Examination of paintings with infrared radiation

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There are different methods that can be used to examine paintings. These include infrared , ultraviolet radiation and photography.

In all cases, the paintings are irradiated during the examination, and the absorption , transmission and scattering of the radiation on color pigments through photographic images is observed at the same time . These methods are supplemented by X-ray examinations and infrared spectroscopy . The former put the painting in a slightly radioactive state, which allows the pigment layers to be examined afterwards. The aim of the investigations is to draw conclusions about pigments on the one hand and about concealed layers of paint or signatures on the other, which provide information about the techniques of the artist of the respective historical epoch. This technology can also be used to unmask counterfeits .

Painting examination with infrared radiation

The interesting wavelength range is 780 nm to 3.5 µm. The infrared radiation (IR) can partially penetrate the individual layers of paint up to the primer and also signatures e.g. B. depict graphite. The composition of the color pigments, which absorb, scatter or transmit the radiation to different degrees, is decisive for the penetration.

Painting construction

Carrier materials essentially consist of the carrier, which consists of canvas, paper or wood. There is a primer on the carrier , for example gesso (similar to plaster of paris and chalk). On the primer is the artist's signature, often made of graphite , on which the paint was then applied. Finally, a layer of varnish can be applied to protect the image.

Color pigments

The properties of a pigment are decisive for the visual effectiveness of a pigment. Depending on the binder, the refractive index also changes when irradiated, so that the visual expression can be quite different. The binders include oil, acrylic, egg (historic), and water. The size of the pigments also plays a role. If the diameter of the pigment is smaller than half the wavelength of light, the pigment appears transparent. The pigment size for an opaque property is between 1/500 and 2/5000 mm.

Painting examination

Optical sensors

Inexpensive sensors are based on CMOS and CCD technologies and are widely represented in the digital camera sector. Silicon is used as the base material . A sensitivity of approx. 360 nm - 1100 nm can be read from the characteristic curve of a photodiode . This means that in relation to the maximum sensitivity there is a significantly lower sensitivity at 1100 nm, which is, however, of interest for the infrared examination.

Examination with the camera

Even if the silicon chip in the camera has three color filters (RGB), they are ineffective in the infrared range and the infrared light can be distributed to all three sensors.

In order to create an image that is as true to life as possible without disruptive infrared effects in the camera, IR cut filters are installed by the manufacturers . But many still allow enough IR radiation to pass through to take an IR image. If this is not the case, the IR cut filters can be removed. In order to only generate the IR radiation in the image, an IR filter must be inserted into the beam path.

Incandescent or halogen lamps are used for irradiation.

Measurement setup

A simple and inexpensive measurement setup can be implemented with a webcam. To do this, however, the inner infrared blocking filter must be removed and an infrared transmission filter must be placed in front of the lens.

See also

literature

  • Franz Mairinger: Radiation examination on works of art. Seemann, Leipzig 2003, ISBN 3-363-00778-7 .
  • Andreas Siejek, Kathrin Kirsch: The signature on the painting surface . Siegel, Munich 2004, ISBN 3-935643-13-6 .
  • Max Doerner: Painting material and its use in the picture. Christophorus Verlag, Freiburg 2009, ISBN 978-3-86230-002-0 .
  • Klaus Mangold: Digital infrared photography. Hüthig Jehle Rehm Publishing Group, Heidelberg 2010, ISBN 978-3-8266-9053-2 .
  • Benjamin Ide, Helmuth Grötzebauch, Volkhard Nordmeier: Painting analysis as a context for physics lessons. In: PhyDid B. Didactics of Physics, contributions to the DPG spring conference . Frankfurt 2014, ISSN  2191-379X online
  • Helmuth Grötzebauch, Volkhard Nordmeier: Examination of paintings - experiments for teaching In: Praxis der Naturwissenschaften, Hallbergmoos 2013, ISSN  1617-5689 [1] .
  • Knut Nicolaus: Painting / Examined-Discovered-Researched. Klinkhardt & Biermann, Braunschweig 1979

Web links

Individual evidence

  1. ^ Franz Mairinger: Radiation examination on works of art. Seemann, Leipzig 2003, p. 90.
  2. Max Doerner: Painting material and its use in the picture. Christophorus Verlag, Freiburg 2009, p. 31.
  3. ^ Klaus Mangold: Digital infrared photography. Hüthig Jehle Rehm Publishing Group, Heidelberg 2010, p. 102.
  4. Helmuth Grötzebauch, Volkhard Nordmeier: Investigation of paintings - experiments for teaching In: Praxis der Naturwissenschaften, Hallbergmoos 2013, ISSN 1617-5689.