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Infrared Reflectography (IRR)

Infrared (IR) is part of the electromagnetic spectrum with a wavelength longer than visible light but shorter than microwaves. Thus it is more energy-rich than microwaves but has lower energy than visible light.
Human eye is not capable to detect infrared radiation, so technical means are needed for IR detection.

IRR is one of several techniques for the visualization of paint layers below the surface. IR penetrates distinct paint layers depending on the used pigments. With means of IRR Pentimenti, Under-drawings, Sketches, and Pauses can be made visible. In specific cases completely repainted picture will be shown. IRR is used for Investigation of the painting process and painting material. The outcomes can give hints and proofs for correlating artist and artifact.

Several proposal exists to subdivide IR radiation. Here the definition from ISO is shown:

wavelength [nm]




visible light

violet, blue, green, yellow, orange, red


NIR     IR-A

part of IR between visible light and water
absorption line


NIR     IR-B

part of IR with longer wavelength then
water absorption line



mid wave IR



far IR - Tera-hertz-radiation: 100-1000µm

Aberration: To reduce the effect of chromatic aberrations that is caused by dispersion, the variation of a lens's refractive index with wavelength the range of used frequencies has to be cut. Thus a filter has to be used to absorb radiation with a shorter wavelength than 1000nm if the sensor can detect this range. On the other side of the spectrum detectors sensitivity is used to be limited up to 1700 nm in special cases to 2400 nm. Lenses with achromatic doublet are not available for IR range.

Prerequisites: This method performs only, if firstly the examined art object shows an IR-transparent main color layer. Neither total absorption nor total reflection must be shown with the used wavelength range so infrared radiation can penetrate through. Thinner layers of main color more useful. A thicker main color layer leads to more absorption thus less radiation can penetrate through. Don't forget that this layer has to be passed twice by radiation. Secondly at least two layers beneath till the primer must show different reflection rate.
The ordinary white primer shows a very sufficient reflection rate and under-drawings with carbon black as pencil or ink, silver pen or crayon, and black chalk give a pretty good absorption [1] so differences in the area can be detected. Other drawing materials like raddle/ruddle can not be detected due to similar reflectance.

History: Infrared Reflectography was introduced by J.R.J Van Asperen de Boer in the late sixties of the XXth century [2]. It was already known that infrared photography using wavelength up to 900 nm helps to show under-drawing. To expand this 900 nm boundary new imaging-methods were needed and found in US military technology. Video cameras with lead sulfide vidicon were used there to visualize IR-A radiation. Both were expensive, the camera itself and the stitching. But with this technology leap so many more finding in art investigations are possible that the result outruns the costs of this new process.

The term "Reflectography" was lent from a copy process name "Reflex Copying Process" invented in 1896. In the fifties of the XXth century it becomes famous. Several vendors were offering Copying machines using improved version of this technology [3]. Today this technology is extinct but the name itself is used as an other word for copying or photograph.

For Infrared Reflectography the screen were photographed and after development the paper images manually stitched and the gained overall picture finished. Two decades later frame-grabbers are used for "copying"
and digital programs were invented to stitch. Here at INTK the first stitching program has been written. Today several vendors for stitching programs exist in Italy, Germany, Russia, France, and Great Britain.
with the next step the invention of digital cameras usage of frame-grabbers becomes needless. And the latest digital cameras provide such a huge resolution that stitching can be reduced to a very minimum.

geometric parameters: To improve results some details should be considered.

For plane art objects:
a/ Camera (i.e. detector and lenses) and art object are moved in parallel where the camera axis is perpendicularly to the moving plane.
The stand for the moving target has other properties than those of a common tripod for photography. A tripod should enable a unfettered movement of camera or object in all directions including rotations. With Infrared Reflectography each single shot should be in equal distance and orientation of camera and object. That is such a stand must be inflexible to rotation while moving to the next shot position. And it must guarantee, that the moving is done along a plane.
The big disadvantage of this approach is, that the gaining images need to be rectified before these become stitched. Thus adjacent images must overlap to gain to cushion reduced size of an single image due to correction. And much more calculation has to be done causing a significant delay while finishing. The big advantage is that cheap cameras can be used.
b/ The detector is moved in the focal plane of lenses. Both lenses and art object are fixed.
Such images can be joined giving the result without correction.
But lenses must support such a big detecting area which makes them very expensive. Also optomechanics supporting movement on micrometer-scale is needed. Increasing costs too. Ans last but not least the exposure takes up to several minutes so the floor must be vibration free.

