Light
Principles
Light is an energy form capable of damaging materials such as organic fibres (textiles and paper), watercolour paintings, dyed materials, coloured leather, botanical specimens and colour photographs.
The human eye can detect only those wavelengths of light that make up the visible spectrum (rainbow colours). Unseen components of light such as ultraviolet (UV) and infra-red (IR) light can also severely affect particular material types (Figure 1).
Figure 1: Light region of the electromagnetic spectrum.
Ultraviolet (UV) Radiation
(10-380 nm)
As wavelengths below 300 nanometres (nm) do not penetrate the earth's atmosphere and glass cuts out UV radiation with wavelengths of less than 320 nm, the UV band of concern is 320-380 nm. This shortwave, invisible UV radiation is highly energetic and is most likely to cause photochemical deterioration. There is a high proportion of UV radiation in daylight and a significant amount is emitted by fluorescent tubes.
Visible Light
(380-760 nm)
Visible light affects an object in two important ways - it can lead to its deterioration and it can affect the way an observer perceives an object.
Deterioration is caused not only by high energy light at the blue end of the spectrum but also by heat produced by lower energy red light. Exposure to brighter, more intense light increases the rate at which light-induced damage occurs.
If colour-corrected light is not used for viewing, a colour shift may occur that human eyes cannot perceive. Examples of this include the apparent change in colour when red meat is removed from specially lit butchers’ displays (metamerism) into the ordinary light of their stores and the yellow tinge imparted to objects by a dimmed incandescent globe. Fluorescent tubes on the other hand, are an excellent source of accurate colour-rendering light. Unfortunately this same light is capable of causing significant photochemical damage to susceptible objects.
Infra-red (IR) Radiation
(760 nm upwards)
IR radiation can cause objects to heat up and accelerate chemical deterioration processes. It is also capable of producing changes in relative humidity levels. Under conditions of reduced relative humidity objects can become brittle, while increased relative humidity will increase corrosion of metals. Such changes in relative humidity correspond to the IR source being turned on and off respectively.
Light Measurement
The human eye responds well to green-yellow light but less so to the blue and red ends of the spectrum. Both UV and IR radiation are invisible to the human eye. Light meters and cameras have been designed to measure visible light in the same way that the eye perceives it.
The brightness or intensity of light is measured in units called lumens. One lumen every square metre is equivalent to one lux. When measuring light intensity the meter must face the direction of the light source. To determine the intensity of light impacting on an object, place the meter directly in front of the object, as close to the surface as possible and parallel to the object’s surface.
UV radiation may be measured either as a proportion of visible light, with units of microwatts per lumen (µW/lumen) or more usefully as a direct measure of actual UV exposure, with units of microwatts per square metre (µW/m2).
IR radiation is not usually measured, with its effect most easily detected by monitoring associated temperature changes.
Advances in technology have led to the development of light and UV monitoring devices that can take either one-off, instantaneous measurements or, by the use of meters with data logging capabilities, can monitor these parameters over extended periods. Data logging instruments allow monitoring of light variations that occur during the day and as the angle of the sun alters from season to season. Use of these instruments to determine light exposure profiles aids in the long-term management of light sensitive objects.
While specialist equipment is available to monitor light and UV levels, the brightness of visible light can also be measured easily using any camera that has a built-in light meter (see Appendix 1).
If, as is often the case, light and UV monitors are not readily available, it is important to be aware of the approximate light and UV radiation levels generated by various light sources. Note that the actual levels of exposure are dependent on the distance between the light source and the exposed object, with increasing distance resulting in reduced light and UV levels.
| Examples of illumination (visible light) levels: | Lux |
|---|---|
| Daylight, fluorescent lights in an office or laboratory | 600-1500 |
| Daylight coming in through a window | 35,000 |
| Direct sunlight | up to 136,000 |
| Examples of UV from various sources: | µW/lumen |
|---|---|
| Fluorescent lights (low UV) | 33 |
| Fluorescent lights (general) | 80-250 |
| Tungsten globe | 60-80 |
| Direct sun | 400 |
| Light overcast sky | 800 |
| Blue sky | 1600 |
If the light intensity (lux) and the UV radiation levels (µW/lumen) are known then the actual UV exposure (µW/m2) can be calculated by multiplying these two values together. For example, a fluorescent tube with a light intensity of 800 lux and a UV output of 100 µW/lumen would be exposing an object to UV levels corresponding to 80,000 µW/m2 (= 800 lumens/m2 x 100 µW/lumen).
Another important aspect of light measurement relates to the measurement of colours in objects. Although this requires the use of specialised equipment (Chroma Meters or colorimeters), it provides an accurate way of quantifying changes that may occur in coloured objects, paintings and prints. In this process, measurements taken at strategic points are repeated after a period of light exposure. Any changes in colour will be detected and can be related to the environmental conditions to which the object was exposed. Chroma Meters range in price and quality and it is possible to use one instrument to measure chromaticity, colour difference, correlated colour temperature and the luminance of light sources.
Light-induced Deterioration
Light has many effects, the most noticeable of which are colour changes induced in objects. For instance the bleaching of ivory, the fading of pigments, dyes and inks and the discolouration of wood, varnishes and lacquers are all due to light exposure (Figure 2).
Different materials react to light in different ways. Some materials fade while others darken. Some types of wood yellow, others bleach and some turn grey when exposed to light. A more subtle effect can be observed in colour photographs or water colours where there may be a colour shift when one of the more light-sensitive dyes is affected by light.
