1. A method for detection of drops appearing in a digital image acquired through a transparent wall by means of an image acquisition system, the method comprising the following steps: b) a gradient image is established from said digital image, by assigning to each pixel the value of the gradient at the position of the pixel; c) said gradient image is filtered to obtain a filtered image by assigning to each pixel a value obtained by applying a rank filter to a neighbourhood of the pixel; and d) a mask is established representing the position of the detected drops in said digital image, in which mask the pixels for which the difference between the value of the pixel in the filtered image and the value of the pixel in the gradient image exceeds a predetermined value are activated.