The time of flight (TOF) escribes the method used to measure the time that it takes for a particle, object or stream to reach a detector while traveling over a known distance. 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 can be used to determine the mass-to-charge ratio. The time-of-flight of electrons is used to measure their kinetic energy. In near infrared spectroscopy, the time-of-flight method is used to estimate the wavelength dependent optical pathlength. With an ultrasonic flow meter measurement, the principle is used to work out speed of signal propagation upstream and downstream of flow, in order to estimate total flow velocity. Optical time-of-flight sensors also exist, but depend on timing individual particles following the flow rather than using Doppler changes in the flow itself (as this would require generally high flow velocities and extremely narrow-band optical filters; see planar Doppler velocimetry). In kinematics, TOF is the duration in which a projectile is travelling through the air. Given the initial velocity u of the particle, the downward (i.e. gravitational) acceleration a, and the projectile's angle of projection θ (measured relative to the horizontal), then a simple rearrangement of the SUVAT equation