UBC Theses and Dissertations
Modulation and enhancement of partial internal reflection at an opitical interface Liptak, Anne Helene
This thesis investigates the mechanisms involved in a reflective display based on the control of partial internal reflection hereafter abbreviated PIR. This work was motivated by the development of a reflective display based on the frustration of total internal reflection.1 Total internal reflection can be prevented by placing an absorbing medium in the evanescent wave region. The reflection is restored when the absorber is pulled out of this region. Heavily coloured pigments suspended in a perfluorinated hydrocarbon serve as absorbers; their motion in and out of the evanescent zone is accomplished using electrophoresis. When light does not undergo total internal reflection, it is not obvious that this method of changing the reflectivity would be effective. Surprisingly, a similar change in reflectance can be observed as the absorbing pigments are brought into and out of the region a few nanometres from the interface. The thesis studies of such changes in partial reflection. The reflectance changes at a single optical interface caused by the motion of pigments were studied for light undergoing total and partial reflections. The mechanism controlling reflectance for partially reflected light is optical interference. The interference system was modeled to emulate experimental conditions with results agreeing well with experiment. The reflectance from a system with multiple reflections was controlled using the pigment suspension. Light undergoes partial reflection on several surfaces, and the compound losses due to transmission significantly reduce the overall reflectance. To increase the overall reflectance, a layer of aluminium was deposited on selected interfaces. To demonstrate the potential use of the modulation of PIR, flashing retroreflective panels were constructed, with the possible application of highway signs in mind. Reasonably encouraging reflectance values and contrast ratios were observed. It is suggested that further development of this technology is warranted, with the goal of creating electronically addressed variable message signs.