A method for inspecting a substrate for defects, including: (a) obtaining an inspected pixel and a reference pixel; (b) calculating an inspected value and a reference value, the inspected value representative of the inspected pixel and the reference value representative of the reference pixel; (c) selecting a threshold in response to a selected value out of the inspected value and the reference value; and (d) determining a relationship between the selected threshold, the reference value and the inspected value to indicate a presence of a defect.