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Tech Bite: How Tissot determines the winner in a photo finish.

Tour de France 2017, Stage 7.
by Peter Chong on July 8, 2017

We often hear the term “photo finish” in a race. What exactly does this mean? And how does the timing sponsor of the sports event determine the winner after such a photo finish. We take a closer look at how Tissot, this year’s timing sponsor for the Tour de France 2017. Our subject example occured in Stage 7 of the Tour. After 213.5km of racing, German rider Marcel Kittel (racing for Quickstep Floors) finished five millimeters (!) in front of Norwegian Edvald Boassen Hagen (racing for Dimension Data) on July 7, making the Tour de France’s stage 7 finish to Nuit-Saint-George one of the tightest in race history. That’s a difference of .0003 seconds at the line over a 5 hour race.

Shown below is the photo finish from the high speed cameras at the finish line. At this point, the riders were going at speeds of up to 75 kph.

 

The photo finish image revealed just how close stage 7 was. Photo: Tim De Waele | TDWsport.com
Read more at Velonews.

 

The photograph above looks like it is taken by a regular camera as the riders sail across the finish line, but it is not. It is taken by a special system supplied by the Tour’s timing sponsor, Tissot.

Part of the system is the photo finish camera. This is a special camera with no shutter, but has an aperture which is only a small vertical slit which is only one pixel wide. This slit is lined up with the finish line. As the camera does not have a shutter which opens and closes to expose the sensor (or in the early days, film), it does not miss anything. The camera takes 10,000 pictures a second of the view of the finish line through this one pixel wide slit. Each frame is a one pixel wide image, like a time lapse photograph. These images are then compiled  to form a single large image, and looks like the above.

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In a normal photo, there would be a background, but there is none in the photo finish picture above. The entire picture is the width of the vertical slit (the camera can only see out into that slit, and nothing else). The whole picture is the finish line, stretched out in time. And we are actually looking at the moment they crossed the line. The spokes of the wheel appear curved as an artifact of the time lapse photography. Also as an artifact of the photography technique, riders and bikes moving faster than the refresh rate would look squashed, and those slower would look stretched out.

The thin red line is a tool on the computer screen used by the judges to see who passed it first. The red line can be moved to check the relative positions of all the riders. They can do this because the entire photograph is of the riders as they crossed the finish line.

From the photograph above, it looks like there is not enough detail to pick out the difference. But the actual photographs used for judging are higher resolution than the one released to the press shown above.

As the rate in which the sensor is refreshing is known, in this case 1/10,000th of a second, we can measure the exact time gaps with using a ruler on the photograph. Each pixel corresponding to 1/10,000th of a second. And the smallest gap which can be measured between two front tyres is thus 1/10,000th of a second. As the gap in this case is 3/10,000th of a second, the camera system can easily resolve this.

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As the tour’s time keeper, Tissot supply the camera and associated computer timing equipment, and their technicians provide the images to the judges. The judges decide who wins, so the decision is still a human process, but one which is aided by Tissot’s technology.

As the Tour’s sponsors, Tissot also released a series of Tour de France watches. Shown below is the Chrono XL Tour de France 2017 edition, with yellow (the leader’s jersey in the Tour is yellow) motifs of a bicycle fork as the design element on the strap. The watch retails for US$350.

 

We leave you with the stage summary from the Tour de France 2017 Stage 7.

 

 

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