Abstract No. 32 Ultraviolet Fluorescence using a deep UV LED source and multiple optical filters new possibilities for advanced on-line surface inspection Arne Bengtson and Tania Irebo Swerea KIMAB AB, Isafjordsgatan 28A, SE-164 40 Kista, Sweden Ultraviolet Florescence (UVF) is used as a sensitive technique to monitor oil and other organic substances on metal surfaces, examples of applications are given below: Control of the application of oil for corrosion protection Checking contamination of tramp oils in cold rolling mills Monitoring surface cleanliness prior to surface treatment and coating processes In this work, a pulsed low power deep UV LED (280 nm) is used as the excitation source, and a highly sensitive photomultiplier (PMT) as detector. An assembly of up to six different optical filters allows obtaining spectral information. The purpose of this is to be able to combine high sensitivity with the capability to identify different fluorescing compounds, i.e. obtain analytical information. In addition, the software is synchronised to an X-Y stage for mapping of flat samples. Performance test have demonstrated detection limits in the few mg/m 2 range, which rivals commercial systems with more expensive laser excitation sources. This makes the system a relatively low-cost alternative for fast surface cleanliness monitoring, where it is only necessary to record the signal in one spectral window. Employing the assembly of optical filters, it has been shown that it is possible to identify different oils and even mixtures of oils, based on the relative intensities of the different spectral windows. Examples of how this can be accomplished by classification algorithms will be shown. Rather than employing conventional quantitative analysis, the system is trained on a limited number of pure oils and oil mixtures depending on the application. For off-line analysis the 2 D mapping by means of an X-Y translation stage is a useful extension of the system, for trouble shooting and quality control. The lateral resolution is approximately one mm. In conclusion it has been shown that a flexible, modular UVF system based on a low power UV LED excitation source is a powerful tool for both on-line and offline surface inspection.
Ultraviolet Fluorescence using a deep UV LED and multiple optical filters new possibilities for advanced on-line surface inspection Arne Bengtson and Tania Irebo Swerea KIMAB AB, Kista, Sweden Basic principles of Ultraviolet Fluorescence (UVF) Ground state molecules are excited to a higher electronic state by absorption of UV photons. The excited molecules revert to the ground state by optical emission (fluorescence). The fluorescence has a broad spectrum due to the large number of vibrational rotational levels in both electronic states.
The first KIMAB UVF system installed at a SSAB slitting line in 1998 Characteristics of UVF for surface inspection Detection of oils and other organic compounds Contactless measurement Robust LED based sensors Optical fiber probes for narrow spaces and complex geometries Easily mounted on robots for automated measurements Excitation Fluorescence
Diode laser or UV LED? 2015 Part 1 Diode laser UV LED This Excitation Emission Matrix (EEM) for a typical oil shows a 10 times higher fluorescence yield at 280 nm compared with 355 nm, The lower yield of the laser is to a large extent compensated by a higher power than a LED, but a laser is considerably more expensive and does not give better detection limits. Diode laser or UV LED? Part 2 Spectra recorded with a laser Spectrum recorded wih a UV LED - A certain drawback with the LED is a rather large bandwidth, with tails extending far from the center wavelength. - This requires to use optical filtering about 50 nm above the LED center wavelength in order to block reflected light. - In spite of this drawback, filtering the LED light is almost as effective as filtering light.cetas the laser
The new multispectral UVF system developed at Swerea KIMAB Pulsed 280 nm UV LED source Photomultiplier detector for highest sensitivity New type of probe with concentric fiber bundle to pick up the fluorescence Motorised filter wheel for flexible use of up to 6 optical filters Programmable X-Y table for 2-D mapping User-friendly software for easy control of the complete system and automated measurement. Key components of the UVF system Probe with concentric arrangement of optical fibers UV LED 280 nm Motorised filter wheel GUI for software UVF SCAN
Applications to surface inspection 2-D mapping of oil and other organic compounds on surfaces
Test of lateral resolution with a plastic tape on a steel sheet Very small oil droplet on a clean steel sheet
Spot of oil contamination on a steel sheet Estimated DL s in the few mg/m 2 range, for oils with a high fluoresence yield. The UVF signal correlates well to measurement of surface carbon by combustion UVF: total int. 0.3E+08 0.2E+08 0.2E+08 0.1E+08 5.0E+06 37 40 44 0.0E+00 49 0 200 400 600 800 1000 1200 1400 1600 46 Surface carbon: (mgc/m2) The DL for surface carbon depends on the fluoresence yield of the oil, but is close to 1 mgc/m2 in several applications. 43
Applications to hydrocarbons (PAH) in water The multispectral dimension coarse spectra obtained by five optical filters CCD spectrum of diesel excited by A 280 nm LED, 335 nm high pass Filter. Optical filter spectra of diesel in distilled water (blue) and untreated lake water (red).
Reduction of background from biological material in tap water by optical filtering Intensitet 2500 2000 1500 1000 500 0 Utan filter Med filter These results show that it is necessary to measure in at least 2 spectral windows in order to monitor contamination of petroleum products in tap water. Five week test to monitor incoming water at a water processing plant The probe was submerged in a bucket, with continous streaming of incoming water. The system was set for automatic measurements every 20 min, alternating between 5 optical filter channels.
Possibility to differentiate compounds in surface inspection Identification of different oils and oil mixtures using multivariate analysis These experiments were carried out on fluid oils using a miniature CCD spectrometer. The results show that it is possible to detect e.g. influx of leaking hydraulic oil by multispectral analysis. However, a CCD spectrometer does not have sufficient sensitivity to detect thin oil films.
Fast switching between optical filter channels is needed for online applications This is an example of a fast optical switch of relatively low cost. Planned development Construction of a pre-commercial system based on the technology of the current development system. Incorporation of a fast optical switch for rapid multispectral detection. Improved correction for backscattered light, utilising the multispectral information. Addition of sensors for special applications, e.g. inspection inside tubes. We can and will do all of this in the near future!
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