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Technical environment

Technical environment

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LED lighting and conservation requirements – the damage factor

Checking the illuminance on the painting.

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.

Overview of the topic damage factor

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What is the damage factor?

In principle, every exposure has a damaging effect: colours fade, change their hue and become darker; materials become brittle, dissolve and form cracks.

The damage factor (Fdm, rel), also known as the damage index, describes how damaging the radiation of a light source is for colours and the composition of materials. For example, sunlight has both invisible UV radiation and visible short-wave light. For this reason, it is significantly more harmful to many materials than artificial light produced by LEDs.

The damage factor depends on the spectrum of a light source and the damaging component of the light it contains. It is the quotient of damaging irradiance (W/m²) and illuminance (lx or lm/m²), and is typically expressed in milliwatts per lumen (mW/lm). Values < 0.2 are considered acceptable for illuminating art. While earlier light sources often significantly exceed this value, current high-quality LEDs are below it. Luminaires with a warm white light colour have an advantage here because their spectrum contains no UV radiation and only a low short-wave blue component. The damage factor of these LEDs is well below 0.2.

How is the damage factor applied?

The damage factor alone only gives an indication of the damage potential of a spectrum. How damaging the light actually is for an exhibit also depends on the following factors:

  1. Relative illuminance sensitivity of the illuminated materials (Hs,dm in Wh/m²)

  2. Irradiance (W/m²)

  3. Irradiation period (ts in h)

  4. Proportion of the infrared component in the spectrum (can be neglected for LED luminaires)

The relative illuminance sensitivity (Hs,dm) describes the amount of radiation above which damage to an exhibit occurs. It depends on the material of the exhibit and must ultimately be individually determined by experts.

Here are a few typical materials and indications of illuminance sensitivity:
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.


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.

How light-sensitive is which exhibit?

For a rough classification of materials, CIE 157 defines four classes which can be used as a rule of thumb for determining the maximum illuminances and exposure times.
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

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