System for measuring the velocity field in a sheet of water during the passage of a tyre casing, the said tyre casing being in a condition of rolling along a trajectory, comprising an apex having a tread in a filled and inflated state, in rotation about a natural axis of rotation, and having a geometrical indentation with a characteristic dimension L of between 0.1 and 200 millimetres on its external surface level with the tread, comprising: - a sheet of water with a thickness he of between 1 and 8 millimetres, comprising particles that are inert in water, the density of which is less than 30% of the density of water and the characteristic dimension of which is D; - a transparent window located under the sheet of water, having a first surface in contact with the sheet of water, which has a characteristic dimension H1 greater than 200 millimetres and the average plane surface of which has a normal N, and a second surface, the average plane surface of which has a normal N' making an angle less than α degrees with respect to the normal N of the first surface, α being less than 60 degrees, preferably 30 degrees, very preferably 15 degrees; - an optical device comprising a first part having a light source, capable of corresponding to the starting point of an optical path of the light, and a sheet generator capable of generating a two-dimensional slice of light, the light spectrum of which extends over a chromatic band C1C2; and a second part comprising a first plane surface arranged to receive the two-dimensional slice of light, and a second plane surface in contact with the sheet of water and arranged to emit a plane sheet of light capable of covering at least a part, located downstream of the trajectory, of the first surface of the window; - an imaging device located under the window, comprising a zone photosensitive to a chromatic band C3C4, capable of corresponding to the arrival point of the optical path of the light, defining an observation zone level with the sheet of water making it possible to take at least two pictures of the observation zone, which are spaced apart by a period T; - a light exposure device, located on the optical path of the light, controlling an exposure time TE of the photosensitive zone of the imaging device; - a synchronisation device comprising a means for detecting the passage of the tyre on the trajectory, which is located upstream of the trajectory with respect to the window, a calculation means capable of evaluating the instant of the arrival of the tyre casing in the observation zone, and a communication means controlling the optical device, the exposure device and the imaging device; characterised in that the particles are photoluminescent, having an activation spectrum contained in the chromatic band C1C2 and an emission spectrum in the chromatic band C3C4, the said chromatic bands not overlapping, in that the characteristic dimension D of the particles is between 1 micrometre and 100 micrometres, preferably from 10 to 50 micrometres, in that the concentration of the particles in the water is between 2.5 and 250 grams/m 3 and in that the optical device generates a plane sheet of light with a thickness ep less than or equal to the height he. - a sheet of water with a thickness he of between 1 and 8 millimetres, comprising particles that are inert in water, the density of which is less than 30% of the density of water and the characteristic dimension of which is D; - a transparent window located under the sheet of water, having a first surface in contact with the sheet of water, which has a characteristic dimension H1 greater than 200 millimetres and the average plane surface of which has a normal N, and a second surface, the average plane surface of which has a normal N' making an angle less than α degrees with respect to the normal N of the first surface, α being less than 60 degrees, preferably 30 degrees, very preferably 15 degrees; - an optical device comprising a first part having a light source, capable of corresponding to the starting point of an optical path of the light, and a sheet generator capable of generating a two-dimensional slice of light, the light spectrum of which extends over a chromatic band C1C2; and a second part comprising a first plane surface arranged to receive the two-dimensional slice of light, and a second plane surface in contact with the sheet of water and arranged to emit a plane sheet of light capable of covering at least a part, located downstream of the trajectory, of the first surface of the window; - an imaging device located under the window, comprising a zone photosensitive to a chromatic band C3C4, capable of corresponding to the arrival point of the optical path of the light, defining an observation zone level with the sheet of water making it possible to take at least two pictures of the observation zone, which are spaced apart by a period T; - a light exposure device, located on the optical path of the light, controlling an exposure time TE of the photosensitive zone of the imaging device; - a synchronisation device comprising a means for detecting the passage of the tyre on the trajectory, which is located upstream of the trajectory with respect to the window, a calculation means capable of evaluating the instant of the arrival of the tyre casing in the observation zone, and a communication means controlling the optical device, the exposure device and the imaging device;