Laser calorimetric absorptance testing of samples with varying geometry

During the 1990s the International Standard for absorptance testing of optical coatings was developed. Based on the method of laser calorimetry and after years of theoretical and empirical work, ISO 115511 was revised and published in its current version in 2003. Laser calorimetry is based on the measurement and evaluation of the temperature increase caused by absorption in the sample exposed to laser radiation. In dependence on the thermal diffusivity of the sample, a temperature distribution develops in the irradiated sample. Therefore, finding a correlation of temperature increase and absorption becomes a complex task. This challenge was met by keeping the sample geometry to a standard size and simulating the thermal distribution for a number of optical materials. By this, LZH developed a calorimetric test set-up that offers fully calibrated absorptance values for sample diameters of 25 mm (or 1 inch) with a total error of below 13 % and a relative measurement error of below 5 %. However, this technique is limited to the mentioned sample geometry. This work presents a new approach to adjust the measurement configuration to numerous sample sizes of standard circular laser components. Finite element analysis and experimental verification is presented for exemplary values of the samples diameters. Based on a new sample mount concept, this procedure allows utilizing all flexibility in test wavelength and angle of incidence, combined with the sensitivity level sufficient for current optical materials.