How can holography show changes in air density around an aerodynamic object?

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hologram graphic — holography Figure 1: Gabor's original method for creating holograms.

Holography visualizes changes in air density around an aerodynamic object by recording the refractive-index variations caused by pressure fluctuations as gas flows around the object.1

In wind-tunnel experiments, this is achieved through pulsed-laser holography. Unlike traditional optical interferometry, which requires highly stable and precise components to detect interference fringes, a holographic system records these changes interferometrically without the same stringent optical requirements. By using the rapid, high-intensity flash of a pulsed laser, researchers can capture the state of high-speed air flow as if it were at rest, allowing for the detailed study of density-related optical changes.1