The time resolved geometrical optics of the thermal lens

The thermal lens apparatus as usually designed for energy transfer and thermal conduction studies, has been used to measure the thermal conductivity coefficients of ethylene and ethyl chloride (2.2 × 10^?2 and 3.1 × 10^?2 J m^?1 s^?1 K^?1). Contrary to previous analyses, a relaxation model based on the IR laser beam diameter must be used in order to get order of magnitude agreement between these data and literature values. The importance of Fresnel diffraction in greatly limiting the accuracy of the above data is determined and it is shown that the traditional design of apparatus is thus wholly unsuitable for a simple qualitative model. A new apparatus design is suggested which is amenable to simple quantitative as well as qualitative interpretation.