Laser calorimetry has been widely used to measure thin-film optical absorption in filmed substrates with total film-substrate system absorptance of 1 x 10-2 and below. This paper will discuss the advantages and disadvantages of the various types of calorimeters presently used to measure witness-size samples in the absorptance range of 1 x 10-2 to 1 x 10-5. Special emphasis will be given to single-layer films. Considered will be the implications of front versus back surface films, film thickness, film-substrate interface absorption, and laser coherence on the interpretation of the measured total absorptance. Throughout this paper, the main concern will be the errors present in calorimetric measurements.