Time of flight

Time of flight (ToF) is the measurement of the time taken by an object, particle or wave (be it acoustic, electromagnetic, etc.) to travel a distance through a medium. This information can then be used to establish a time standard (such as an atomic fountain), as a way to measure velocity or path length, or as a way to learn about the particle or medium’s properties (such as composition or flow rate).

The traveling object may be detected directly (e.g., via an ion detector in mass spectrometry) or indirectly (e.g., by light scattered from an object in laser doppler velocimetry).

In electronics, one of the earliest devices using the principle are ultrasonic distance-measuring devices, which emit an ultrasonic pulse and are able to measure the distance to a solid object based on the time taken for the wave to bounce back to the emitter. The ToF method is also used to estimate the electron mobility.

Originally, it was designed for measurement of low-conductive thin films, later adjusted for common semiconductors. This experimental technique is used for metal-dielectric-metal structures  as well as organic field-effect transistors. The excess charges are generated by application of the laser or voltage pulse.

For magnetic resonance angiography (MRA), ToF is a major underlying method. In this method, blood entering the imaged area is not yet saturated, giving it a much higher signal when using short echo time and flow compensation. It can be used in the detection of aneurysm, stenosis or dissection.

In time-of-flight mass spectrometry, ions are accelerated by an electrical field to the same kinetic energy with the velocity of the ion depending on the mass-to-charge ratio. Thus the time-of-flight is used to measure velocity, from which the mass-to-charge ratio can be determined. The time-of-flight of electrons is used to measure their kinetic energy.

In near infrared spectroscopy, the ToF method is used to measure the media-dependent optical pathlength over a range of optical wavelengths, from which composition and properties of the media can be analyzed.

In ultrasonic flow meter measurement, ToF is used to measure speed of signal propagation upstream and downstream of flow of a media, in order to estimate total flow velocity. This measurement is made in a collinear direction with the flow.

In planar Doppler velocimetry (optical flow meter measurement), ToF measurements are made perpendicular to the flow by timing when individual particles cross two or more locations along the flow (collinear measurements would require generally high flow velocities and extremely narrow-band optical filters).

In optical interferometry, the pathlength difference between sample and reference arms can be measured by ToF methods, such as frequency modulation followed by phase shift measurement or cross correlation of signals. Such methods are used in laser radar and laser tracker systems for medium-long range distance measurement.

In neutron time-of-flight scattering, a pulsed monochromatic neutron beam is scattered by a sample. The energy spectrum of the scattered neutrons is measured via time of flight.

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