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FAQ - Process parameter fluorescence for parts cleanliness inspection

What is fluorescence?

The fluorescence which is stimulated by an ultraviolet radiation is a special form of the luminescence. Thereby, electrons of fluorescent molecules reach a higher energy level by absorbing photons (as shown in the picture). Such a stimulated condition is energetically unfavorable and unstable. The electrons return immediately into their ground state where the absorbed energy is released again. At the same time an emission of the fluorescence light takes place. Due to the fact that a part of the energy is dissipated into heat, the emitted radiation has less energy and thus, a longer wavelength. Organic materials which are exposed to the UV light, often show a high self-fluorescence. Therefore, it is possible to detect contamination such as oil and grease films as well as surfactants on surfaces. Furthermore, it is possible to detect defined wax and corrosion protective layers as well.

What influence do roughness and material of the surface have?

Compared to measurement methods based on reflection, surface roughness and material have a smaller influence on the measurement result. Typical production-related variations in surface properties are negligible, completely different surfaces - e.g. blasted or polished, steel or brass - should be taken into account. Ideally, limit values should be defined specifically for the component and the subsequent processing.

The SITA CleanoSpector is based on the confocal measuring principle. This simplifies positioning and distance setting. The excitation light and the light emitted by fluorescence follow the same parallel radiation path. Reflection and scattering of the excitation light are filtered out by optical filters. The excited fluorescence light on the contaminated surface is emitted diffusely in all directions. A part of this light is detected by the SITA CleanoSpector and measured due to its intensity.

Why does the device display increasing cleanliness values when repeating the measurements at one point?

When fluorescent-active materials get excited to fluoresce, it comes to the so called photo bleaching effect. The lifetime of fluorescent molecules is limited due to irreversible photochemical processes. The photo bleaching effect is caused by a photo oxidation where excited molecules with molecular oxygen from the ground state react in a chemical destruction process. This chemical destruction process produces excited singlet oxygen (the electrons of the oxygen reach a higher energy level). Under this condition, the molecule can be oxidized irreversibly from the oxygen. The lifetime of excited fluorescent molecules depends on the intensity of the excited radiation and the temperature.
The SITA CleanoSpector was designed in a way so that, in general, the detecting duration of one measuring point does not provoke a significant photo bleaching effect. If you repeat the detection of the cleanliness at one point, it is possible that the device displays an increased cleanliness value.

 

 

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