Abstract: An ion milling end point detection method for etching coated substrates is disclosed comprising the use of a light measurement of light passing through a monitor substrate (e.g., glass coated with the same material at about the same thickness as the coated substrate to be etched) during the milling process, comparing the light values with a reference value and determining the end point of the milling process based on said values. Coated and uncoated monitor substrates may also be employed together and the values compared to determine the end point. An apparatus for determining the end point of an ion milling process is also disclosed.