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A laser-speckle method and a white-light image-correlation method are used for strain mapping. A schematic model of the correlation function of two speckle patterns is proposed for investigation of strain influence on displacement-measurement accuracy. A specific software has been developed to calculate by direct correlation the displacement values between two pictures with a grainy pattern at any point on an object's surface. Its efficiency is demonstrated in several tests. Moreover, theoretical results are checked through experimental measurements. The limitations and performances of both optical techniques are discussed.