A method of controlling process variables, for a fiberizing assembly including a rotary fiberizing disk in the manufacture of fibers from a high temperature, molten, clear or translucent, thermoplastic, fiberizable material, utilizes an optical sensor assembly. The optical sensor assembly includes a water-cooled optical fiber sensor probe which, in effect, only gathers light emitted from the external sidewall surface of the rotary fiberizing disk. The light is conducted from the probe to an electronic unit that converts the light energy into a temperature value. This temperature value is used to monitor the process and to make any changes in process variables, such as but not limited to heat input to the fiberizing disk, rate of rotation of the fiberizing disk, burner air/fuel ratio, required to produce fibers having desired fiber properties.