Corrosion of structures is a serious problem involving man and material safety. Over the years, though several methods of monitoring corrosion have been devised with some success, but there is a persistent need for devising non-destructive and in-situ techniques for monitoring corrosion in structures. Fiber optic techniques are capable of meeting these requirements, besides offering several other important advantages. Fiber optic corrosion sensors have thus become quite attractive and are currently being investigated to address the high costs associated with the existing structural maintenance procedures. Fiber optics based direct absorption spectroscopic techniques investigated by some groups for estimating corrosion have used single fiber elements for recording the signal reflected from specimen at different wavelengths. As the light coupling efficiency of the single fiber elements is relatively poor in comparison with that of fiber bundles and the signal available for processing is weak, the paper presents a simple and alternate technique based on the color matching principle of fiber optic colorimetry to detect corrosion induced color changes. It employs a thin Y- shaped fiber optic bundle which increases the quantity of light energy coupled from a whitelight source. The light reflected off the sample is made incident on a PIN photo- detector through a complementary filter. A series of such probes can be safety embedded and or bonded to structures at pre-determined locations. The experimental set up for this sensor was implemented and feasibility of in-situ corrosion detection in structures demonstrated. Measurement data was acquired for steel samples corroded both in concrete embedded and open ambience conditions and results analyzed.
Simplified techniques of recording rainbow and full-field cylindrical holograms by using flexible fiber optic light beam delivery systems are described. The method used in recording full-fieId cylindrical holograms is similar to that of the conventional technique, where the object is kept at the centre of a cylinder and the hologram recording film is wrapped around the object forming a cylindrical shape. There are, however, some practical problems in realising these holograms: (a) proper illumination of the object and (b) achievement of a balanced intensity ratio between the object and the reference beams. These problems have conveniently been overcome by employing flexible multiple fiber optic beam delivery systems, where both front and the back sides of the object have been illuminated separately using a practically non-overlapping fiber optic end geometry coupled with an independent reference beam. Rainbow holograms in both one-step and two-step recording geometries have also been successfully realised, where the advantage of flexibility offered by fiber optic beam delivery system is gainfully utilised for providing the desired vertically inclined (overhanging) reference beam. Methods for recording inbuilt reference beam and large size rainbow holograms by the use of optical fibers in the recording geometries are also being studied.
The present review highlights various white light holographic dis-'' play techniques which were investigated in the recent past at CSIO Chandigarh. New methods have been proposed for recording inbuilt reference beam rainbow holograms multiplexed restricted aperture reflection holograms fullview rainbow-reflection and composite holo grams and large s ize rainbow holograms. 1.
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