A gas absorption spectroscopy system comprising: a sealed chamber having a reference gas with a known moisture concentration therein; a tunable laser diode disposed in the sealed chamber and configured to generate an illumination beam; a measurement cell coupled to the sealed chamber and configured for exposure to a gas sample such that illumination travelling through the measurement cell passes through the gas sample ; a process window disposed between the sealed chamber and the measurement cell , the process window being configured to receive the illumination beam from the tunable laser diode and reflect a first portion of illumination while allowing a second portion of illumination to pass into the measurement cell ; a reference detector disposed to receive the first portion of illumination and provide a reference detector signal; a measurement detector disposed to receive the second portion of illumination after the second portion of illumination has passed through the measurement cell and provide a measurement detector signal; and a controller coupled to the reference detector and the measurement detector , the controller being configured to provide a compensated moisture output based on the reference detector signal and the measurement detector signal; laser control circuitry coupling the tunable laser diode to the controller ; and wherein the controller is configured to control at least one operating parameter of the tunable laser diode and to correct at least one parameter for maintaining factory calibration of the instrument, based on the reference detector signal by itself, thereby using the known moisture concentration in the sealed chamber to provide calibration of the instrument. a sealed chamber having a reference gas with a known moisture concentration therein; a tunable laser diode disposed in the sealed chamber and configured to generate an illumination beam; a measurement cell coupled to the sealed chamber and configured for exposure to a gas sample such that illumination travelling through the measurement cell passes through the gas sample ; a process window disposed between the sealed chamber and the measurement cell , the process window being configured to receive the illumination beam from the tunable laser diode and reflect a first portion of illumination while allowing a second portion of illumination to pass into the measurement cell ; a reference detector disposed to receive the first portion of illumination and provide a reference detector signal; a measurement detector disposed to receive the second portion of illumination after the second portion of illumination has passed through the measurement cell and provide a measurement detector signal; and a controller coupled to the reference detector and the measurement detector , the controller being configured to provide a compensated moisture output based on the reference detector signal and the measurement detector signal; laser control circuitry coupling the tunable laser diode to the controller ; and wherein the controller is configured to control at least one operating parameter of the tunable laser diode and to correct at least one parameter for maintaining factory calibration of the instrument, based on the reference detector signal by itself, thereby using the known moisture concentration in the sealed chamber to provide calibration of the instrument. <EOS>