incoherent illumination in one example the illumination source 140 is plasma in addition a chuck or holding stage 130 may be provided for supporting or holding the apparatus 120 the apparatus 120 may include an optical element 122 one or more optical sensors 124 and a substrate 126 the data processing unit 150 may be located on the substrate 126 as part of the apparatus 120 specifically the optical element 122 may be made of a material that allows transmission of the wavelength of interest the optical element may have a known thickness variation by way of example and not by way of limitation the optical element 122 may be a dielectrical wedge as shown in fig3 a or a transparent windows as shown in fig4 a the one or more optical detectors 124 may include an array of sensors e g a linear array for sensing optical signals passing through the optical element 122 coupled to the apparatus 120 the data processing unit 150 such as a computer or processor includes an data acquisition application 152 that would process and analyze the data collected from the apparatus 120 to develop and obtain a spatial and time resolved contour map the data processing unit 150 may be coupled to the one or more optical detectors 124 the data processing unit 150 may further include a memory 154 to store data collected from the apparatus 120 such as reference waveforms ref and error term e m for further processing a computer 180 running an off line application may be coupled to the data processor unit 150 e g by a wireless link 190 to download a stored data session e m for processing the data it is noted that.