Infrared Reflectography: a Method for theExamination of Paintings

J. R. J. van Asperen de Boer

A method has been developed to detect underdrawings in paintings more completely than is possible withir photography. A modified Barnes ir camera has been used equipped with a lead sulfide detector.

The results can be explained in terms of an increase in hiding thickness of paint layers with the wave-length. Practical aspects of reflectography of paintings are briefly described and an example is given.

Introduction

The technique of mediaeval painting in Europe canbe studied through historical sources and by examina-tion of actual paintings.

In Early Netherlandish painting, first a mixture ofchalk and animal glue was applied on a carefully con-structed oak panel.' In Italian painting, the woodis frequently poplar and the ground consists of gypsumand glue.2 Such white grounds provide an excellentbackground for a preliminary design. In the processof painting, this underdrawing was gradually coveredby paint layers. Unfinished paintings illustrate thistechnique.3 Paints were prepared in the artists'studios by hand-grinding pigments in oil or egg tem-pera. The drawing has been found in many cases toconsist of bone black or carbon black applied in anegg medium with a brush.4

The underdrawing may sometimes be partly ob-served with the naked eye. Infrared photographyhas been used to improve the detectability of under-drawings, but the results are mostly confined to redand whitish areas in the painting. Green and otherdark areas appear blackish in such photographs.

The underdrawing is an important element in theart historical judgement of a painting, because it mayprovide complementary information. Art historianshave occasionally tried to use ir photographs in at-tribution problems.'

Because underdrawings can only be partly revealedin ir photographs, it was attempted to improve thisdetection method.'

Determination of the Hiding Thickness of Paintsin the Near Infrared

The ability of a paint layer to hide a backgroundwith reflectances RB and Rw (RB < R,) depends

The author is with the Central Research Laboratory for Objectsof Art and Science, Amsterdam, The Netherlands.

Received 8 March 1968.

primarily on the scattering properties of the paint andthe paint layer thickness.

For nonabsorbing particles, small compared withthe wavelength X of the incident radiation, scatteringdecreases rapidly with X (Ref. 7).

Since in the nondestructive examination of paintingsthe layer thickness can obviously not be modified, itwas tried to decrease the hiding ability by using radi-ation with longer wavelengths.

Blevin' showed that the thickness XD of a layer thatjust hides a background with reflectances RB and R,is found from

y(kRw - RB) - y(l -)(1 - RBR)+ k(1 - aRw)(a -RB) - (1 - aRB)(a - Rw) = 0, (1)

where y = b cothbSXD,; S is the scattering coefficientof the paint layer; a and b are functions of the re-flectance R0. of an infinitely thick paint layer. Thefactor k is the ratio of the reflectance of a paint layerapplied over a black and over a white background.Its value depends on the detector system used to ob-serve the contrast. Following GureviW9 and Launer,"values for S and R0. were calculated from transmittancemeasurements on paint layers prepared with pigmentssuch as found in mediaeval paintings.'"

A Zeiss PM Q III spectrophotometer with an in-tegrating sphere equipped with a lead sulfide detectorwas used. Monochromatic radiation was incident at60° from the normal, which theoretically should beequivalent to diffuse radiation. In this way, diffuseincident and diffuse transmitted radiation was provided,a necessary requirement for the Kubelka-Munk theoryfrom which Eq. (1) is derived.12 No account was takenof surface reflections.

It was found that the paint reflectance (particularlyfor light paints) decreases beyond 1.0 y, an effect alsoobserved by de la Perelle et al.1" This means that thewhitish chalk glue and gypsum glue grounds becomedarker. The reflectance of the black underdrawingmay vary, especially when the drawing is very thin.

September 1968 / Vol. 7, No. 9 / APPLIED OPTICS 1711

200

-

Z 160

)X 120toU)wz2 80I-

z° 40

0.8 1.2 1.6 2.0 2.4WAVELENGTH (MICRONS)

Fig. 1. The hiding tickness XD of various paints applied over achalk glue ground and a drawing with RB = 0.1. All pigmentsground with linseed oil. The pigment volume concentrationsused have been indicated in brackets. (1) vermillion (PVC =11 %); (2) lead white (PVC = 28%); (3) azurite (PVC = 30%);(4) malachite (PVC = 21%); (5) raw ochre (PVC = 31 %), and

(6) raw sienna (PVC = 21.5%).

