The human eye was the first. It is doubtless reliable, but the path to the hand is beyond our control. Muybridge's cut thread introduced in 1879 was the first automatic detector and trigger. The starting gate, also derived from chornophotography, was first used by E.J Marej in 1881. From the 'twenties onwards, a pneumatic tube was used. The light barrier also appeared around the same time, so-called because, to activate a photoelectric cell, a light ray is used which, like Muybridge's thread, delimits the reference line. The photoelectric effect, which is an emission of electrons released from metals by light, was suspected by César Béquerel in 1835 and discovered by Willoughby Smith in 1873. Heinrich Hertz, too, had an inkling of it, and his pupil, Philipp Leonard, studied it in 1892. The theory was formulated by Albert Einstein in 1914. His revolutionary theory of light quanta called into questions the nature of the light wave and was also behind the development of television. For the hippodromes of North America in 1929, K. Crook was the first to apply it in measurement of time and determination of the winner. In the 'forties, infrared or "invisible" light was used. The light ray was no longer subject to the influences of outside light. The first autonomous infrared mobile cells were used in 1948 at the Olympic Winter games in St. Moriz. The photoelectric cells checks only one point and one competitor. To control any point on a body and several competitors in parallel, the photo finish technique is used. Born in 1880 with instant photography, it uses the continuously recorded electronic image today.