Holography

Overview

Holography is a technique by which the image of a three dimensional object is recorded on film so that, upon reconstruction, the constructed image of the object is three dimensional. Each portion of the hologram stores an encoded message about the whole object. Creating a simple hologram involves the use of two beams of light originating from a single laser. This laser light is of a single frequency (monochromatic) and forms waves that are in phase with one another (coherent). The light beam originating from the laser is split into an object and a reference beam by a partially reflective mirror. The object beam, illuminates the surface of the object. and reflects onto the film. The reference beam, strikes the film directly. These two beams interfere on the film, producing a complex diffraction grating containing the phase and amplitude information of the object.

Applications

Holograms allow the storage of a three dimensional volume of information on 2-D media. This three dimensionality of holography, high resolving power, and short exposure times of pulsed lasers (<20 ns) make it an excellent technique for evaluation of dynamic events such as atomization processes, impacts, and explosions. A large area of interest can be studied in great detail with all of the elements of that volume in focus. By double pulsing the laser, velocity information can also be obtained. MetroLaser has fielded holographic recording systems used to further the understanding of complex physical processes. These holographic systems are providing answers in research labs and industry where other diagnostic instruments have had limitations.

Because the holographic process records the phase information of the light reflected from an object, structural changes can be visualized if two or more holograms are taken of an object before and after deformation. This technique, called holographic interferometry, can be used to watch crystal growth on a microscopic level or storage tank vibration modes in a macroscopic view.. Interference fringes show the magnitude and the location of localized deformation. Interferometry can also be used to visualize gasdynamic and aerodynamic flows in shock tunnels and wind tunnels. In double pulse holographic interferometry two wavefronts are recorded in the same hologram and the resulting interferogram depicts the changes in flow between these two times. This differencing allows rejection of steady or slowly varying errors in the test facility. Another application of interferometry is in the field of Holographic non-destructive testing. A variety of customer supplied structural components can be evaluated in MetroLaser’s non-destructive testing laboratory.

Holography is also used in creating optical elements, such as diffraction gratings. The holographic diffractive element can be used to re-direct light instead of using bulky and weighty mirrors and refractive materials. MetroLaser also uses the phase information in holograms to develop novel approaches for security measures. Recording a specific wavefront on a hologram provides a unique key for accessing sensitive systems. It is also a method to create a complex, 3-D image to impede forgery and counterfeiting.