The Fluorescence measurement method utilises the property of organic substances to fluoresce. UV light is focused onto the surface, causing the organic substances to fluoresce. As the thickness of the organic layer increases, so does the intensity of the fluorescence. The intensity is measured in RFU: Relative Fluorescence Unit.
Fluorescence is a special form of luminescence. It is excited by ultraviolet radiation. Electrons in fluorescent molecules absorb photons and reach a higher energy level. This excited state is energetically unstable. The electrons quickly return to the ground state, releasing the absorbed energy. This results in the emission of fluorescence light. Since part of the energy is converted into heat, the emitted radiation is less energetic and therefore has a longer Wavelength.
Measurement of Fluorescence Intensity
Organic substances such as oils, fats, or emulsions are excited to fluoresce by a UV light source. A photodiode measures the intensity of the radiation emitted by fluorescence at a specified wavelength in the blue light range. The intensity is measured in RFU (Relative Fluorescence Unit). The higher the RFU value, the higher the intensity of the fluorescence light.
Due to calibration with SITA fluorescence standards, the measured RFU values are comparable.
SITA fluorescence measuring devices operate on the confocal measurement principle. The excitation light and the light emitted by fluorescence follow the same parallel path. The fluorescence measured in the liquid or on surfaces is a cumulative parameter of all fluorescent substances present. If mixtures of substances are present, the measured fluorescence cannot determine which substances are present or their individual concentrations.
Fluorescence of Contaminants
Manufacturing aids such as oils, fats, coolants, and release agents are technical fluids. Their fluorescence is mainly due to unsaturated hydrocarbon structures and added additives with aromatic ring systems. Carboxylic acids, their esters, and aliphatic ketones also fluoresce.
Fluorescence measurement can detect the smallest amounts of fluorescent substances. Even fingerprints, consisting of less than 2% skin fats, can be detected. Typical detection ranges for manufacturing aids are 10 mg/m² … 10 g/m² (1 μg/cm² … 1 mg/cm²).
Exceptions are substances that do not fluoresce under UV excitation, such as silicone oils, saturated organic compounds, short-chain unsaturated hydrocarbons, metals, and their oxides. Whether contamination can be detected in the amount relevant to the process (bonding, welding, etc.) can be tested with a simple fluorescence test.
By adding fluorescent pigments or dyes as fluorescence markers, weakly or non-fluorescent substances can also be detected. Fluorescence markers are used to lower the detection limit and capture layer thicknesses below 10 mg/m² (1μg/cm²).
Fluorescence of Substrates
Metallic and ceramic surfaces do not fluoresce. Glass surfaces may fluoresce due to impurities 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.
Photobleaching
Photobleaching is a dynamic process in which fluorescent structures are photochemically destroyed by UV excitation. A vivid example is the discolouration of paper pages by sunlight, where the bright white is destroyed.
The intensity of the photobleaching effect is substance-dependent. The measurement time of SITA fluorescence measuring devices is so short that the scanning duration of a measurement point keeps the photobleaching effect to a minimum.
In practical measurements, the photobleaching effect only occurs when the same spot on the surface is repeatedly measured without moving the sensor. Such practice is not recommended. It is more sensible to measure at several different points and average the results to evaluate the surface.
For repeated measurements as part of a gauge capability study, we recommend using SITA fluorescence standards. They exclude cross-influences from photobleaching and positioning.