Additional approaches for curved surfaces:
a/ Enlarging the depth of field to have a sharp vision of all parts by means of a very low stop. Thus extremely less light is detects causing a increasing of noise. To reduce noise exposure time must be increase.
So you need a vibration free floor.
b/ Focus each exposure separately giving images of different magnification. For correction and stitching huge amount of calculation power is needed. Fortunately this can be afforded with the latest generation of PCs.

IR Camera:

The simplest and cheapest solution are CCD cameras where the infrared filter usual covering the CCD array has been removed. These cameras can be obtained from mass market and supports up to fifty million pixels.
Sensitivity is up to 1000 nm, special designed arrays can detect radiation up to 1200 nm to the expense of noise. So this is a complete replacement for IR film which is not produced any more. Like IR photography you need a IR filter blocking visible light.

Also almost extinct are Vidicon cameras gaining an analogue video signal. Theses have low resolution up to 700 lines and show ghost pictures when moving the target which are gone slowly. So you must rest half a minute before shooting a photo. A frame-grabber must be used to store the camera signal to a file. Only sensitivity is better than for CCD cameras. sensitivity is up to 1800 nm or 2200 nm depending on material. Because sensitivity starts in visible range an IR filter has to be used to. And sensitivity decreases hardly with wavelength.

The typical NIR camera is using an InGaAs detector with a good sensitivity over the whole range from 900-1700 nm. Detector arrays are offered in size of 320 and 640 lines. A detector line has up to 4096 lines.

The latest improvement are MCT (HgCdTe) detector arrays with a sensibility for 850-2500 nm, with increasing sensitivity for longer wavelength.

All cameras above do not need cooling. Optional Thermo Electric Cooler
(TEC) improves noise so the usage is recommended.

PtSi detectors are the favorite choice when homogeneity among all array pixels is required. But this detector-type needs cooling down to 77K (-196°C). This is done by liquid nitrous or by a Stirling-engine. Cheap TECs are not sufficient.

Typical outcome of an IR camera is a 12 bit image corresponding to 4096 shades of gray. For comparison the human eye can detect about 60 shades which is about 6 bit.
And to establish homogeneity for InGaAs and MCT cameras, elaborate digital corrections are established.

IR-light sources: Sunlight is the natural source of infrared radiation.
But neither it is not available on demand nor it provides a standardised illumination. An due to its destructive power art obejcts should not be exposed to sunlight. Therefor artificial IR-light sources are used.

Incandescent lamps were the first artificial light source. Providing a continuous spectrum of electromagnetc radiation the yield of IR radiation is strong enough to support IRR. A diffusor should be used to spread to spotted source of light, otherwise a mirror image of the filament can be dedected. But the high amount of heat produced by this IR source discourages the use of it.

Halogen cold-light lamps providing radiation with a reduced rate of heat as the name suggests. Nevertheless there is sufficient amount of NIR radiation supporting IRR. Halogen light supports a continuous spectrum too covering the whole frequency range of IR detectors.

Latest evolution are IR-diodes. Developed for data transfer over fiber optic cable it is produced in huge quantities making it cheap. Each diod provides monochromatic light thus no heat radiation is produced.
An other advantage is, that no abberation exists when monochromatic light is used. Supporting radiation for covering the total frequency range of an IR detector different diodes supporting different wavelength have to be combined.

Common to all artificial IR-light sources is that the used voltage has to be stabilized. Otherwise interference pattern are shown. On the other hand exposer time could be synchronized to the frequency of voltage.

IR-lenses: Lenses used with IRR have to provide IR transparency of course! Simple glass lenses show too much absorption so they can not be used. Quartz lenses must be used instead. More suffisticated materials than silicon like Germanium or Zincselenide are used for IR-transparent lenses too.



[2] Infrared Reflectography: A Method for the Examination of Paintings
J. R. J. van Asperen de Boer
Applied Optics, Vol. 7, Issue 9, pp. 1711-1714 (1968)


David Owen, "Making Copies," Smithsonian, Aug. 2004, pp. 91-97.

IR Vidicon cameras