Double-exposure holographic interferometry
Holographic interferometry - Principles

The technique employed is similar to that used to produce a conventional hologram (simple exposure). Let's assume we've made the holography setup shown in figure 1 a. A first exposure is made, with the pressure inside the thruster being p₁ (state 1). The internal pressure is then raised from p₁ to p₂, without touching anything else. A second pose is then carried out to register the thruster in state 2 (pressure p₂).

After photographic development, we have a plate containing the sum of two holograms, inconsistent with each other since they were produced at different times.

However, when rendered (figure