Abstract: An apparatus and method for monitoring precursor flux is disclosed herein. The apparatus comprises an optical cell configured for electromagnetic radiation spectroscopy and has a precursor reservoir or deposition chamber configured to provide a flow of a vapor deposition precursor therethrough, a first inner window sealing a first optical opening in the precursor reservoir or deposition chamber, a first outer window in optical communication with the first inner window, a first vacuum chamber disposed between the first inner window and the first outer window, a second inner window sealing a second optical opening in the precursor reservoir or deposition chamber, a second outer window in optical communication with the second inner window, a second vacuum chamber disposed between the second inner window and the second outer window. Each window being disposed to be in optical communication with one another, a electromagnetic radiation or light source, and an optical detector.