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The diffraction equations describing the wave propagation from a rough surface to an observation plane are derived. Through the statistical analysis of speckle patterns, an analytical expression relating the rms (root-mean-square) roughness to the speckle contrast is obtained. The limitations of the traditional speckle contrast method used for slightly-rough-surface roughness measurement are discussed. A specular intensity component method (SICM) is proposed as a complement to the speckle contrast method, which overcomes the disadvantages of the speckle contrast techniques. The measuring principle of the SICM is clarified in detail. An experiment is given to illustrate the whole calculation and calibration process, which shows that the SICM can be further developed for on-line measurement in the manufacturing automation industry.