laser having a wavelength selected to be for example 1 1 microns by first selecting an approximated expected energy source having a given wavelength and then selecting a predetermined thickness of the polysilicon layer in accordance with the approximated energy source the trial and error procedure required to select a combination of layer thickness and energy source wavelength which will provide accurate edge detection can be reduced considerably regarding the selection of an appropriate approximated energy source wavelength reference is made to a book entitled u201c a user 39 s guide to ellipsometry u201d by harland g tompkins the contents of which are hereby incorporated by reference in their entirety wherein an absorption coefficient a u03b1 is provided for different forms of silicon a polysilicon layer is indicated in the tompkins book to have an absorption coefficient u03b1 of approximately 2 u00d7 10 3 centimeters u2212 1 where photon energy of the polysilicon is below 1 2 ev when the photon energy is 2 0ev the absorption coefficient of polysilicon is four times greater i e 8 u00d7 10 3 cm u2212 1 having selected a predetermined thickness of the opaque layer relative to an approximate wavelength range of the energy source to be used a more exact wavelength to be used for the edge detection can be determined based on an absorption coefficient of the opaque layer for example the transmissivity of the layer.