Photothermal

The eombination in a compact system of an infrared sensor and a laser as excitation source is called a photothermal camera. The surface heating is aehieved by the absorption of the focused beam of a laser. This localisation of the heating permits a three-dimensional heat diffusion in the sample to be examined. The infrared (IR) emission of the surface in the neighbourhood of the heating spot is measured by an infrared detector. A full surface inspection is possible through a video scanning of the excitation and detection spots on the piece to test (figure 1). 

Figure 2 Surface-breaking crack signal of a Photothermal camera (theory)...

The principle of the photothermal camera is known for decades (see ref. 1) but did not offer sufficient performances to permit this instrument to find an industrial issue. The main reason of that lack of success Is the poor efficiency of the previous configurations when applied on industrial products (figures 3, 4, 5) ... 

To overcome these limitations, the new photothermal camera developed by FRAMATOME and ONERA (figure 6) ... 

This new design of photothermal eamera is being patented. 

The new design of photothermal eamera has been thoroughly evaluated on different kinds of cracked metallic (mainly steel) parts in order to compare its sensitivity to the one of penetrants. For that purpose, an experimental set-up has been built. 

Figure 7 New photothermal camera versus penetrant testing on a crack (width 5 pm) at the surface of Fig 4. part The large arrow shows that the PT indication almost extinguishes at thinner width (approx. I pm) locations along the crack the photothermal signal does not. The bottom little arrow indicates a place where the crack splits in two, which is not revealed on the PT image because of the spreading of the penetrant liquuid.

Figure 8 Sensitivity of the new photothermal camera to small depth defects. lmage of an EDM notch of 1mm long, 100 pm width and 200 pm depth on ANSI 304 stainless steel with a bad surface condition (ground surface, "Vi 2 -6 ).

Such requirements are meant to change the usual configurations and technologies usually associated with photothermal set-up. These changes mainly affect the IR detection devices, the optical components associated with the excitation and detection flux and the signal acquisition hardware and software. Figure 9 presents a sketch of the so-called pre-industrial demonstrator built from those different improvements. 

A new design of an old concept of thermal surface inspection permits the photothermal camera to be considered as an interesting alternative to NDT methods like penetrant or magnetic testing, particularly when the testing has to be made in a remote or automatic way. 

For a few months, results obtained on a pre-industrial version of this new photothermal camera allow this technology to enter into the industrial step of its development. 

Development of a photothermal measuring technique for the determination of hardness profile in steel ... 

S. Bialkowski, Photothermal Spectroscopy Methodsfor ChemicalAnalysis,]ohxi Wiley Sons, Inc., New York, 1996. 

Coatings for photothermal solar converters with high infrared reflectance, which use Reaction (3) and a 1000°C anneal,... 

Water soluble starch capped nanoparticles proved to be efficient non-cytotoxic bactericidal agents at nanomolar concentrations. The investigation also suggested that starch capped CuNPs have great potential for use in biomedical applications such as cellular imaging or photothermal therapy. 

S. Link and M. A. El-Sayed, Shape and size dependence of radiative, non-radiative and photothermal properties of gold nanocrystals. Int. Rev. Phy. Chem. 19(3), 409-453 (2000). 

Huang, X., El-Sayed, I.H., Qian, W. and El-Sayed, M.A. (2006) Cancer cell imaging and photothermal therapy in the near-irrfrared region by using gold nanorods. Journal of the American Chemical Society, 128, 2115-2120. 

As aforementioned, laser ablation of polymer films themselves and dopant-induced laser ablation of polymer films have been extensively investigated. The photochemical or photothermal mechanism has been discussed. The feature of the dopant-... 

The applicability of alternative photothermal densitometric techniques, such as PAS, for characterisation of TLC plates with particular emphasis on the in-depth distribution of compounds in the sorbent, has been investigated [776], No specific applications for polymer/additive systems appear to have been reported so... 

Photothermal Spectroscopy Methods for Chemical Analysis. By Stephen E. Bialkowski Element Speclatlon in Bioinorganic Chemistry. Edited by Sergio Caroli Laser-Enhanced Ionization Spectrometry. Edited by John C. Travis and Gregory C. Turk Fluorescence Imaging Spectroscopy and Microscopy. Edited by Xue Feng Wang and Brian Herman... 

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