Influence of Defocus on Area Measurements

Finding the area of an object from image is an essential problem in many applications of machine vision, but there is usually error in area measurement because of defocus. In this work, influence of defocus on area measurement has been researched experimentally and theoretically. The results of experiment show that the relative error is 31.3% when the object plane deviates 10mm from conjugated plane of image plane. A novel method has been developed to amend this error from defocus, according to the relation between maximal image gradient and defocusing amount. Firstly, the relation between maximal image gradient and defocusing amount is calibrated. Secondly, the maximal gradient of image is calculated. Thirdly, the defocusing amount is deduced. Finally, the area of defocused image is amended according to the defocusing amount. After amendment, the relative error of image area is less than 3.8%, when the defocusing amount is within 10mm.