Near-Infrared Luminescence of Cadmium Pigments: In Situ Identification and Mapping in Paintings

Abstract

A comprehensive study of the luminescence properties of cadmium pigments was undertaken to determine whether these properties could be used for in situ identification and mapping of the pigments in paintings. Cadmium pigments are semiconductors that show band edge luminescence in the visible range and deep trap luminescence in the red/infrared range. Emission maxima, quantum yields, and excitation spectra from the band edge and deep trap emissions were studied for sixty commercial cadmium pigments that span the color range from yellow to red (reflectance transition 470 to 660 nm). For paints containing cadmium pigments, luminescence from deep traps was more readily observable than that from the band edge, although the yield varied widely from zero to around 4.5%. Optimal excitation for emission is found to be in the visible for both pigments in powder form and mixed with a medium. The maxima of the deep trap emission shift with the band gap energy, providing a potentially useful way to assign pigment type even when used in pigment mixtures. The usefulness of the results of the study on mockups was demonstrated by the mapping of cadmium pigments of different hues with the aid of calibrated luminescence imaging spectroscopy in a painting by Edward Steichen, entitled Study for 'Le Tournesol' (1920). Analysis of the luminescence image cube reveals at least six unique spectral components, associated with emission from white pigments, paint binder, and cadmium red and yellow pigments. The results were compared with those from X-ray fluorescence spectrometry (XRF) and fiber-optic reflection spectroscopy (FORS) and the results obtained on paint samples containing cadmium pigments. These results show that, when present, the emission from traps can be used as an analytical tool to identify cadmium pigments, to distinguish among cadmium sulfide, cadmium zinc sulfide, and cadmium sulfoselenide, and to map cadmium pigments, even in mixtures.

Visible and infrared imaging spectroscopy of paintings and improved reflectography

“…The emission spectrum from the white flower petals (spectrum #1 in Fig. 4c) decreases from 650 to 900 nm, characteristic of zinc or lead white in a paint binder such as oil [4,5,12,13]. Emission spectrum #2 from the red flower petals shows a similar trend.…”

“…Luminescence Imaging Spectroscopy: As previously described [4,5], an illumination and multispectral camera system assembled in-house, operating from 650 to 950 nm, was used to obtain image cubes calibrated to relative radiance. Excitation illumination was done using two Kodak Ektagraphic III AT carousel slide projectors (OSRAM 300 W EXP bulbs) equipped with F/D 2.5, 152 mm Buhl optical lenses set up at 45 degrees from the normal of the painting at a distance of approximately 3.6 m. The excitation bandpass was 380-520 nm defined by 50 mm 2 Kron/Cousins B astronomy filters inserted into the two slide projectors.…”

“…Little or no luminescence is detected in the relatively pure yellow areas of the flowers. Using in-scene known luminescence standards [4,5] allows the luminescence image cube to be calibrated to relative spectral radiance. This procedure allows for the comparison of emission spectra from each of these areas.…”

“…These include reflectance [2][3][4], luminescence [4,5], and x-ray fluorescence (XRF) [6] spectroscopies. Reflectance and luminescence spectroscopies provide information about electronic transitions [5,[7][8][9][10][11][12][13]. Reflectance spectroscopy in the SWIR provides information about vibrational transitions, which are mostly overtones and combination bands whose fundamental transitions occur in the mid-IR.…”

“…Efficient macroscale imaging of paintings using line-scanning imaging spectrometers [2-4, 15, 16] or filter-based imaging systems [5,10,18] has allowed conservation scientists to collect reflectance and luminescence images of paintings in the visible (400-750 nm), NIR, and SWIR. Using highly sensitive focal plane arrays, the images can be acquired under lighting conditions suitable for examination of paintings and even illuminated manuscripts [15,18].…”