The Time of flight method of measuring particle mass is done as follows. An ion of known electrical charge and unknown mass enters a mass spectrometer and is accelerated by an electrical field of known strength. The time that it subsequently takes for the particle to reach a detector at a known distance is measured. This time will depend on the mass of the particle (heavier particles reach lower speeds). From this time and the known experimental parameters one can find the mass of the particle. This method of analysis is a powerful tool for finding the atomic composition of individual molecules.
In near infrared spectroscopy Time of Flight method is used to estimate the wavelength dependent optical pathlength.