Navigating the Future the Two Is Better thanofOne: True Color Museum Imaging Using Dual Illumination
Spectral imaging has long been recognized to outperform conventional color imaging for color accurate reproduction of cultural heritage objects and materials.1-4 Many such materials are color inconstant, meaning that, when the lighting changes, their appearance changes, sometimes dramatically. This happens because seeing color and capturing color are not identical processes. Keeping everything else exactly the same, if you substitute your eyes for a conventional color camera, the picture you take will not exactly replicate what you see – your eyes and conventional cameras are not equivalent sensors. There are standard methods of transforming RGB camera signals to trichromatic human color perception, called profiling. Creating an objective camera profile is essentially the process of determining a mathematical map between these two sensors that transforms camera capture to match human vision as closely as possible, but problems remain in ‘as closely as possible’. When working to create a master file, from which digital and print reproductions can be tuned for specific viewing and illuminating conditions, ‘as closely as possible’ may not be good enough. Spectral imaging, in comparison, involves sampling the color spectrum more finely than conventional color capture.5-6 However, the cost and complexity of the
Figure 1. Top: diagram illustrating the process of capturing a pair of RGB images under two different illumination conditions that are then combined to create a six-channel spectral image stack. Bottom: example six-channel sensitivity of an RGB camera + LED-based dual illumination spectral imaging system.