1. A method for analysing in real-time in a paper machine or board machine the surface structure of a web produced in a paper machine or a board machine, which method comprises: that an imaging system is arranged above or below the web and is directed towards a pre-determined area of the web, that an illumination system is arranged above or below the web in order to illuminate the area from a pre-determined direction with obliquely incident light, and that an image analysis system is arranged in association with the imaging system, wherein the method further comprises: an image capture step in which the imaging system is caused to take several digital images of the web under the said oblique incident illumination and during a pre-determined period, when the web passes in front of the imaging system, which images form an image sequence that images a series of surface sections along a band in the web, and an evaluation step that is carried out by the image analysis system and which comprises an image analysis step, that comprises a first analysis operation in which the variance of the pixel values in each pixel row in each image in the image sequence is determined within a pre-determined wavelength band, and a second analysis operation in which the mean value of the variances of all pixel rows of all images in the image sequence is calculated.