Specifications

Submitted to Studies in Conservation, March 2006

tested using colored targets and an oil painting, Pot of Geraniums by Henri Matisse.

Comparisons were made with an imaging spectrometer consisting of a monochrome sensor and a

liquid-crystal tunable filter and a commercial RGB digital camera. The new, practical system had

the highest colorimetric accuracy of the three systems and equivalent spectral accuracy to the 31-

band imaging spectrometer.

Introduction

Since 2001, the Munsell Color Science Laboratory has had a research program with the aim of

developing spectral-based imaging, archiving, and reproduction of cultural heritage. The

program is called Art-SI, standing for Art Spectral Imaging, and can be accessed online at

http://www.art-si.org/. Three imaging approaches have been studied: spectral measurement using

a monochrome sensor and a liquid-crystal tunable filter (LCTF), spectral estimation using a

monochrome sensor and six optimized absorption filters, and spectral estimation using a color-

filter array (CFA) sensor and two optimized absorption filters. Berns has summarized each

approach and provided an extensive listing of publications [1]. This last approach has the

greatest potential for day-to-day usage in a cultural-heritage-institution imaging department such

as a museum, archive, or library, principally because it is based on a simple modification of

commercially available professional-grade digital cameras and the department’s workflow can be

streamlined. (We will use the term “museum” for the remainder of this publication.) The purpose

of this publication is to describe the approach and present a comparison with both the LCTF

system [2] and a color-managed RGB system. Greater details about the comparison can be found

in reference 3.

Technical Approach – General Overview

For those active in spectral-based imaging research, it is well understood that a camera should

have more than three channels (e.g., RGB) when used to estimate the spectral properties of

reflecting objects as a general solution. We have determined that six channels are sufficient for

our application [4]. One way to achieve six channels is to use a digital color camera with a color

filter array (CFA) sensor and sequentially place two absorption (colored) filters in the optical

path. This results in a pair of RGB color images, equivalent to six channels. (Of course, one