For the most extreme applications, LED and fiber-optic lighting offer respectable, if not expensive, alternatives. In a fiber-optic system, both UV and IR must be filtered out before entering the fiber, or the port's fiber end will be damaged over time. The absorption of UV by glass or plastic fiber also helps ensure literally no UV transmission through it. With LEDs, there is no radiation above 400 nanometers from almost any white LED, and LEDs can not emit IR either. While LEDs are easier and less costly, glass fiber systems still offer the most perfect display light, because the heat source is also removed from the display environment.
Daylight contains an extremely high percentage of UV light and, when introduced at high light levels, typically provides the greatest risk of photodegradation in architectural applications. Technically careless daylighting designs and ill-advised placement of artwork near windows and skylights are common problems. More than one twentieth-century museum has inappropriate amounts of daylight with little or no filtration, due largely to ignorance and architectural fashion. But most curators are technically astute, and they demand that today's most exciting museum designs are technical masterpieces where UV management is tantamount.
As in photography, reciprocity is the principle by which exposure time is multiplied by exposure intensity to determine total exposure. In the case of art, artifacts, and book and archival collections, the lux-hours of exposure are the metric (to obtain footcandle-hours, divide lux-hours by 10.764). In other words, an object illuminated to 50 lux for 1,000 hours receives 50,000 lux-hours of exposure.
Curators will often set specific limits for each individual piece based on a number of factors, including the current state of the object, its useful life, and the intended number of display hours per year. There are some artworks, such as metal and stone, that have no practical limits, although the specific paint or finish may cause a curator to assign a limiting value. But for most artwork and collectables, typical values range from less than 50,000 lux-hours per year for sensitive materials to over 500,000 lux-hours per year for more durable pieces, including stable dyes, wood finishes, leather, and some plastics.
The lux-hours of allowed exposure are, of course, adjusted for the light source. The curator's reference hours are based on using tungsten lamps with basic UV filters. For example, using a tungsten PAR lamp with a basic UV filter to display a painting, a curator might set a limit of 50,000 lux-hours per year. The lamp itself is rated at 75 uW/lumen. To display this painting using a ceramic metal halide lamp rated 98 uW per lumen, you can either reduce the light level or display period by about 25 percent (75/98), or you can employ a better filter.
Better, more universal standards are constantly under discussion amongst the museum curatorial and preservation communities. For example, a new metric called the "Blue Wool" test has been developed in conjunction with the International Standards Organization (ISO). This is a simple but effective test, in which particular blue wool samples, which are extremely sensitive to light, can be readily tested in the actual setting. Standardized samples and other measurement devices are commercially available. Likewise, the amount of light and UV can be predicted for any source, including daylight, through basic illuminating engineering calculations with factors addressing the percentage of UV content. Acceptably accurate designs for museum daylighting, for instance, can be created using a modern lighting program with daylighting capabilities, plus some basic math accounting for filtering through transmission and absorption.
As a basic rule, try to limit the amount of light on important pieces to about 5 footcandles. This will cause minimum damage to even the most fragile artworks, even if illuminated for over 2,000 hours per year. But remember, the light contribution includes both electric and natural light, and in many ordinary settings, extremely high levels of exposure can be the result of a nearby window. Limit the amount of light through filtering rather than dimming, as dimming causes light color shift. And above all, consider controls as a very effective way of limiting exposure. Simple time-of-day switches are a great way to start, but to address the issue in detail, employ motion sensing to dim display lights when no one is present.
Very few lighting practitioners design museums, and for them these comments are hardly important. But many of us are called upon to illuminate private collections and important pieces in residences and many different corporate and institutional settings. Consideration of these important lighting criteria early on in a project's development will not only lead to its success, but it will also safeguard the very objects these spaces are meant to celebrate.