Laser schlieren deflectometry

Laser schlieren deflectometry (LSD) is a method for a high-speed measurement of the gas temperature in microscopic dimensions, in particular for temperature peaks under dynamic conditions at atmospheric pressure. The principle of LSD is derived from schlieren photography: a narrow laser beam is used to scan an area in a gas where changes in properties are associated with characteristic changes of refractive index.

Source: Wikipedia — Laser schlieren deflectometry (CC BY-SA 4.0)