Calibration accuracy of laser calorimetry for common crystal geometries

Abstract

An established method for precise determination of optical absorption is the so called laser calorimetry. According to ISO 11551 1 laser calorimetry is preferred to other photothermal test methods, because of its capability to deliver absolute calibration. Many optical materials have low heat conductivity, which can affect the calibration significantly. The timeand spatial dependent temperature profile in a sample of materials with low heat conductivity requires accurate temperature measurement strategies to determine material-independent and absolutely calibrated absorption values. For thin cylindrical samples, ISO 11551 provides a strategy to compensate heat conductivity effects. The optimal temperature sensor position, where accordingly calibrated measurement results 2 can be obtained, is simply based on the symmetric sample geometry. For thick geometries an additional temperature distribution along propagation direction of the heating beam must be considered. The current version of ISO 11551 does not provide a sophisticated solution for this problem, because the heating scheme of a sample is usually unknown. Therefore, a reliable calibration procedure can only be applied to samples of well-known absorption properties of surfaces and bulk material. Utilizing such kind of specifically prepared reference samples in combination with Finite Element Method (FEM) calculations, a general measurement and data evaluation concept based on laser calorimetry is presented, that allows deriving absolutely calibrated absorption measurement results for rectangular sample geometries.