text: The most thorough fluid mechanics studies in gas have been performed using the RELIEF scheme and the APART scheme. Both techniques can be used in ambient air without the need for additional seeding.
  In RELIEF, excited oxygen is used as a tracer. The method takes advantage of quantum mechanical properties that prohibit relaxation of the molecule so that the excited oxygen has a relatively long lifetime.
  APART is based on the "photosynthesis" of nitric oxide.  Since NO is a stable molecule, patterns written with it can, in principle, be followed almost indefinitely.
  Another well-developed and widely documented technique that yields extremely high accuracy is hydroxyl tagging velocimetry (HTV). It is based on photo-dissociation of water vapor followed by visualization of the resulting OH radical using LIF. HTV has been successfully demonstrated in many test conditions ranging from room air temperature flows to Mach 2 flows within a cavity.