Thin film, in-situ measurement through transparent crystal and transparent substrate within processing chamber wall

A system comprising:

• a transparent crystal, wherein at least part of the transparent crystal is embedded within a wall and a liner of a processing chamber;
• a transparent thin film deposited on a surface of the transparent crystal that is exposed to an interior of the processing chamber;
• a spectrometer configured to:
• receive light reflected from a surface of the transparent thin film and a surface of a process film layer deposited on the transparent thin film;
• and
• detect a first spectrum within the received light that is representative of the process film layer;
• and
• a processing device coupled to the spectrometer and configured to:
• receive, from the spectrometer, the first spectrum;
• receive, from the spectrometer, a second spectrum when a source of the received light is off;
• generate a reflectometry signal based on a reference spectrum and a difference between the second spectrum and the first spectrum;
• and
• fit the reflectometry signal to a thin film optical model to determine information comprising one or more optical film property of the process film layer.