Terahertz Imaging Application to art conservation

1. Terahertz Imaging Application to art conservation Ayesha Younus J.C. Delagnes, E. Abraham, P. Mounaix Centre de Physique Moléculaire Optique et Hertzienne, UMR 5798 UNIVERSITE BORDEAUX I 1

2. Contents I- Introduction II- Experimental setup Emission and Detection of THz radiation Transmission imaging, Time-dependent THz signal and Spectrum Reflection imaging Longitudinal & Transversal Resolutions III- Results Image gallery Art conservation (graphite, painting pigments) IV- Conclusions & Perspectives

3. Introduction Freq 0.1-10 THz Wavelength 0.03-3 mm Energy 0.4-40 meV • Non destructive and contactless Imaging • Transparent to amorphous materials, clothing, plastic, tissues, papers, wood, semiconductors... Strong interaction with polar molecules • Negligible heating or ionizing effects • THz Imaging in Bordeaux : pulsed THz imaging with a femtosecond laser • Application to art conservation (Centre de Recherches et de Restauration des Musées de France)

4. THz Generation LT-GaAs : Low Temp GaAs

5. delay THz Detection LIA: lock in Amp Varying the time delay Map the THz field in time domain

6. Sample Pump Probe D S E Laser Broadband Spectroscopic THz imaging Raster scanning THz sample Temporal response Spectral response

7. Beam splitter Sample PC Emitter PC Detector Pump Probe Paper Mirror THz Imaging THz : Reflection Setup

8. 500µm 410µm Transversal Resolution Images at 2 THz 200µm 270µm ABmin = 0.66λ/NA ABmin = 0.66*150/0.44 ABmin =225µm

9. Results • Image Gallery - THz Imaging of various objects - Extraction of spectroscopic information • Investigation of graphite properties (pencil leads) - THz spectroscopy of graphite - THz imaging of documents written with pencil leads • Investigation of painting pigments (Centre de Recherches et de Restauration des Musées de France) - THz spectroscopy of pigments - THz imaging of paint layers on canvas, imaging of hidden graphite sketch

10. Image gallery

11. Graphite on paper THz transmission spectra Transmission as a function of page numbers, detection limit Differentiation between 2 pages

12. Graphite on paper: Grades from HB to 8B Image at 2 THz Phase Image at 2 THz “Broadband terahertz imaging of documents written with lead pencils”, accepted to Optics Communications (2009)

13. THz imaging and art conservation : Painting pigments THz transmission spectra Samples: Talens, « Van Gogh acrylic colours » 1: titanium white PW6 2: oxide black PBk11 3: ultramarin PB29 4: brilliant blue PB15/PG7/PW6 5: carmin PR23 6: naphtol red light PR112/PO34 7: burnt sienna PR101 8: azo yellow light PY3/PY74 9: azo yellow deep PY74/PO43 10: permanent green deep PG7/PY128 Frequency (THz)

14. Graphite on canva and 4 colors (same thickness) ultramarin Titanium white Azo yellow light Brilliant blue Transmission imaging Imax-Imin Phase

15. Graphite on canva & ultramarin (3 diff thicknesses) 1st layer: 30-40 µm 2nd layer: 290-350 µm 3rd layer: 510-550 µm Transmission imaging 30 µm 270 µm 515 µm Image at 1 THz Phase

16. Reflection Imaging canva 1st layer: 30-40 µm 2nd layer: 290-350 µm 3rd layer: 510-550 µm De=340 µm De=600 µm Reflection Transmission

17. Graphite on canva and multiple colors Transmission imaging Phase Image at 1 THz Image at 2 THz Imax-Imin

18. Conclusions and perspectives • THz radiation posses unique properties which distinct it from existing technologies (optics, ultrasounds, x-rays) • Interest of femtosecond technology for the generation and detection of pulsed THz radiations, importance of broadband spectroscopic measurements • Perspectives: painting pigments: investigation of old pigments (C2RMF) Inspection of various kinds of wood: determination of wood properties (aging, origin, arche) from the analysis of ring sections of stem Projects: Art-Tera : CPMOH, C2RMF et LOA Tera-Scan : 3D Tomographic THz Imaging, CPMOH, LaBRI New detectors / new sources (GES Montpellier) * * Wave field effect transistor as a resonant detector for 1 Terahertz imaging applications, accepted to Optics Communications (2009)