Fluorescence is an optical phenomenon which involves light being emitted following the absorption of short-wave light. In order to utilize fluorescence for measurement, a light source such as a laser must be directed at the object, which then begins to glow at a wavelength which is higher than the light source. Optical filters separate the fluorescent light from the excitation light. Cameras or photomultipliers, for instance, are used for detection purposes.
Many materials and substances fluoresce. A basic distinction is made in this respect between substances with so-called intrinsic or auto-fluorescence and special fluorescent dyes of various wavelengths which are used as markings. As many functional coatings, but also impurities, have a characteristic auto-fluorescence, they can be detected by means of fluorescence analysis, especially on metal that does not fluoresce.
In addition to the pure detection of organic impurities on surfaces, an evaluation of the spectra is also very informative, as many of the substances to be examined in production can be identified on the basis of their characteristic spectra.
Fluorescence measurement technology in production
In production, the use of fluorescence measurement technology for the examination of surfaces is of great importance in terms of quality control. For instance, it must be ensured that surfaces are free of contamination before coating or painting; components must be inspected to ensure that they are clean prior to assembly and medical products must be inspected to ensure that they are sterile. Ultimately, the usability of a product frequently depends on compliance with the purity requirements in the production process.
Fraunhofer IPM develops high-sensitivity imaging fluorescence measurement systems for a wide range of applications. Some examples include the 100% inspection of oil-impregnated metal sheets and the purity control of metal components prior to welding, soldering and adhesive bonding. The systems can be used directly in the line thanks to the high measuring and evaluation speed. The systemic adaptations required for this purpose (mechanics, optics, electronics, software, interfaces) are customized.
Using laser scanners for large components and complex geometries
As test specimens are sometimes so large that they can no longer be examined with a camera-based fluorescence measuring system in the time available or as handling requires too much effort, an alternative method based on a laser scanner was recently developed. This involves scanning the component with a UV laser whose beam is deflected two-dimensionally via a scanner mirror. A sensitive photodiode records the previously spectrally filtered fluorescent light. By means of software developed specifically for this purpose, the image of the test specimen is assembled and overlaid with the fluorescence data.