Time resolved digital holographic interferometry through disturbed phase objects

Sammanfattning: Digital holographic interferometry is an optical measurement technique based on the work by Powell and Stetson in 1965. The basic principle of the method is that the whole light wave (both amplitude and phase) from an opaque surface or transparent object can be captured and stored using a single camera. By comparing the phase of the light waves captured at different times it is possible to detect very small surface deformations (for opaque objects) or refractive index changes (for transparent objects). The fact that the method is sensitive also for transparent object is a problem when measuring surface deformations that occur on the same timescale as the random fluctuations in the surrounding medium (most often air) since these effects will be added together in the measurement. In a controlled laboratory environment the levels of air disturbances can often be kept at reasonably low levels, but an interesting new application of the technique would be for process supervision in the manufacturing and process industry where the levels of disturbances are much higher. The purpose of this research has been to develop methods for separating the effects of object deformations and air disturbances from each other by digital processing of the measured data. A large part of the work has consisted of constructing experimental setups, developing algorithms and performing numerical simulations. Air disturbances tend to have fluctuations on a very wide range of time scales. To capture the fast fluctuations a high-speed holographic imaging system has been used throughout this work. The slow fluctuations are captured using long time sequences. This creates an enormous amount of data and handling and pre-processing this data has been one of the initial challenges.Air disturbances are very different depending upon how they are generated. Much of the work has therefore been to gain some understanding of different types of air disturbances such as convection flows surrounding hot objects, gas ejection from heated material (black liquor) and more controlled channel flows with fully developed turbulence. The type of imaging system used will also influence how a certain air disturbance will affect a measurement. The difference between telecentric and conventional imaging systems has been discussed and in connection with that a method of depth-resolved velocity measurements in channel flows has been devised.