Lighting is usually one of the last steps in the process of putting together an art exhibition. Preparators first build or arrange walls to partition the gallery room--often the partitions help the viewer categorize the artworks into certain time periods or artistic media. Preparators also think about sight lines and how visitors walk through a space. Preparators make lighting decisions after planning the space and hanging the artworks. Often, preparators will use as many as thirty lights to illuminate a single artwork.
Scott Rosenfeld, the lighting designer for the Smithsonian American Art Museum, found lighting "Color Field" painting a particular challenge. American Color Field painters of the 1940s and '50s used abstract and nonrepresentational forms to explore color properties. Rosenfeld noted, "Our objective when lighting our modern galleries, especially the one dedicated to Color Field painting, is to help artworks appear as a natural extension of the white wall. The lighting should subtly 'pop' works off the wall without making them look 'spot lit.' If the relationship between the wall and the work is perfect, it will appear almost to float on the wall." To light one painting in the Smithsonian's Color Field gallery, Rosenfeld focused fifteen spots and flood lights on a single painting in order to illuminate the work evenly without casting shadows.
Spotlights project a strong beam of light onto a surface. They create the strongest illumination. Spotlights must be tempered so as not to leave certain spots of the painting brightly lit while leaving others darker; this interferes with viewing the artwork.
Flood lights create a broader and more diffused light. Often lighting designers place a few spotlights towards the center while surrounding the spotlights with floodlights to avoid creating shadows. Both spotlights and floodlights can be turned brighter or dimmer.
Lighting of any sort eventually damages artworks, which is why museums and galleries tend to have more of their collection in storage. Natural light tends to fade colors on paper quickly. Fluorescent lights give off a high degree of UV rays, which are harmful. Incandescent lights are less damaging but they emit more light in the warm spectrum, so they do not illuminate paintings rendered in a cool palette. Most lighting designers and preparators use a combination of incandescent and halogen lights and do not keep artworks displayed for long amounts of time.