A laser calorimeter, which is able to measure absorption as los as 1 - 10-6 of incident laser power even with the use of an argon laser of less than 1 W at 514.5 nm, was developed. The required stability of the temperature background is achieved by a special design of the measurement system minimizing heat flow. Evaluation of absorption is realized by numerical simulations taking into account all relevant processes of heat generation and dissipation. Furthermore, absorption is determined according to ISO/DIS 11 551. Interference influences from the substrate are taken into account.