In a simplified type of portable system, a tripod-mounted laser and spatial filter
project light directly through a holographic plate fastened to the test object. A reflection hologram is formed by
interference between the light traveling through the plate and the light reflected back to the plate from the object. In this
system, the reference beam and the object beam strike the emulsion from opposite sides of the plate, resulting in the
reconstruction of a virtual image produced by reflection. This configuration differs from the holographic systems
described earlier, in which the two recording beams strike the plate from the same side and, during reconstruction, the
virtual image is produced by transmission.
An important consideration in designing reflection holographic systems is that reflection holograms are more sensitive than transmission holograms to photographic emulsion shrinkage, which may take place during the development and drying processes. This shrinkage causes the image formed during reconstruction to be produced at a slightly shorter wavelength (a hologram recorded with red laser light will reconstruct best in yellow or green light). Unless the reconstructing light matches this shorter wavelength, the image will be quite faint. Therefore, white light, which contains all the required wavelengths for efficient image production, is often used as the reconstructing reference beam instead of laser light.
Because, as described above, the film plate also serves as a beam splitter and can be mounted to the test object itself, reflection holographic systems can be quite insensitive to object vibration. The major critical stability requirement is the relationship between the object and the holographic plate fastened to it. If the emulsion shrinkage is fairly uniform and the holographic plate is in close proximity to the test object, the image formed will be bright and clear. The plate must be dried carefully, however, to avoid variations in emulsion thickness, which would cause variation in the color of the image. Because the diffraction of light changes with color, variations in color will cause smearing of the image and loss of resolution. The greater the distance from the object to the plate, the greater the smearing.
An important consideration in designing reflection holographic systems is that reflection holograms are more sensitive than transmission holograms to photographic emulsion shrinkage, which may take place during the development and drying processes. This shrinkage causes the image formed during reconstruction to be produced at a slightly shorter wavelength (a hologram recorded with red laser light will reconstruct best in yellow or green light). Unless the reconstructing light matches this shorter wavelength, the image will be quite faint. Therefore, white light, which contains all the required wavelengths for efficient image production, is often used as the reconstructing reference beam instead of laser light.
Because, as described above, the film plate also serves as a beam splitter and can be mounted to the test object itself, reflection holographic systems can be quite insensitive to object vibration. The major critical stability requirement is the relationship between the object and the holographic plate fastened to it. If the emulsion shrinkage is fairly uniform and the holographic plate is in close proximity to the test object, the image formed will be bright and clear. The plate must be dried carefully, however, to avoid variations in emulsion thickness, which would cause variation in the color of the image. Because the diffraction of light changes with color, variations in color will cause smearing of the image and loss of resolution. The greater the distance from the object to the plate, the greater the smearing.
Portable systems can be used in the following cases:
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Field inspection
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When the size or configuration of a test object is such that it is more practical to attach the holographic
system to the object than vice versa
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When the test object is in an environment or in a structure required as part of the experiment
Portable holographic systems are usually designed to inspect a specific part or a range of small parts. The questions that establish criteria for designing a portable system are the following:
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How is the system to be powered?
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How is stability between the system and the test object to be maintained both during and between
exposures?
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How can critical adjustments be made or eliminated?
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How is the photographic plate to be handled and processed?
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How can the holographic components and the stressing fixture be designed into a workable system
within the definition of portable?