In this book, an analytical method for on-line determination of absorptance and emittance of materials is discussed. It uses data from sample heating and cooling histories obtained from an absorption calorimeter. The calorimeter is a box chamber having a window, sample holder, tungsten halogen lamp and a microcomputer with instrument interface to control the opening and closing of the shutter window. It is automated with a Z80 microprocessor system that acquires data from the sample through thermocouples, filters, amplifiers and analogue-to-digital converters for temporary storage in an on-board RAM. Analytical solutions of heat balance equations of the calorimeter are computed for the data using numerical methods. Newton''s iteration procedure for systems of nonlinear equations is used with the assumption that the thermophysical properties are constant in the temperature range considered. The IBM PS/2 model 50z microcomputer has been employed to process the data and display the results. The book can be used by students, researchers and workers in various fields to model low temperature thermal processes like in solar Photothermal applications.