A method and apparatus for obtaining at least one normalized baseline spectrum for a processing volume of a processing chamber; calculating a distribution value of the normalized baseline spectrum; obtaining a plurality of normalized cleaning process spectrums; calculating a distribution function of the plurality of normalized cleaning process spectrums; comparing the distribution value to the distribution function; and determining an end point by identifying when the distribution function approaches the distribution value. A method includes: initiating a cleaning process in a processing chamber, flowing a cleaning gas into a processing volume of the processing chamber; obtaining a normalized baseline spectrum; measuring a plurality of intensity spectrums; calculate a plurality of normalized cleaning process spectrums; comparing a distribution value of the normalized baseline spectrum to a distribution function of the plurality of normalized cleaning process spectrums; and determining an end point by identifying when the distribution function approaches the distribution value.