42nd Annual Meeting – Paintings Session, May 29 “Illumination For Inpainting: Selecting an Appropriate Color Temperature” by Steven Weintraub

So why do conservators prefer northern daylight for inpainting? In Steven Weintraub’s talk titled “Illumination For Inpainting: Selecting an Appropriate Color Temperature” which he presented in the Paintings Session at the 42nd Annual Meeting in San Francisco, he answered this question, as well as a few others about light sources and selecting an appropriate color temperature.
Steven explained how, in his opinion, it is the distribution pattern of skylight that helps make it so ideal. When diffuse skylight from a north-facing window enters a room, there is a “soft” directionality. This type of light distribution avoids the problem of harsh shadows typically associated with point sources of light. It also avoids the opposite problem of flatness due to the absence of shadows, a condition associated with diffuse sources such as over-head fluorescent lamps.
Using only skylight or daylight for inpainting, however, sets the conservator up for two problems: The first, as Steven mentioned in his abstract, is that the availability and control of northern daylight limits the amount of time, and the location in which it can be used. This becomes a real problem if you happen to be facing a tight deadline during the short winter days in the northeast, or like myself, have constantly changing lighting conditions, such as those during the bout of thunderstorms that we often have here during Houston summers. The second problem, which Steven explained in more detail during his talk, is that the spectral power distribution of typical sources of gallery lighting is pretty much the opposite to that of daylight. The result is that if inpainting is done only with daylight, it increases the risk of metamerism when exhibiting the artwork in an electric lighting situation utilizing a warm color temperature source.
It is for both these reasons that many conservators opt for a mix of electric light and daylight for inpainting, and when possible, test the matching of inpainting in the lighting conditions for which it will be displayed.
But, how does one choose an appropriate electric light source? Steven explained his research and suggested that selecting an appropriate color temperature with adequate color rendering properties was a key.
Apparently, color temperature in the range of 3800° Kelvin is the magic number. From Steven’s talk, I learned that color temperature within the 3800°K range is the transition point between warm to cool on the color temperature scale. Within this color temperature range, Steven theorizes that one gets the best balance of saturation with warm and cool colors.
Steven concluded his talk by illuminating a pair of reproductions of a very well known painting by René Magritte, demonstrating the difference in appearance of the image in various color temperature lighting conditions. This showed how the moderate (3800°K) range really did look the best.