The present invention relates to a device for detecting scratches, in particular on a transparent material surface, such as glass, with an illumination device and a reception device which detect a scanning line on the surface of the material and which are displaceable relative to the material surface, and a related method.
Rough scratches on glass are produced by material being removed with a sharp object. When the material is removed, the glass shatters into irregular, small particles from the smooth surface. This results in irregular structures, which scatter light in all directions, regardless of the direction out of which the incident light comes. Rough scratches of this type are referred to as “rub”, “abrasion”, “scuffing”, “ice-like scratches”, and the like. In addition, there are other scratches, which are generally finer, which are produced in the glass surface via plastic flowing under the pressure of a very sharp object. These finer scratches have smooth flanks, which reflect light only in a certain direction, and which are classified as “groove”, “flute”, “hairline”, “ridge”, or the like. When you look at the glass from a defined direction, the scratch must be illuminated from a certain direction for it to be seen. Since the scratches can have any possible orientation in the glass, a glass surface to be inspected for scratches must be illuminated from all sides, so that all of the scratches can be identified. Since nearly all scratches transition back into themselves when rotated 180°, it is sufficient to illuminate 180° around the glass surface. It is not necessary to illuminate 360° around the glass surface.
It is difficult to attain illumination of this type with curved glass surfaces in particular. A typical example of an application of this is in the detection of scratches in windshields. Since windshields are generally curved spherically, it is necessary to illuminate at least a certain region uniformly from all sides, the region being located around the scanning line of a reception device, e.g., a digital camera, which advantageously extends from top to bottom. In addition, it is not possible to move all regions of a spherically curved windshield past the camera at the same distance. The illumination must therefore have the required properties of diffuse illumination across a certain depth range. In addition, the reception camera must have sufficient depth resolution in order to deliver sharp images of the scratches. The illumination must therefore be adequately bright. To produce contrast that is distinct enough for scratches to be detected, the camera must generally look into a dark light trap. Light enters the camera only when a scratch is illuminated. It must therefore be ensured that neither light directly from the illumination device nor light reflected by the intact material surface enters the camera. This cannot be accomplished using previous sampling and scanning devices, particularly when they are used with curved, e.g., spherically curved, windshields.
Publication U.S. Pat. No. 5,598,262 makes known a method for inspecting transparent material, with which a camera looks through the transparent material and onto a background which is illuminated uniformly with neon tubes, to detect flaws enclosed in the material. To also detect dust particles lying on the surface of the material, the surface of the material facing the camera is illuminated with a lamp which, due to its design, generates parallel light beams, which therefore generates parallel light transversely to the scanning line and diffuse light in the direction of the scanning line.
A method for detecting flaws in a transparent material is described in publication U.S. Pat. No. 3,814,946, with which every side of the surface of the transparent material is illuminated with parallel light transversely to the scanning line and diffuse light in the direction of the scanning line. The surface of the material is observed with a camera; the illumination device located on the side with the camera detects flaws in the surface, and the illumination device located on the side opposite to the side with the camera is used to detect flaws enclosed in the transparent material.
Publication U.S. Pat. No. 5,746,176 describes a design for generating a light beam using several, adjacently positioned LEDs, which generates light which is parallel in one spacial direction and is diffuse in a spacial direction orthogonal thereto.