Photocells

From FIS Freestyle wiki

Photocells The finish beam is wired to the timer’s clock so that when the beam is broken by the passage of a competitor across the finish line, the circuit causes the time of day to be recorded and printed on a paper tape.

The finish beam is wired to the timer’s clock so that when the beam is broken by the passage of a competitor across the finish line, the circuit causes the time of day to be recorded and printed on a paper tape.

The start time is then subtracted from the finish time to yield the elapsed time that is recorded and printed on a paper tape along with either the competitor’s start number or bib number; this is the competitor’s official time.