Viewing a painting illuminated by visible light only allows us to see the surface paint layers. But the real secrets of a painting and insights into the working methods of the artist are hidden under the surface. The aim of most of the imaging methods is to penetrate the surface layer and let us peek into the inner layers of the artwork.
Very usefull and common methods for this purpose are infrared reflectography (IRR) and Infrared photography (IRP). Both methods make use of the fact that infrared radiation can penetrate the paint layers and thus reach the underdrawing layer. The image below shows the whole spectrum of electromagnetic radiation where it can be seen that the red underlined infrared (IR) region reaches from about 700 nm up to 4000 nm. One nanometer (nm) being one billionth of a meter.
The illumination source in this case is a common incandescent lamp radiating enough infrared rays for this purpose. The reflected IR-radiation is then recorded either on infrared film (infrared photography) in a camera equiped with an infrared cutoff filter. The filter absorbs visible light which would ruin the image on the film and lets the infrared radiation pass. The commercially available IR-films are sensitive to radiation with a wavelength up to 1100 nm. The other possibility is recording the reflected IR-radiation in a camera with a sensor sensitive to infrared part of the spectrum. Modern cameras can record IR-radiation of wavelengths up to 2400 nm (infrared reflectography).
References
(1) R. van Asperen de Boer, Infrared Reflectography: a Method for the Examination of Paintings, Applied Optics, 7(9):1711-1714, 09/1968. DOI: 10.1364/AO.7.001711.
Instruments
(1) Charles M. Falcoa, High resolution digital camera for infrared reflectography, Review of Scientific Instruments 80, 071301 2009. Available as pdf.
(2) Raffaella Fontanaa, Maria Chiara Gambinoa, Marinella Grecoa, Luciano Marrasa, Marzia Materazzi, Enrico Pampalonia, Luca Pezzatia, Pasquale Poggia, New high resolution IR-colour reflectography scanner for painting diagnosis, Istituto Nazionale di Ottica Applicata,Firenze, Italia. Available as pdf.
(3) Antonino Cosentino, Panoramic Infrared Reflectography. Technical Recommendations, International Journal of Conservation Science, Volume 5, Issue 1, January-March 2014: 51-60. Available as pdf.
(4) Peccenini, Eva, A scanning device for wide band infrared reflectography. PhD Thesis, Università degli studi di Ferrara, 2012. Available as pdf.
(5) D. Bertani; M. Cetica; P. Poggi; G. Puccioni; E. Buzzegoli; D. Kunzelman; S. Cecchi, A scanning device for infrared reflectography, Studies in Conservation, Volume 35, Issue 3 (01 August 1990), pp. 113-116.
(6) Antonino Cosentino, Panoramic Infrered Reflectography. Technical Recommendation, International Journal of Conservation Science, Volume 5, Issue 1, January-March 2014: 51-60. Available as pdf.
(1) Chad Weiner, Improved Acquisition Technique of Underdrawings in Oil-Paintings Using IR-Reflectography, Thesis, Rochester Institute of Technology, Center for Imaging Science, 1998.
(2) Molly Faries, Techniques and Applications – Analytical Capabilities of Infrared Reflectography: An Art Historian s Perspective, in Scientific Examination of Art: Modern Techniques in Conservation and Analysis, Sackler NAS Colloquium, 2005. Available as pdf.
(3) Claudia Daffara and Raffaella Fontana, Multispectral Infrared Reflectography to Differentiate Features in Paintings, Microsc. Microanal. 17, 691–695, 2011doi:10.1017/S1431927611000031. Available as pdf.
(4) R. van Asperen de Boer, Infrared Reflectography: a Method for the Examination of Paintings, Applied Optics, 7(9):1711-1714, 09/1968. DOI: 10.1364/AO.7.001711.
(5) John K. Delaney, Jason G. Zeibel, Mathieu Thoury, Roy Littleton, Kathryn M. Morales, et al. “Visible and infrared reflectance imaging spectroscopy of paintings: pigment mapping and improved infrared reflectography“, Proc. SPIE 7391, O3A: Optics for Arts, Architecture, and Archaeology II, 739103 (July 07, 2009); doi:10.1117/12.827493.
(6) Daffara C1, Pampaloni E, Pezzati L, Barucci M, Fontana R., Scanning multispectral IR reflectography SMIRR: an advanced tool for art diagnostics, Acc Chem Res. 2010 Jun 15;43(6):847-56. doi: 10.1021/ar900268t.
(7) J.R.J van Asperen de Boer, J. Dijkstra, R. van Schoute, Catalogue of Paintings Examined by Infrared Reflectography, Nederlands Kunsthistorisch Jaarboek 01/1990; 41(1):59-63. DOI: 10.1163/22145966-90000173
(8) Franz Mairinger, UV-, IR- and X-ray imaging, in Non-Destructive Microanalysis of Cultural Heritage Materials, Comprehensive Analytical Chemistry, Volume 42, 2004, Pages 15–71.