shows a comparison of the two methods as a function of the time history of the entire control process in accordance with the present invention red green blue intensity u201c rgb intensity u201d in any given unit a u over time the characteristic curve of the method in accordance with the german examined specification de 102 56 909 b3 being additionally indicated the lower light gray curve in stage i shows the time history of the reflection intensity of the color green from the rgb signal and the upper thick black curve shows the time history of the reflection intensity of a laser spot color red in accordance with the present invention in comparison the middle thin black curve shows the characteristic signal curve from the method according to the german examined specification de 102 56 909 b3 in stage i a very similar result is discernible in the case of the measurements compare the upper two curves the lower curve for the method according to the present invention shows the general insensitivity of the laser signal during stage i given a relatively smooth substrate layer however the measurement in accordance with the german examined specification de 102 56 909 b3 is highly error prone since the presence of rough layers must be assumed in the process in this context one problem is that laser signals i e coherent radiation are more.