Method and apparatus for measuring the position of a phase interface during crystal growth

In the method and apparatus for measuring the position of the phase interface during growth of a crystal from a melt in a crystal growth container according to the VGF method an incident optical signal is propagated to the phase interface between the melt and the crystal through a window ( 16 ) in the container ( 10 ) and a received optical signal reflected from the phase interface ( 14 ) is measured to determine the position of the phase interface. The position of the phase interface is established from the reflected signal by triangulation with a confocal optic system, by interferometric balancing or by transit time of the optical signal. The window ( 16 ) is preferably mounted in a preferably tilted orientation at the end of a tube ( 15 ), which is immersed in the melt ( 12 ).