Method and apparatus for detection of defects in and transparent objects
It is possible to clearly identify poorly contrasted defects independently of a predetermined object structure which was previously impossible with the use of dark field illumination which is known per se.
Abstract
The invention relates to the detection of defects in and on transparent objects, such as lenses of vehicle headlights, which are optionally structured themselves. Typical defects are i.e. trapped gas bubbles or foreign particles, bent cup-shaped shoulder segments, differences in the thickness of the material and surface deviations that alter the index of refraction. The aim of the invention is to provide as simple a means as possible of identifying defects with poor contrast and bearly visible defects in particular, and of differentiating between the defects and predetermined object structures. The inventive method should be suitable for carrying out the visual check of the test piece as well as an objective evaluation with an image processing system, optionally while the objects to be examined are being produced; so that objects with an unacceptable defect can be selected. According to the method, the test object (1) is transilluminated with sufficiently coherent radiation from a point source (3) using at least one optical imaging element (4) and a camera objective (5) is focused on one or preferably various planes of the object (1). For each selected objective focussing operation (a, b, c), each camera image is evaluated according to the existing half-tones. In this way, all the types of defects can be detected. In particular, it is possible to clearly identify poorly contrasted defects independently of a predetermined object structure. This was previously impossible with the use of dark field illumination which is known per se.