Time-interval measurement

(horology) A process that consists either in calculating the duration between two known epochs, or in counting the repetitions of a recurring phenomenon from an arbitrary starting point, as with an electronic digital-reading counter, which counts the cycles of an oscillator.

A determination of the duration between two instants of time (epochs). Time intervals are measured with high precision with a digital display counter. An electronic oscillator generates pulses; the count begins with a start signal and ends with a second signal. Two atomic clocks can be compared in epoch to 1 picosecond (1 ps = 10⁻¹² s) by electronic interpolation. See also Atomic clock; Digital counter; Oscillator; Oscilloscope.

Rapid motions can be studied at short intervals by means of a large variety of high-speed cameras, including stroboscopic, rotating film-drum, rotating mirror, streak, and image converter cameras. An electronic streak camera can separate two pulses 1 ps apart. See also Photography; Stroboscopic photography.

Ultrashort laser pulses are used to study rapid processes caused by the interaction of photons with an atom or molecule. Pulses as short as three wavelengths of 620-nm light, with τ = 6 femtoseconds (1 fs = 10⁻¹⁵ s), have been formed. See also Laser; Laser photochemistry; Optical pulses; Ultrafast molecular processes.

Radioactive decay is used to measure long time intervals, to about 5 × 10⁹ years, concerning human history, the Earth, and the solar system. See also Geochronometry; Radiocarbon dating.