The speckle elongation effect of speckle pattern produced at the farfield diffraction plane by roughness surface under illumination of polychromatic light can be utilized to measure surface roughness. A roughness measurement of a shotblasted surface by means of polychromatic speckle autocorrelation is introduced. In consideration of the isotropic radial structure of polychromatic speckle pattern, an appropriate digital image processing algorithm is poposed. Several factors affecting the characteristics of the speckle elongation are discussed, which include the determination of characteristic length of local autocorrelation function, the size of local window for digital image processing, and the exposal saturation ratio of speckle image. By capturing and processing polychromatic farfield speckle images obtained from many points on each sample surface, optical roughness index values of sample surfaces with roughness parameter R_{a of 0.4, 0.8, 1.6 and 3.2 μm are calculated, respectively. It is shown that the optical roughness index can be used in scaling well the roughness degree of sample surfaces.}