The reflectance of a thick bone black layer was foundto be about 0.05-0.1 u up to 2.5 .

In Fig. 1, the hiding thickness calculated from Eq.(1) using an electronic computer has been plottedagainst the wavelength for various paint layers over achalk glue background and a drawing with RB = 0.1.The curves show that for light paints such as leadwhite and vermillion, not much improvement is to beexpected at longer wavelengths, but that darker paintsmade with azurite, malachite, and the earth pig-ments show considerable increase in X.D with X and amaximum at about 2.0 .

The XD values depend on the pigment volume con-centration among other factors, but for the trans-mittance measurements, rather high pigment volumeconcentrations were taken. The accuracy is not verygreat and errors in XD may rise up to 20%, but this issufficient to explain the observed results.

Figure 1 illustrates that a much better detection ofunderdrawings is to be expected around 2.0 ,u, becausethe thickness of the paint layers in mediaeval paintingis generally less than the XD values found in this spec-tral region.

The hiding thickness between 0.8 g and 0.9 isfound to be lower than the actual paint layer thicknessin mediaeval painting, except for lead white and ver-million. For such paints, XD is of the same order asthe layer thickness encountered and this explains whyir photographs frequently reveal an underdrawing inareas painted with these pigments.

Instrumentation

To produce an image of reflected ir radiation in the2.0-u region, a Barnes ir camera was selected. Thisinstrument has been designed to record emitted heatand is normally used with a thermistor-bolometer inthermography. 14

Modified to provide 350 lines/raster, and equippedwith a room temperature lead sulfide detector peakedat about 2.0 A, the system is used to produce a two-dimensional picture of translated reflected radiation andis thus converted into a reflectograph. The resultingdocuments are therefore called ir reflectograms, * alsoto prevent confusion with ordinary ir film photographsor long wavelength thermograms.

For larger paintings, the resolution is satisfactory,but small paintings cannot be examined in detail.This could be improved with short range optics.Diaphragms are used in front of the radiometer aper-ture to increase the sharpness of the image.

Weihel" showed that the noise equivalent radiationdifference of a photoconductive detector is proportionalto the square root of the detector area and the squareroot of the solid angle scanned per second, but in-versely proportional to the instantaneous field of view.In modifying the instrument, it was required to ob-tain a high resolution and thus a small instantaneousfield of view. This was obtained by the manufacturerin selecting a 0.066 mm X 0.066 mm detector area andincreasing the scanning time to 24 min per reflectogram.

The high system responsivity thus obtained requiresonly very low illumination levels. At 1 m, for example,reflectograms can be obtained with spotlights used inphotography providing an illuminance of only 14 lxat the painting. This is much lower than the maxi-mum allowable level of 150 lx in museums.'6 Paint-ings may warp if subjected to high illumination levelsand in museum practice this is clearly not acceptable.'7System responsivity would therefore be an importantcriterion for successful application of other ir imagingsystems.

It is a fortunate coincidence that PbS detectors aresuperior to other detectors in the vicinity of 2.0 , be-cause the hiding thickness was found to have also amaximum in this spectral region.

Paint media and varnishes would absorb betweenapproximately 2.8 and 3.5 ,4, due to 0-H and C-Hvibrations, and at longer wavelengths, the radiationemitted by the painting itself becomes more importantand would interfere with the reflected radiation.There is thus no point in using a detector system with amore extended spectral response in the ir for the pro-duction of reflectograms.

Infrared Reflectography of Paintings. ResultsIn reflectographing a painting, the Barnes ir camera

is mounted on a tripod that can be moved in the verticaland horizontal directions. In this way, a painting canbe reflectographed in sections with identical areas.Reflectograms are later assembled. The illuminationlevel is regulated by a variable resistor to provide afixed glow tube current output at an attenuation that

* The author is indebted to J. R. Yoder, Barnes EngineeringCompany, Stamford, Conn., for originally suggesting this termi-nology.

