How can double-exposure holography show elastic deformation in an object?

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

Double-exposure holography is a powerful application of holographic interferometry used to visualize elastic deformation in objects.

The process works by superimposing two wave fronts on a single hologram: one recorded before the object is deformed and one recorded after the deformation has been introduced. When this double hologram is reconstructed, the object appears crossed by interference fringes. These fringes represent the slight modifications in the wave front caused by the object's change in shape, allowing researchers to map complex surface distortions that would be impossible to measure using classical interferometry.1

Alternatively, for time-varying studies, a hologram of the object in its unstressed state can be compared against a second hologram of the stressed object. By viewing the stressed hologram through the original, the resulting superposition produces the same interference pattern as a double exposure, revealing the nature of the deformation.1