A new method and optical system have been developed to measure the strain distribution in sheet metal bending. The method relies on a simple pattern, i.e., a single line of certain width, or an area having a boundary line, marked on the sheet before deformation. The line traverses a width of the sheet metal at an angle. The sheet metal is deformed and an equivalent two-dimensional image of the line after deformation is obtained. By comparing the two-dimensional equivalent of the line after deformation with that before deformation, the strain on the surface of the deformed region is determined. Experiments using sheet metal flanging and hemming were conducted to verify the effectiveness and determine the limitation of the proposed method.