A method includes receiving light, by a light coupling device and along an optical path, reflected back from a reflector mounted on a liner of a processing chamber. The method further includes detecting, by a spectrometer within the received light, a first spectrum representative of a deposited film layer on the reflector. The method further includes aligning, using an alignment device, the light coupling device in two dimensions with reference to the reflector along the optical path until maximization of the light received by the light coupling device.