Holographic interferometry

rdf:langString Holographic interferometry

rdf:langString Holographic interferometry (HI) is a technique which enables static and dynamic displacements of objects with optically rough surfaces to be measured to optical interferometric precision (i.e. to fractions of a wavelength of light). These measurements can be applied to stress, strain and vibration analysis, as well as to non-destructive testing and radiation dosimetry. It can also be used to detect optical path length variations in transparent media, which enables, for example, fluid flow to be visualised and analyzed. It can also be used to generate contours representing the form of the surface. Holography is the two-step process of recording a diffracted light field scattered from an object, and performing image rendering. This process can be achieved with traditional photographic plates or with a digital sensor array, in digital holography. If the recorded field is superimposed on the 'live field' scattered from the object, the two fields will be identical. If, however, a small deformation is applied to the object, the relative phases of the two light fields will alter, and it is possible to observe interference. This technique is known as live holographic interferometry. It is also possible to obtain fringes by making two recordings of the light field scattered from the object on the same recording medium. The reconstructed light fields may then interfere to give fringes which map out the displacement of the surface. This is known as 'frozen fringe' holography. The form of the fringe pattern is related to the changes in surface position or air compaction. Many methods of analysing such patterns automatically have been developed in recent years.