Atomic absorption optical-based flux monitor (OFM) has been implemented as a non-invasive in situ measurement tool for production MBE operation. For our setup, the optical probe beam samples the group III beam fluxes (Al, Ga, and In) by passing just in front of the substrate platen. The multi-species atomic absorption profile taken during MBE becomes a record for each epi-layer growth. For InGaAs critical layers in devices such as GaAs-based PHEMTs and InP-based HBTs, OFM is used to monitor the In mole fraction. Data sets of InxGa1-xAs composition measured by OFM are taken from two different sizes of multi-wafer reactors and compared to the results from X-ray diffraction measurements. The standard deviation of the difference between the two measurement techniques for x = 15%, 20%, and 53% InGaAs composition are all within 0.7% range and better in some cases. The longest data span covers a period of 5 months. (C) 2002 Elsevier Science B.V. All rights reserved.