The present invention provides a method and apparatus for improving optical sensing of a plasma process through the use of a fiber optic sensor placed within a standard showerhead hole of a standard gas showerhead positioned in an upper electrode of a plasma system during the plasma processing of a substrate. A film property can be calculated based on the measured plasma emission from the surface of the substrate. The film property can be film deposition rate, refractive index, film thickness, etc. Based on the measured film property, the plasma processing of the substrate can be adjusted and/or terminated. In addition, a window is provided that is positioned in the upper electrode assembly for viewing the plasma emission through the standard showerhead hole.