Conventional photography represents two-dimensional (2D) records of three-dimensional (3D) scenes. Here the distribution of the light intensity passing through or reflected by the scene is registered on a photosensitive surface. By recording this way, information on the phase distribution of the light waves is not required. On the contrary, holography depends on recording the amplitude and the phase of the light waves. It is then possible to reconstruct the scene in three dimensions as well. This makes holography a special imaging method.
Because recording materials are only sensitive to light intensity, the phase information has to be transformed into a light intensity code. This can be done by using coherent light for object illumination and by adding to it a reference wave from the same light source as the object wave. Both waves, or light beams, interfere at the recording medium producing an interference pattern in which the local fringe density describes a function of the phase distribution and the gradient of darkness of the fringes is proportional to the intensity of the object wave. The recorded scene can be reconstructed if the photographic plate, which after development, will be called hologram, is illuminated with the reference wave. The information stored in the photo plate is encoded and can be shown as a true 3D image of the original scene. An observer cannot distinguish between the reconstructed and the original wave field.
Holograms can be classified according to the arrangement of the optical components in the holographic camera or according to the features of the developed photographic plate, as shown in Figure 1.
With respect to the arrangement of the components
In-line. A plane wave illuminates a particle or a particle field. A small amount of light is diffracted by the particles forming an object beam, which is able to interfere with the undisturbed wave.
Off-Axis (Counter Light). A light beam illuminates a transparent medium containing the object to be holographed. The object beam is recorded simultaneously with a reference beam on a photo plate.
Off-Axis. An object being illuminated reflects part of the light to a photo plate. Simultaneously the plate is also exposed to a reference beam. The path length of both beams is almost equal.
Reflection. The object beam meets the photo plate at the emulsion side while the reference beam incides on its glass side. It can be reconstructed with white light.
Rainbow. With the aid of two cylindrical lenses a laser light sheet is produced and sharply focussed onto a ground glass plate in order to obtain diffuse illumination of the object. The light travels through the partial transparent object and finally meets the photo plate. A plane wave is used as a reference. It can be reconstructed with white light.
With respect to the developed photo sheet
Amplitude. Once the recording has been made, the photo plate is developed and fixed. The photo emulsion shrinks a little but remains uniformly thick. The amplitude of the reconstructed wave results in a function of the darkness distribution in the photographic sheet.
Phase. This fixing process is replaced by bleaching the photo plate until total transparency. During this process the silver ions are removed from the photo emulsion producing an irregular thickness distribution on the photographic sheet, which depends on the recorded intensity distribution. The reconstruction wave behind the plate results in phase modulation.
Pulsed laser holography is a very suitable nonintrusive method to visualize disperse flows, when the particle size in the dispersed phase is bigger than 10 times the wavelength. Spray geometry, drop size and distribution, droplet velocities and trajectories can be measured and evaluated by means of digital image processing. The purpose of digital image processing is to reflect the main features of a picture more clearly and informatively than in the original and to judge the contents of an image quantitively by employing pattern recognition algorithms.
Chavez, A. and Mayinger, F. (1988) Single and double pulsed holography for characterization of sprays of refigerant Rl13 injected into its own saturated vapor, Experimental Heat Transfer Fluid Mechanics and Thermodynamics, 848-854.
Hariharan, P. (1984) Optical Holography, Cambridge University Press.
Mayinger, F. (1994) Optical Measurements–Techniques and Applications, Springer Verlag, Berlin, Heidelberg, New York.
Nishida, K., Nurakami, N., and Hiroyasu, H. (1978) Holographic measurements of evaporating diesel sprays at high pressure and temperature, JSME Int. Journal, 30-259, 107-115.