This poster was presented at the 3D Digital Documentation Summit held July 10-12, 2012 at the Presidio, San Fransisco, CA.
Color and Spectral Archiving Using Dual-RGB Imaging
by Roy S. Berns and Jim Coddington
Imaging in museums serves multiple end uses from publication to detailed documentation. Commercial RGB digital cameras have, at best, adequate color accuracy , demonstrating a need for a spectrally based approach. However, true spectral imaging systems are complex and require advanced understanding of imaging science , placing them outside routine use in museums and conservation facilities. At RIT, an approach was developed that bridged spectral and RGB imaging [3-5]. By using either two different colored light sources or two different colored filters (“Dual-RGB” imaging), a high-resolution RGB camera was transformed into a six-channel abridged spectral imaging system.
The system consists of a camera with the IR filter removed thereby extending spectral sensitivity beyond 600 nm, and two filters, each of which is used to capture images. The optimal filter pair were Schott BG 39 and GG475 optical glass, each with an AR coating and a glued UV and IR cut filter. Interestingly, the BG 39 is a common IR filter; thus the camera can be used as either an RGB or Dual-RGB camera.
Photoshop plugin software was written in 2007 that performed all the color and spectral processing and was used with a modified PhaseOne H20 and RIT-built computer controlled filter wheel. During 2011 a second generation Dual-RGB system was developed at RIT in an effort to improve performance and to also incorporate it into RIT’s three light 3d imaging system.
Four computational steps are required for the Dual-RGB approach: flat fielding, image registration, colorimetric processing, and spectral processing. Flat fielding includes spatial low-pass filtering, effectively removing noise artifacts from the sensor and smoothing any texture or dirt of the diffuse white board. The image registration is calculated at the sub-pixel level. The colorimetric and spectral processing achieves high color accuracy, reasonable spectral accuracy, and minimal noise propagation. All these steps are now combined into a GUI-based executable Matlab program, usable on any OS. The archived image is a nine-channel ProPhotoRGB Tiff file. The first three channels are ProPhotoRGB encoded data. Channels 4-6 are image data from filter 1 and channels 7-9 are image data from filter 2. Channel 4-9 data have been flat fielded, registered, rescaled, encoded nonlinearly using the sRGB standard, and quantized to 16 bits.
Roy S. Berns is the Richard S. Hunter Professor in Color Science, Appearance, and Technology and Director of the Munsell Color Science Laboratory within the Center for Imaging Science at Rochester Institute of Technology, USA. He received B.S. and M.S. degrees in Textiles from the University of California at Davis and a Ph.D. degree in Chemistry from Rensselaer Polytechnic Institute (RPI). He is the author of the third edition of Billmeyer and Saltzman’s Principles of Color Technology. Berns’ main research focus is using color and imaging sciences for the visual arts.
Jim Coddington is Agnes Gund Chief Conservator at MoMA. He has a B.A. from Reed College and an M.S. from the University of Delaware/Winterthur Museum Conservation Program.