Introduction

Optical measurement techniques have been receiving an increasing attention during the last years. Although most of the techniques which are described in this book were developed decades ago, their application to the analysis of phenomena in heat- and mass-transfer becomes more and more common. This originates from several reasons. The most important one - common to all optical techniques — is that they work non-intrusively and therefore do not influence the investigated process. Even highly transient and/or sensitive phenomena can be analyzed with a high spatial and temporal resolution. Furthermore, in most cases it is possible to visualize directly (on-line) the processes in the test section. This yields the very high accuracies of the measurements and new insights in heat- and mass-transfer phenomena. Both are required to analyze and to improve processes in chemical and power engineering. Secondly, the rapid improvements in computer technology allow to process and to store the huge amount of data within a reasonable time that is obtained by applying optical measuring techniques. High development rates can also be found in the field of light detector, laser, and semiconductor technology.