although the super-slow motion effects cameras lately coming into use, and offering up to a thousand frames a second instead of broadcast cameras' 25 or 50, could potentially improve the system even more. The Hawk-eye cameras are from the Danish manufacturer, JAI, which produces high quality digital cameras for specialised and industrial applications. It also, rather confusingly, produces a surveillance and alarm system called 'Hawkeye Network' which has nothing whatsoever to do with the game of cricket.

The cameras are monochrome, high resolution and operate at 100 frames per second to get a precise take on each ball.

Not surprisingly, there have been objections to the system from amongst cricket traditionalists and others. Many of their objections are based on a misunderstanding of how the system work. For example, they suggest that there's no way that the system can correctly account for things like the ball sliding off the pitch, and swinging through the air.

But, says Hawk-eye inventor Paul Hawkins, the system doesn't need to do that. All it does is track the ball from the time it leaves the bowler's hand to the time it passes the stumps at the other end. If it slips on the wicket surface, then this simply becomes part of the tracking information. Likewise if it swings sideways during its trajectory, or spins off the pitch.

It is only when the ball's flight is interrupted, typically by the batsman's pad, that the system performs any projections. In that case, it takes the flight data leading up to that point, inserts it into a well-established computer model for objects flying through the air, and calculates what the path would have been from that point to the stumps.

Whether or not Hawk-eye should enter the formal decision-making processes of the umpires remains debatable. But for informing the television viewing audience and embarrassing the umpires, it is peerless.

© 2002-2009, Stephen Dawson