The setup used for measuring the deposition and etch rates is presented in the figure below:

It usually consists of a CW He-Ne or diode laser with power of a few mW, focusing lenses and a photodiode with appropriate wavelength response. The laser beam is focused on the specific point of interest at the surface of the growing film or the material to be treated. The angle of incidence is our case is usually in the range 77–82^ο  and depends on the specific reactor configuration, the distance between the two electrodes and the distance to the optical windows. The reflected beam is collected from the opposite  side of the reactor and it is focused on the photodiode. The signal is then recorded, analyzed and stored in a personal computer using a simple A/D card.