This paper describes a new approach for the functional characterization of the surface topography of metallic products. The aim is to determine in a quantitative and unbiased way the roughness parameter which describes best the property or the function of the material. To achieve this, a new numerical statistical treatment had to be defined and a specific software had to be designed and developed. With these tools, more than 100 roughness parameters can be calculated. The most pertinent one for the description of the relationship between brightness and roughness of cold rolled steel sheets is the fractal dimension.