A detection method for a spot image based thin line detection is disclosed. The method includes a step for constructing a band limited spot image from a transmitted and reflected optical image of the mask. The spot image is calibrated to reduce noise introduced by the one or more inspection systems. Based on the band limited spot image, a non-printable feature map is generated for the non-printable features and a printable feature map is generated for the printable features. One or more test images of the mask are analyzed to detect defects on such mask. A sensitivity level of defect detection is reduced in areas of the one or more test images defined by the non-printable feature map, as compared with areas of the one or more test images that are not defined by the non-printable features map