TOM_ac is a ThermoOptical measurement system (TOM) for components or samples with a maximum size of about 30 mm which is operated in a controlled atmosphere. It has a telecentric beam path for precise dimensional measurement. Dimensional changes are detected optically with a resolution of 0.3 µm. The user defines which measurements he intends to follow via measuring windows. Numerous measurement windows can be set simultaneously. The test setup in a vacuum-tight, graphite heated stationary furnace is very flexible and allows the study of various phenomena. Thus wetting properties of melts can be measured as well as the sintering kinetics of green bodies or the infiltration of melts into porous preforms. Using additional modules numerous other measurements are available: creep rates, changes in weight, viscosity, etc. Measurements are defined via job files and carried out fully automatically - including the computer control of the furnace atmosphere.