9 Light
Without light it would not be possible to see objects in a room or enjoy artworks.
The colors, distribution and design of lighting as well as the arrangements of lights and luminaires create different lighting ambiances – light spaces that serve the various technical needs of an exhibition.
The conservation aspect is particularly important here – protection from light plays a key role in all exhibition spaces. Daylight and artificial light contain rays that can cause exhibits to fade, dry out or become discolored or deformed if they are exposed for long periods. Conservation measures can protect against damage, but only if they are rigorously applied.
Protection from light damage
Fading, yellowing, darkening, discoloring, brittleness, deformation, curling, splitting, tearing, swelling, drying out, shriveling, disintegrating – this list gives the impression that extremely destructive influences are at work. In fact, exhibits displayed in daylight or under artificial lighting are often exposed to more than just one of these risks.
Following a brief introduction to optical radiation, the threats posed by photochemical changes and thermodynamic processes will be discussed and possible protective measures presented.
Optical radiation
The risks of exposing light-sensitive materials to optical radiation – the part of the spectrum covering short-wave ultraviolet (UV) radiation (100 to 380 nm = nanometers), visible light with a wavelength of 380 to 780 nm and long-wave infrared (IR) radiation (780 nm to 1 millimeter) should not be underestimated. Optical radiation triggers photochemical or thermodynamic (physical) processes. With its high UV content and the heat radiated from the sun, daylight always poses a considerable threat.
Lighting engineers and other scientists have studied these phenomena (cf. the references listed in the knowledge base). As well as empirical values and associated tips for conservation measures to protect objects from light, such research has yielded an extensive body of formulae which, although they make it possible to calculate the harmful effects, are not particularly illuminating for non-specialists. No formulae or mathematical calculations will therefore be listed here.
Two points are important to note:
1. It is not incident radiation, but the radiation that an object absorbs that causes harm to it. Photochemical changes take place very slowly. Light damage is however cumulative, that is to say no material ever “forgets” radiation, its duration and intensity.
2. UV radiation and short-wave light are generally more harmful than long-wave light and infrared radiation. This means that radiation in the visible spectrum (visible light) can also cause harm.
I. Photochemical changes
Organic materials in particular are susceptible to photochemical changes. Inorganic materials are much more rarely affected. In museums, the greatest fear is of changes in color, i.e. the fading, yellowing or darkening of color pigments, binders and varnishes used on watercolors and oil paintings, or of paper, textiles and wood. Materials are categorized on the basis of their sensitivity to photochemical damage:
| Category | Description |
| 1. Insensitive | The object is made of solid materials that are not sensitive to light. Examples: Most metals, minerals and glasses, stone, true ceramics, enamels. |
| 2. Low sensitivity | The object contains durable materials that are slightly sensitive to light. Examples: Oil and tempera paintings, frescoes, undyed leather and wood, horn, bone, ivory, lacquer, various plastics. |
| 3. Moderate sensitivity | The object contains less durable materials that are sensitive to light. Examples: Costumes, watercolors, pastels, wall hangings, prints and drawings, manuscripts, tempera paintings, wallpaper, gouaches, dyed leather and many historical objects made of natural materials such as botanical specimens, skins and feathers. |
| 4. High sensitivity | The object is made of highly light-sensitive materials. Examples: Silks, newspapers, known less permanent dyes. |
Table 1: Material categories according to CIE publication 157 (Control of Damage to Museum Objects by Optical Radiation, Technical Report, CIE 157:2004)
The images below show the influence of photochemical changes on textiles.
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| Cushion before radiation | Cushion after radiation |
The most important parameters that contribute to photochemical processes are:
- Irradiance of the object.
- Irradiation time: Length of time an object is exposed to irradiation. The higher the irradiance and the length of exposure to it, the greater the potential risk.
- Spectral radiation distribution of the light source (daylight or lamps): Each wavelength of light has a specific spectral color associated with it. White light comprises a multiplicity of spectral colors of differing intensity. As well as for daylight, this spectral radiation distribution is characteristic for each type of lamp. Incandescent lamps, for example, produce a light dominated by the red colors of the spectrum while daylight is dominated by the short-wave blue colors.
- Relative spectral sensitivity: Designates the dependence of an object’s light sensitivity on the wavelengths of the reference radiation. Although UV radiation (< 380 nm) has the greatest potential for damage, sensitivity in the visible light range, in particular at the blue end of the spectrum, should not be ignored.
