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Process parameter fluorescence

Fluorescence is a special form of luminescence. Fluorescence is excited by radiation in an ultraviolet range. When the electrons of fluorescing molecules absorb photons, they attain a higher level of energy. This excited state is then energetically instable. The electrons quickly return to their initial condition, causing the previously absorbed energy to be released once again. This release of energy results in the emission of fluorescent light. Because some of the energy is transformed into heat, the emitted radiation has less energy, and therefore a longer wavelength.

 

 

Fluorescence of contamination

The fluorescence of production aids refers to aromatic ring systems contained as additives as well as the unsaturated structures of oils and greases. Carboxylic acids and their esters as well as aliphatic ketones fluoresce, too. The fluorescence measurement detects slightest traces of fluores-cent substances. Even finger prints that consist of less than 2 % fat from the skin are detectable. Production aids tools can be detected within the range of 10 mg/m² – 10 g/m² (1 μg/cm² - 1 mg/cm²).

Exceptions are substances that do not fluoresce when excited with UV light. Those include some silicon oils, saturated organic compounds as well as short unsaturated hydrocarbons, metals and their oxides. Processing aids such as oils, greases, cooling lubricants and release agents are technical liquids that also contain numerous additives, which can likewise contribute to fluores-cence. Whether a contamination can be verified in the amount that is relevant for the process (bonding, welding etc.), can be assessed by a simple fluorescence test. Substances that do not fluoresce can be detected by adding fluorescent pigments or colouring agents as fluorescence marker. Using a fluorescence marker can detect production aids in quantities even smaller than 10 mg/m² (1 μg/cm²).

Fluorescence of substrates

Metal and ceramic surfaces do not fluoresce. In the case of glass surfaces, fluorescence is possible due to contamination in the amorphous structure. Other materials such as paper, textiles and plastics tend to fluoresce more strongly due to their complex structure of organic molecules.

Photo bleaching

Photo bleaching is a dynamic process in which the contamination is photo-chemically destroyed by the UV excitation, which reduces its ability to fluoresce. The intensity of the photo bleaching effect depends on the material. The SITA CleanoSpector is designed so that the scanning period of a measuring point usually does not cause a significant photo bleaching effect.

In practice, the photo bleaching effect is of secondary importance in the cleanliness inspection of parts. A multiple measurement of the same spot is not practical. Instead, one should measure on several different spots in order to obtain an average, which then makes it possible to assess the cleanliness of the part. We recommend conducting a measuring system analysis with multiple measurements using the SITA Calibration Standards to eliminate adverse effects from photo bleaching and positioning.

 

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