A Fringe Projection System for measurement of Condensing Fluid Films in Reduced Gravity
Deepti Tulsiani
MS Thesis, Dept. of Electrical and Computer Engineering, Worcester Polytechnic Institute, November 22, 2005.
SUMMARY
The thesis describes the design of a fringe projection system to study the dynamics of condensation with potential application in a reduced gravity environment. The desired properties of the system are its simplicity, portability, compact size and robustness to vibration, making it suitable for use in spaceflights. The concept is that an optical system for imaging the condensation layer enables extraction of valuable data from the image because of the ability of the optical system to image the perturbations in the condensation films.
COMPLETE ABSTRACT
The thesis describes the design of a fringe projection system to study the dynamics of condensation with potential application in a reduced gravity environment. The desired properties of the system are its simplicity, portability, compact size and robustness to vibration, making it suitable for use in spaceflights. The concept is that an optical system for imaging the condensation layer enables extraction of valuable data from the image because of the ability of the optical system to image the perturbations in the condensation films. By acquiring a sequence of images of the deformed fringe pattern, the change in the surface topology can be observed over time, giving greater understanding of condensation dynamics in reduced gravity.
The principle behind the system is fringe projection, which is a simple way of optically mapping a surface in which fringes are projected onto the object under test in one direction and are viewed from another direction. The projected fringe pattern gets changed in accordance with the surface topology of the object. This change manifests itself in the form of angular and spatial frequency carrier shift in the Fourier domain. Thus the extraction of surface topology of the object becomes a matter of linear frequency domain filtering. The deformed fringe pattern is thus analyzed by a Fourier transform based fringe analysis technique followed by phase unwrapping.
The system hardware setup was developed, and a series of experiments were performed which validate the working of the system. A condensation chamber was developed and integrated with the system to allow study of condensation. The process of condensation was implemented by cooling the reference copper block by circulating ice cold water though it and by passing water vapor over it in a controlled manner.
The fringe projection system was implemented using laser light, which involved the generation of fringes with the use of a beam splitter and two reference mirrors. The results obtained were promising except for the presence of excessive amount of speckle noise in the images. As an alternative, fringe projection was also implemented using a white light source and a Ronchi ruling. It was found that the fringes obtained from the white light source were of low intensity and contrast and the camera was not sensitive enough to capture images good enough for further analysis. Suggestions have been provided for further improvement of these images as further work.
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