Specifications

Submitted to Studies in Conservation, March 2006

pixels that were adjacent to either red or to blue pixels. For the three-plane image, only the red,

blue, and red-adjacent green pixels were used.

The physical movement of the two filters resulted in a registration difference between the

pair of three-plane images, typically on the order of ten pixels (about 0.25%). Using the public

domain software library, ITK [7], and assuming that the images varied only in translation, the

images were registered by applying a transformation calculated from a small user-defined region

of the image. (A more robust approach is a future activity.)

A multiplicative spatial correction was applied to compensate for spatially non-uniform

lighting, commonly used to accentuate surface topography such as impasto. The correction was

based on an image of a neutral background, shot under the same conditions as the object.

A transformation matrix was derived from a calibration target of known spectral

reflectance factors. The transformation matrix was used to convert the six-channel image to a 36-

channel image, resulting in an estimate of the object’s spectral reflectance factor as a function of

position. The calibration procedure is described in the next section.

For most applications, the spectral image was rendered for a specific CIE illuminant and

observer using conventional colorimetric calculations. For the case of an ICC color-managed

workflow, this was CIE Illuminant D50 and the 1931 standard observer. The rendering produced

a digital master saved as a 16-bit CIELAB TIFF file with linear encoding with respect to

CIELAB (similar to reciprocal gamma encoding of 1/2.4).

For archiving purposes, we anticipate storing the registered spatially-corrected six-

channel images along with corresponding calibration meta-data using the DNG file format.

Technical Approach – Details

The multi-filter approach [8] was first demonstrated using an IBM Pro/3000 digital camera based

on a monochrome linear CCD with colorimetric filters. For the current Kodak KAF 22000CE