A crystal puller includes a heater, a silicon melt, a measurement system, and a reflector defining a central passage through which a crystal is pulled. The measurement system includes a laser to selectively transmit a coherent light beam to a target object to produce a reflection on a surface of the silicon melt. A detector array directed to the surface of the silicon melt captures light through an opening in the crystal puller. An optical modulator pulses the coherent light beam of the laser into discrete coherent light beams having a period. A control system controls the optical modulator to pulse the coherent light beams of the laser into discrete coherent light beams, controls the detector array to capture light through the opening, and controls the detector array to filter the discrete coherent light beams having the period from the captured light.