A more sophisticated technique for optical flow measurement relies on the principle of
scintillation, whereby the fluid itself warps the path of light passing through, rather than entrained
particulate matter scattering the light. Scintillation is the same phenomenon responsible for the
“twinkling” of stars and city lights viewed from a long distance: as air passes between the light
and the observer, pockets of air having different density (due to differences in temperature) and/or
sufficient turbulence cause some of the light to be refracted away from an otherwise straight-line
path, making it appear as though the light source is randomly vibrating or oscillating. In fact, the
phenomenon of scintillation has been used for the measurement of air velocity (anemometry) for
many years before its application to industrial flow measurement.