Specifications

Submitted to Studies in Conservation, March 2006

units. This index is a ∆E

value for CIE Illuminant A following a slight spectral adjustment [14]

such that for CIE Illuminant D65, the colorimetric data are identical, that is, a ∆E

of zero. [The

spectral adjustment is the Fairman parameric correction, shown in Eq. (4).] The spectral root-

mean-square (RMS) error over the wavelength range of 380-730 nm was also calculated. The

Quantix-LCTF system had higher spectral accuracy than the MCSL-Sinar system. For the

ColorChecker DC, the ColorChecker, and the Blues targets, the Quantix-LCTF system was

superior at an α of 0.01 (based on evaluating RMS error); for the Gamblin target, there was not a

statistically significant difference. The superior performance was the expected result since the

Quantix-LCTF system was a true spectral device whereas the MCSL-Sinar was an abridged

device. What was unexpected was that the performance of the MCSL-Sinar system was so close

to the Quantix-LCTF system and for the Gamblin target, the two systems were equivalent

statistically. Furthermore, the Best Case results and the Quantix-LCTF system were not

significantly different except for the Blues target. To be fair, it needs to be pointed out that much

more effort had been put into development of the MCSL-Sinar system. Once we achieved an

acceptable result for the spectral camera, we began looking at more practical approaches. It is

likely that if we had selected the Quantix-LCTF system as the recommended system, better

performance would be reported herein.

The GretagMacbeth ColorChecker Color Rendition Chart has become a de facto imaging

standard. The spectral estimation accuracy of this target for the MCSL-Sinar and Quantix-LCTF

systems are shown in Figure 6. The average spectral difference as a function of wavelength is

plotted as the blue solid line. The Quantix-LCTF system had a nearly flat curve quite close to

zero, the desired result whereas the MCSL-Sinar system had a strong undulation. Plots of the

neutral colors would show excessive undulation. This is a common result for such an abridged

spectrometer. We have performed some preliminary research to address this problem including

pigment mapping [23] and treating the camera system as a conventional spectrophotometer [24].

At every wavelength, a scatter plot could be made comparing the spectrophotometer and

camera system and a line fit to these data. A correlation coefficient of the line fit would indicate

the amount of scatter. In order to have a number with similar magnitude to the spectral

differences, the correlation coefficient, ranging between zero and unity, was subtracted from