Reflections of a thermal wave imager: Two decades of research in photoacoustics and photothermal phenomena

From time to time during the course of the first ten international conferen ces, my colleagues have presented overviews of trends in the field and sub- fields of photoacoustics and photothermal phenomena. For example, during th e meeting in Baltimore, Boccara and Fournier discussed the "Why, When and H ow?" of photothermal investigations. Spectroscopy ("Where do they Win?") wa s reviewed by Miller(2) at Heidelberg, and trace gas monitoring application s were reviewed by Sigrist(3) at the same meeting. At the most recent previ ous meeting in Rome, Power(4) gave a survey of issues in inverse problem th eory, Balageas et al reviewed industrial applications of IR thermography, a nd Mills(5) provided a review of biological and medical applications. Becau se of the breadth of the field, the following discussion will be restricted to the topics of thermal wave imaging and determinations of thermal diffus ivities of solids.