Abstract
A method and an apparatus are described which allow a nondestructive indication of the progress of the drifting operation. While drifting, pulsed light is shone onto the face opposite the lithium diffused layer of the device. The photosensitivity of the detector is used to obtain a signal whose amplitude is inversely proportional to the window thickness, i.e., the thickness of the undrifted region. A charge sensitive amplifier and a peak rectifier convert the pulses into a dc signal which is used for window thickness indication.

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