A method and apparatus for cleaning a CVD chamber including optoelectronic detection of the completion or endpoint of the cleaning procedure once a ratio of emission lines reaches a threshold value. The method comprises the steps of: providing a plasma of a cleaning gas into the chamber and creating a plasma from the cleaning gas. The intensity of emission lines of the cleaning gas and of at least one background gas in the chamber are monitored. A ratio of the intensity of the cleaning gas emission line to the intensity of the background gas emission line is determined and monitored as a function of time. The determined ratio is compared to a preset threshold calibration value. The flow of gas is controlled based on the comparing step. The apparatus includes a cleaning gas supply with a valved inlet providing an entrance to the interior of the chamber for passing cleaning gas to the interior of the chamber. A detector having an optical input is disposed for sensing the electromagnetic radiation. The detector has a first channel for detecting a relative intensity of an emission line corresponding to the cleaning gas and a second channel for detecting a relative intensity of the emission line corresponding to the background gases. Software or circuitry is employed to determine a normalized signal using a signal from the first channel and a signal from the second channel. The value of the normalized signal is substantially invariant with respect to simultaneous corresponding changes in the intensity of the signal measured by the first channel and the intensity of the signal measured by the second channel.