The use of large numerical apertures in spectroscopic ellipsometer (SE) or reflectometer (SR) designs allows for smaller measurement targets, but it may introduce side effects like the propagation of high diffraction orders into the collection pupil. In the normal optical critical dimension (CD) measurement condition, the zeroth-order diffracted light is the only light illuminating the pupil. However, when the pitch of the measurement target is large enough, the high-order diffracted light can also enter the pupil and mix with the zeroth-order diffracted light. This contaminates the measurement and leads to inaccuracies in fitting the target's model to the data. <p> </p>In this paper, we propose a method that retains good measurement results even when the collected spectrum includes contamination due to high-order diffraction. In this approach, we treat the high-order diffracted light contamination as the primary source of error in the measurement. We modify the maximum likelihood estimation using one of three different additional weighting schemes that lead to optimal measurement results. In general, we have found that not all the wavelengths in the affected wavelength zone have the same contamination impact<p> </p>. The above concept was validated using a synthetic and an actual large pitch CD measurement, successfully demonstrating that the proposed method virtually eliminates the inaccuracy. These results show that this method effectively allows the user to use the full wavelength range of interest from 190-800nm, in spite of the fact that a significant fraction of this wavelength range includes the presence of higher diffraction orders. This method thus allows for accurate measurements of large pitch SRAM targets and enables yield improvement for N5 and newer logic devices.