- Effective threshold irradiation: Measure of the absolute sensitivity of an object. Changes start to occur in light-sensitive materials on first exposure to radiation – at first invisibly, then visibly. The threshold at which change starts to be noticeable is the measure used to rate the material’s light sensitivity. The threshold irradiation time (i.e. the time taken until the threshold is reached) is stated for individual types of material under daylight or the light of different lamps.
The effective irradiation is calculated mathematically from the values for optical radiation (spectral distribution), irradiation and the relative object sensitivity.
The following properties and conditions also play a role in photochemical processes:
- Spectral absorption characteristics of the material and its specific disposition for secondary reactions. The radiation is only photochemically effective if it is absorbed by the material. After the photochemical effect commences, the green color, e.g. produced by mixing the blue and yellow pigments, will turn increasingly blue because the blue pigments absorb less energy-rich short-wave radiation (blue) than the yellow pigments. The blue pigments are consequently less affected and therefore dominate the resulting color.
- Ambient and object temperature
- Moisture content of the object and its surroundings
- Pollutants or dust deposits on the object
- Properties of the dyes and pigments used
Potential damage
The damaging irradiance and the illuminance at the exhibit have a fixed ratio to one another. This ratio is a measure of the potential harm. It is the key parameter used to describe the damage that could be caused to specific exhibits and materials given a lighting scenario with particular light sources and filters.
The relative potential damage of daylight and lamps listed in Table 2 relates to the effect of the unfiltered light of low voltage halogen lamps (reference value: 100 percent).
| Light source | Without edge filter Filter edge (nm) |
Window glass | ||||
| 380 | 400 | 420 | Single | Double | ||
| Daylight | 235 | 155 | 130 | 110 | 205 | 190 |
| GLS incandescent lamp | 85 | 75 | 70 | 65 | 80 | 75 |
| Low voltage halogen lamp | 100 | 80 | 75 | 70 | 90 | 90 |
| HID metal halide lamp | 220 | 175 | 145 | 110 | 210 | 210 |
| Neutral white fluorescent lamp | 100 | 85 | 80 | 70 | 95 | 90 |
| Warm white fluorescent lamp | 90 | 75 | 70 | 60 | 85 | 85 |
| Cool white LED | 80 | 80 | 80 | 75 | 80 | 80 |
Table 2: Relative potential damage of light sources with and without filters. For wavelengths shorter than the threshold wavelength stated, the edge filters do not allow any radiation to pass. Light with longer wavelengths should be allowed to pass unimpeded in order not to affect the color rendering. The values specified for the relative potential damage are stated for edge filters where in close proximity to the stated threshold wavelength the filter transmittance rises from 0% to the maximum value (approx. 85%). Edge filters can be realized as “dichroic filters”, e.g. by means of high-quality optical glass or coatings.
An object that is exposed, for example, to the light of a halogen metal vapor lamp for 1000 hours at 200 lux with an edge filter at 380 nm (simple UV protection) will experience 180 percent of the damage – i.e. almost twice as much – than with an unfiltered low voltage halogen lamp.
Conversely this means that to attain the same degree of damage, the object can be exposed to the unfiltered light of a low voltage halogen lamp for almost twice as long or with twice the intensity than with the filtered light of the halogen metal vapor lamp.
Pre-exposure to light
Studies show that pre-exposure to light can also play a role in the selection of lighting for an exhibit. Even small doses of effective irradiation damage objects which have never been exhibited or pre-exposed to light, whereas it takes much higher doses to cause the same damage to older, pre-exposed material that has already undergone change.
Many molecular degradation processes gradually slow down and finally even come to a halt. In such cases, it is possible to take account of the pre-exposure time and reduce the light protection measures accordingly. The best way to calculate pre-exposure time is if all irradiation times (and types) are documented. To establish pre-exposure by means of comparison measurements, parts of the object must be unexposed.
Measures to protect against photochemical changes
Where photochemical processes need to be prevented from occurring in the first place, or at least retarded, protection from light means reducing the effective irradiation. In particular, exposure to highly damaging short-wave radiation – especially UV radiation – needs to be reduced or blocked altogether.
There are a number of effective ways of doing this:
- Select a suitable light source: Very sensitive materials should be illuminated with light that is less potentially damaging (cf. Table 2).
