Can We Trust Track & Field Records? – How accurate are they?: Interesting anaylsis of the technology used to time track and field events. This is in the news lately because a Jamaican just broke the 100 meter record by .01 seconds. Not a big margin.
The best systems use a digital camera positioned on the stripe that marks the finish line. This camera has a very narrow field of view and records only the thin slice of space directly above the line. As runners cross the line and into its field of view, the camera takes a rapid series of time-stamped pictures. These create a composite image that shows when each athlete passed through the slice and over the line.