Specifications

Submitted to Studies in Conservation, March 2006
11
unity. Perfect correlation would yield zero. These values as a function of wavelength are plotted
in Figure 6 as the dashed red line. For both systems, large scatter occurred for short wavelengths.
This was caused largely by the calibration targets containing titanium dioxide white. It has very
low reflectance in this wavelength range and the spectral variation of the target patches was very
small. This resulted in large uncertainty when estimating these wavelengths using each camera
system. A second reason concerned the low quantum efficiency of CCD sensors in this
wavelength range, also increasing uncertainty.
Experimental Verification – Henri Matisse, Pot of Geraniums
When the Quantix-LCTF system was tested at NGA, three paintings were imaged: Alvise
Vivarini, Saint Jerome Reading; Alexej von Jawlensky, Murnau; and Henri Matisse, Pot of
Geraniums; all among the permanent collection of NGA. The Matisse proved to be the most
challenging [2, 15], having the widest range of pigments and spectral properties. This painting
was again imaged using the MCSL-Sinar and NGA-Sinar systems. A GretagMacbeth EyeOne
had been used to measure 43 positions on the painting. The painting and the measurement
positions are shown in reference 2.
Preliminary evaluations were performed where each test target was used to derive the
calibration transformation converting camera signals to spectral reflectance factor and CIELAB
for the MCSL-Sinar system. As described above, the results were affected by the choice of
calibration target. Not surprising, the Gamblin target was the most successful. There were two
reasons. First, the spectral properties of this target span the spectral properties of the painting.
Second, this target was painted on canvas board, resulting in similar surface characteristics to the
painting. Because the NGA lighting was so directional, having similar surface properties
between the calibration target and the painting helped improve spectral and colorimetric
accuracy. This second reason was more important than the first, as evinced by the Gamblin target
having superior performance to the combination of the ColorChecker DC and Blues targets.
The National Gallery of Art’s imaging workflow included visual editing where local and
global color corrections were made using Adobe Photoshop to improve the color-matching
accuracy between a work of art illuminated with high color rendering daylight-fluorescent