This laser beam is tuned to a wavelength that causes the hydroxyl molecules to fluoresce in the UV spectrum.
By delaying the pulse of the fluorescence laser and the camera shot, an image of the grid that has now displaced downstream can be captured.
By dividing the displacement by the known time delay the two dimensional velocity field (in the plane of the grid) can be determined.
Flow ratios, however, are shown to affect the impingement locations, where increased air flow ratios can reduce the required combustor size by isolating reaction products solely within the secondary cavity.
OTV consists of an initial "write" step, where a 193-nm pulsed excimer laser creates ozone grid lines via oxygen (O2) UV absorption, and a subsequent "read" step, where a 248-nm excimer laser photodissociates the formed O3 and fluoresces the vibrationally excited O2 product thus revealing the grid lines' displacement.