Holographic interferometry with additional shadowgraph visualization

This paper describes the development of an optical system that can simultaneously generate an interferogram and a shadowgraph of a refractive index field (in this case, a compressible fluid flow). Such a combination allows one to record both the density and its second derivative of the same object at the same time. This can be achieved by splitting the object beam of a holographic interferometer before superimposing it with the reference beam. One half of this beam forms the hologram together with the reference beam, while the other is led into an imaging unit that generates an image of a plane that is located either before or behind the test object (the so-called shadow plane). This arrangement allows one to generate simultaneously two pictures, which provide more reliable quantitative and qualitative data about the investigated flow than each of the visualizations alone.