source 20 structure according to the invention including an optical temperature sensor and associated electronics in source 20 optical light pipe 21 is disposed through hermetically sealed passthrough 22 in flange 23 with the light collecting end 21a disposed near or in contact with the bottom of crucible 24 the external end 21b of light pipe 21 is optically connected through fiber optic connector 25 or other suitable optical means to optical detector and associated measurement and data display instrumentation 26 light pipe 21 must be constructed of material having suitable high temperature low vapor pressure inertness and non contaminating characteristics needed for use within an mbe chamber suitable materials for light pipe 21 may therefore include sapphire quartz various optical glasses or other material as would occur to the skilled artisan guided by these teachings in a system built in demonstration of the invention sapphire was used as light pipe 21 light pipe 21 may be hermetically sealed in passthrough 22 by brazing or other suitable vacuum sealing means placement of detector 26 remote of heater coil 27 and the mbe chamber 12 39 containing source 20 renders source 20 substantially immune to electrical noise associated with the mbe process in an alternative configuration passthrough 22 may be configured to accommodate thermocouple 28 passthroughs 29a 29b provide for connecting heater coil 27 to external power source 30 detector 26 may be a photoconductive or photovoltaic detector thermopile or other optical detector as would occur to the skilled artisan practicing the invention a.