Light, and especially the UV component in the spectrum, causes the colours and materials of artworks to fade and age. The degree to which the spectrum of a light source, such as an LED, damages art, is specified with the damage factor. Because high-quality LEDs emit neither ultraviolet nor infrared radiation, they are particularly suitable for illuminating sensitive exhibits.
| Newsprint | Hs,dm = 5 Wh/m² |
|---|---|
| Watercolour on paper | Hs,dm = 175 Wh/m² |
| Textiles | Hs,dm = 290 Wh/m² |
| Oil paint on canvas | Hs,dm = 850 Wh/m² |
The irradiance (Edm) measures the harmful irradiation that hits the object (W/m²). The value is part of the damage factor of a spectrum.
Finally, the exposure time (ts) is also important. In addition to the illuminance (E), this factor defines the time span until damage occurs. Note that light damage is cumulative, so 1 hour at 1000lx has the same effect as 1000 hours at 1lx. In museums, lighting designers and curators thus have the challenge of illuminating an exhibit sufficiently brightly so that details can be seen, but without running the risk of damaging it (too quickly) by excessive exposure.
Example:
Use this formula to determine how long an object can be exposed before damage occurs:
ts = Hs,m / (Fdm,rel x E)
An oil painting on canvas is to be illuminated to 200lx by a 3000K CRI 97 LED luminaire with a damage factor of 0.155mW/lm. According to the above table, the illuminance sensitivity of the exhibit is 850Wh/m².
ts = 850Wh/m² / (0.000155W/lm x 200lx)
ts = 27,419h
In contrast, when illuminated with daylight through a window, the damage factor of the spectrum is 0.333mW/lm. All other factors being equal, the calculation would look like this:
ts = 850Wh/m² / (0.000333 W/lm x 200lx)
ts = 12,763h
Due to the high-energy wavelengths in the UV and bluish spectral range of sunlight, the exhibit can only be exposed to sunlight for half as long.
| Sensitivity of exhibit, object |
Description | Max. illuminance | Limit for annual exposure duration |
|---|---|---|---|
| 1. Insensitive | The object is made entirely of durable materials, i.e. they do not react to light. Examples: most metals, stone, most glass, real ceramics, enamel, most minerals. |
unlimited | unlimited |
| 2. Low light sensitivity | The object is made of durable materials that react easily to light. Examples: oil and tempera painting, frescoes, undyed leather and wood, horn, bone, ivory, lacquers, certain plastics. |
200lx | 600,000lxh/a |
| 3. Medium light sensitivity | The object contains volatile materials that are moderately sensitive to light. Examples: costumes, watercolours, pastel drawings, tapestries, prints and drawings, manuscripts, miniatures, distemper paintings, wallpapers, gouache, dyed leather and most natural history objects, including botanical specimens, furs and feathers. |
50lx | 150,000lxh/a |
| 4. High light sensitivity | The object contains highly light-sensitive materials. Examples: silk, newsprint and very volatile dyes. |
50lx | 15,000lxh/a |
