As with many things in the art world, the subject of lighting often attracts pretentious theories and explanations. In reality, lighting artworks is becoming increasingly easier and far more effective due to the advances in LED lighting technology which effectively eliminates the problem of potential heat damage, allowing for greater levels of illumination that previously possible. With many of the great works of art being relit recently maybe we should revisit the basic principles.
Its important to differentiate between the approach to lighting paintings and lighting sculptures and other art installations. Surprisingly, these are often confused but the facts are simple:
PAINTINGS - These should ideally be illuminated in isolation to draw the observer into the painting. The lighting should be even across the entire surface of the painting and should exhibit a high CRI so that the colours reproduce accurately. (The precise colour temperature of the light source depends on many factors including the type of light the original painting was conceived in but a good starting point is around 4000 K).
Opinions differ as to the requirement for even illumination of a painting but in my option this is page one! An artist goes to considerable lengths to create depth, light & shade in a painting so the last thing the artist wants is for the lighting designer to put a different spin on it by altering the brightness in some areas of the painting. This is equally true if its lit from the wrong angle causing surface flare which reduces the effective contrast of the painting and over-exaggerates the texture of the paint and canvas.
Some would say that this doesn't apply to murals, especially when they're more of a trompe-l'oeil but in this case, the lighting of the room should be made to match the painting, not the other way round.
SCULPTURES - The lighting of three dimensional artworks is a completely different matter since they rely on the way the natural light plays on them at different times of day to bring out different features so they provide an ideal subject for more creative lighting rather than basic illumination.
A good way to light sculptures in a gallery is by use of various individual hard light sources programmed to fade in at different times of day to replicate the movement of the sun. This should of course be combined with a certain amount of indirect soft light to maintain a suitable contrast ratio.
INSTALLATIONS - This type of art embraces the use of light more than any, since it is vital that the installation is designed to work within the space allotted and the lighting tuned to ensure that the installation is viewed and experienced in exactly the manner intended by the artist. Installations can range from video walls to a pile of bricks but whatever the type, the artist is as much a lighting designer as artist, working closely with lighting technicians and AV specialists to achieve the goal.
I feel sure that if the old masters were alive today they would embrace the new media and be determined to exercise control over the way their work is exhibited as do many modern artists, especially those working on installation art.
FINAL THOUGHT - When lighting artworks just pause to consider what the artist intended (if they're alive ask them!); its the least we can do.