This invention seeks to provide methods and apparatus that can improve the accuracy of measured parameter data used for processing workpieces. One aspect of the present invention includes methods of measuring process conditions with low distortion of the measurements caused by the measuring apparatus. The measurements include data for applications such as data for monitoring, controlling, and optimizing processes and process tools. Another aspect of the present invention includes apparatus for measuring substantially correct data for applications such as generating data for monitoring, controlling, and optimizing processes and process tools.