- Filter out harmful radiation: If other lamps are to be used or radiation blocked altogether, short-wave radiation can be filtered out. Although halogen lamps for line and low voltage applications are available with integrated UV blockers, this is not enough to meet conservation requirements. Special filters are preferable: glass filters, absorption filters, dichroic filters, plastic lenses or films – the range is wide, appropriate filters are available for every application. They can be used to filter out short-wave radiation up to 380 nm. If the filter also eliminates light in the short-wave blue range up to the threshold wavelength of 420 nm, protection from photochemical changes can be improved without noticeably impairing color rendering. If other wavelengths > 420 are filtered out, this can no longer be ensured. An alternative to light exit filters on lamps is to use display case glass or picture glass that filters out UV radiation.
- Limit exposure to light
See Table 3 for the maximum annual exposure to light depending on the sensitivity of the object. This gives the maximum illuminance for the objects, assuming 300 days open per year and 10 hours on display each day. Since the irradiation, i.e. the product of the irradiance and the irradiation time, determines the degree of damage, the illuminance can be increased accordingly if the annual display time is shorter. Given light levels of less than 50 lux (lx), it will not be possible to properly discern either the color or details of an object. For highly sensitive objects therefore, it is advisable to reduce their exposure to light to 300 hours per year (h/a) rather than reducing the illuminance to below 50 lx.
Exhibits and exhibition spaces should be illuminated for as short a time as possible. Outside opening hours, darkness is best. During cleaning, setting up/dismantling exhibitions and repair work, separate non-damaging lighting is advisable; at least any lighting purely for presentation purposes should remain switched off during such operations and the general lighting should also be dimmed. In many cases, presence sensors are useful for limiting irradiation during opening hours. Timely dimming makes for a pleasant transition between light and dark. A sufficiently long time delay before deactivation should be programmed.
| Material category | Illuminance threshold in lx | Annual display time in h/a | Light exposure threshold per year in lx h/a |
| 1. Insensitive | Unlimited | Unlimited | Unlimited |
| 2. Low sensitivity | 200 | 3.000 | 600.000 |
| 3. Moderate sensitivity | 50 | 3.000 | 150.000 |
| 4. High sensitivity | 50 | 300 | 15.000 |
Table 3: Threshold values for annual exposure to light (in lx h/a) and illuminance (in lx) (from CIE Publication 157, cf. Table 1)
4. Protect from daylight
Effective protection from light also means limiting daylight illuminance. For sensitive exhibits, UV radiation and short-wave light also need to be filtered out. Such protective measures include daylight-limiting systems such as curtains, blinds and UV protective film, for example. These are generally used in combination. Daylight-limiting systems prevent the direct incidence of sunlight and reduce light levels, while films filter out UV radiation while at the same time additionally lowering light levels.
II. Thermodynamic processes
Thermodynamic processes affect organic materials almost exclusively: wood, textile fibers, parchment, leather and others. The thermal load on the exhibit occurs as a result of light and IR radiation being absorbed and mostly gives rise to drying processes. These reduce tensile strength, elasticity and volume, inducing mechanical stresses which cause first the surface, then often the entire object to become deformed.
The physical changes induced by heat radiation are more serious if at the same time photochemical processes occur which are accelerated by the heat and interact with the thermodynamic processes. Even if the temperature and humidity change, e.g. due to light sources being turned on and off, the physical change is also accelerated.
Unlike the molecular change that takes place in photochemical processes which can come to a halt, the thermal load on an object as a result of irradiation always remains harmful. The thermodynamic effects of radiation on an object are determined by its thermal sensitivity and the irradiance, with the radiation absorbed playing a crucial role. For the thermal load on an exhibition room, the key determinant is the product of the luminous efficacy of the lamps and the utilization factor of the lighting installation.
Measures to protect against thermodynamic processes
Measures to protect objects from thermal loading are mostly identical to those for photochemical processes (see above). Effective measures:
- Select a suitable light source: For heat-sensitive materials, only lamps that emit light containing low infrared radiation are suitable. Where low-voltage halogen lamps are used, cool-beam reflector lamps are preferable. The light output by fiber-optic lighting systems and light-emitting diodes (LEDs) does not contain any infrared radiation.
- Filter out harmful radiation with IR filters
- Limit exposure to light
- Dissipate the heat: Even in the case of lamps that produce a luminous flux with a low heat content, luminaires and their immediate surroundings can heat up. This is relevant in display cases, for example. To ensure this secondary IR radiation does no damage, it needs to be dissipated. If necessary, fans can be installed to increase air circulation.
- Protect from daylight
The infrared radiation of daylight is just as damaging as that of lamps so direct sunlight must always be blocked.