Lack of precision and accuracy of in-process critical dimension (CD) measurements of linewidth continues to be a serious problem at micrometer and submicrometer dimensions. Even with highly repeatable optical linewidth measurement systems, variable "offsets" or errors have been shown to occur with changes in process variables such as thickness of the patterned layer and sublayers and changes in the indices of refraction of the materials. All of these variations result in a change in the optical phase difference that occurs on reflection at the line edge and, therefore, result in changes in the structure of the optical image. Although an accurate coherent optical edge-detection method has been developed, it requires accurate knowledge of this phase difference, which is not always possible in CD measurements. This paper proposes a new dual-threshold method for edge detection and focusing, based on image theory, which can be adapted to most optical microscope based measurement systems. It does not require knowledge of the phase discontinuity at the line edge. The accuracy of this criterion is compared to two more widely used criteria, (1) the minimum and (2) 50% threshold, and it is concluded that, when the phase difference is unknown and varies with normal processing, the new dual-threshold method is the superior method.