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I I I I I I I I I

1-

F

1-

l I I I II II I I

Fig. 2. Detail of a paint-ing attributed to Geertgentot Sint Jans (Amster-dam, Rijksmuseum. Cat.950- Al). Panchromatic

film.

Fig. 3. Detail of a paint-ing attributed to Geert-gen tot Sint Jans. In-

frared photograph.

Fig. 4. Detail of a paint-ing attributed to Geertgentot Sint Jans. Reflecto-

gram assembly.

September 1968 / Vol. 7, No. 9 / APPLIED OPTICS 1713

produces almost no visible noise in the reflectogram.Contrast can be enhanced by subtracting or adding aninternally generated signal, which allows small re-flectance differences to be imaged.' 4

Figure 4 shows the improvement obtained with thismethod in detecting the underdrawing compared withconventional ir photography. Illustrated is a detailof a picture in the Rijksmuseum, Amsterdam, (Cat.950-Al) representing the Adoration of the Magi(Fig. 2). This painting is usually attributed to Geert-gen tot Sint Jans, a painter who worked in Haarlemduring the second half of the fifteenth century.

In the ir photograph (Fig. 3), an underdrawing ispartly visible, e.g., in the architecture, but it is diffi-cult to appreciate the actual forms. The reflecto-gram assembly (Fig. 4) shows that especially the treesand the background architecture were lightly sketchedand more carefully worked out during the paintingprocess.

In mediaeval painting, various layers were appliedsuccessively and improvements could thus be madegradually. A noticeable improvement is the camelin the cortege of one of the Magi, because originally ahorse was drawn there. This may have been done foriconographical reasons to stress the oriental char-acter of the Magi. The head of the white horse justright of the camel changed its position. The paintlayers at the surface are not completely eliminated asthe trunks of the trees are seen in the reflectogram bothpainted and drawn. The dark foliage of the trees isalso still visible. The black verticals correspond tojoints between the planks of the panel.

Infrared reflectography could contribute to a betterunderstanding of earlier painting techniques and

workshop practice, especially if used in conjunctionwith other methods for the examination of paint-ings, such as x radiography and analysis of paintingmaterials. However, much material is generally re-quired before useful conclusions can be drawn and itis therefore to be expected that such a developmentwill be slow.

References1. P. Coremans, L'agneau mystique au laboratoire (De Sikkel,

Antwerpen, 1953).2. R. J. Gettens and M. E. Mrose, Stud. Conserv. 1, 174

(1954).3. A. P. Laurie, The Painter's Methods and Materials (J. B.

Lippincott Co., Philadelphia, 1926).4. P. Coremans, R. J. Gettens, and J. Thissen, Stud. Conserv.

1, 1 (1952).5. R. van Schoute. Over de techniek van Jeroen Bosch,

Jheronimus Bosch. Bijdragen bil gelegenheid van deherdenkingstentoonstelling te 's Hertogenbosch 1967, pp. 72-79.

6. J. R. J. van Asperen de Boer, Stud. Conserv. 11, 45 (1966).7. H. C. van de Hulst, Light Scattering by Small Particles

(John Wiley & Sons, Inc., New York, 1957).8. W. R. Blevin, J. Oil Colour Chem. Assoc. 41, 850 (1958).9. M. Gurevi6, Phys. Z. 31, 753 (1930).

10. H. F. Launer, J. Opt. Soc. Amer. 32, 84 (1942).11. J. Plesters, Stud. Conserv. 2, 110 (1956).12. P. Kubelka, J. Opt. Soc. Amer. 38, 448 (1948).13. E. T. de la Perelle, T. S. Moss, and H. Herbert, Infrared

Phys. 3, 35 (1963).14. R. W. Astheimer and E. M. Wormser, J. Opt. Soc. Amer.

49, 184 (1959).15. W. K. Weihe, Proc. Inst. Radio Eng. 47, 1593 (1959).16. G. Thomson, Stud. Conserv. 6, 49 (1961).17. F. Kollmann, A. Schneider, and R. Teichgriber, Holz als

Roh- und Werkstoff, 19, 41 (1961).

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