Non-contact systems and methods for determining distance between silicon melt and reflector in a crystal puller

A measurement system includes a target object at least partially visible through an opening in a crystal puller. The crystal puller has a silicon melt in a crucible and a reflector defining a central passage through which a crystal is pulled. A detector array captures light through the opening. The detector array is directed to a surface of the silicon melt in the crystal puller and to the target object, and a laser selectively transmits a coherent light beam through the opening to the target object to produce a reflection of the target object on the surface of the silicon melt. An optical modulator pulses the coherent light beams of the laser into discrete coherent light beams having a period, and a lock-in amplifier is connected to the detector array to filter discrete coherent light having the period from captured light.