In addition to inducing colour changes in objects, exposure to light is also responsible for weakening fibres in textiles and paper.
Light has the potential to damage organic materials whether they are in pure form or make up part of a composite object. Natural organic materials such as wood, fibres and biological specimens are all vulnerable, as are modern polymeric materials (plastics) and even metal objects which contain an organic component such as paint, lacquer or an inlay. The inlaid ivory handle of a hand gun for instance, would be susceptible to light damage.
For composite objects the most sensitive component must be considered when planning for display and/or storage. The relative sensitivities of different materials may be gauged by comparing the recommended maximum lighting levels for these materials (see below). Expose the most sensitive objects to lower light levels to minimise photochemical damage.
Reciprocity Rule
The same amount of damage will be produced by strong light in a short time as will be done by a weaker light over a longer period. For example, 500 lux exposure for 10 hours will cause an equivalent amount of deterioration to 50 lux exposure for 100 hours.
Monitoring Deterioration
The impacts of lighting conditions can be monitored on site using light dosimeters such as blue wool scales or the recently developed LightCheck®. A blue wool scale is a small card comprising a series of dyes of differing light fastness (Figure 2). Half of the card is covered and it is then placed in the area in which lighting impacts are to be determined. After a fixed exposure period, the blue wool scale is removed and the cover removed from the card. A comparison can then be made between the exposed and unexposed dyes and a clear indication gained as to the likely damage that will occur to light sensitive objects exposed at the test site.
Figure 2: A blue wool test strip where the left side of the strip was covered while the right side was exposed to full sun for 30 days (Copyright Catharine Ellis).
An alternative system, LightCheck®, can be used in a similar fashion. A LightCheck® strip is placed next to an object on display and regularly inspected. The extent of light damage is determined by comparison with a colour scale that can then be related to an equivalent exposure level. LightCheck® strips are available in two formats, one for shorter duration and testing in areas holding very light sensitive objects and the other for areas in which less sensitive objects are exposed. The LightCheck® strips are considered to be more sensitive to light exposure than the blue wool scales.
Recommended Maximum Light and UV Levels
These values are set so that objects can be viewed while minimising the risk of photochemical damage. Note that the UV levels recommended below correspond to 30 µW/lumen for highly sensitive objects and 75 µW/lumen for sensitive and insensitive objects.
Highly Sensitive Objects - 50 lux and 1500 µW/m2
Textiles, costumes, watercolours, tapestries, prints and drawings, manuscripts, miniatures, distemper in frescoes, wallpapers, gouache, dyed leather, most natural exhibits, botanical specimens, fur and feathers.
Sensitive Objects - 200 lux and 15,000 µW/m2
Oil, tempera and acrylic paintings, undyed leather, horn, bone, ivory, most wood, oriental lacquer and other painted or coloured objects.
Light Insensitive Objects - 300 lux and 22,500 µW/m2
Stone, metal, glass, ceramic, jewellery, enamel and wooden objects that have largely been used outdoors or have otherwise lost their natural colouring.
Recommended Overall Exposure Levels
When developing a lighting strategy it is essential to consider overall exposure levels and not just maximum light and UV levels. Overall exposure levels can be determined by multiplying the actual light exposure levels by the number of hours for which objects are exposed. For example, an object exposed to an average 50 lux for 8 hours a day, 300 days of the year would have an overall exposure of 50 x 8 x 300 lux hours (= 120,000 lux hours or 120 klux hours). For highly light sensitive objects, Thomson (1986) proposes overall exposures of 200 klux hours, for sensitive objects 650 klux hours and no overall limit for light insensitive objects. For these latter objects overall light levels will be largely determined by the environmental requirements of nearby objects.
Determining overall exposure levels is not easy however and may require extensive data logging. If daylight impinges on display spaces, consultation with local meteorological offices or educational institutions may be necessary to determine daylight availability. Daylight availability varies with the changing seasons due to the duration of available light and the angle of the sun.
Controlling Light Levels
Of all the agents that contribute to the deterioration of cultural materials, light is probably the easiest to control. The problem is achieving the right balance between minimising photochemical damage by limiting light levels and still maintaining appropriate colour rendering of objects and allowing objects to be viewed without reducing surface details. Loss of object detail can be a significant problem for older visitors to museums.
Light levels may be controlled by using some or all of the following strategies:
- exclude all daylight;
- place the most light-sensitive objects furthest from light sources;
- use low UV light sources;
- use UV filters over light sources or in display cabinets;
- alternate sensitive materials between display and low-light storage;
- use partitions to create ‘shaded’ areas;
- use copies for display; and
- use automatic light switching.
Ideally, all daylight should be excluded from storage and display areas. This has three beneficial effects. It reduces overall light levels significantly, it allows the intensity of interior light and the amount of UV exposure to be controlled and also gives greater opportunities for creative lighting effects to be introduced.
Light control may be achieved by using appropriate artificial lighting sources such as low UV fluorescent lights and arrangements such as reflected rather than direct lighting. Whatever the light sources and arrangements, the end result must be conditions that meet the required specifications for the illuminated objects and allow the objects to be viewed satisfactorily.
Options to eliminate daylight include the use of blinds, shutters, curtains and even paint. While UV-absorbing films and tints can also play a part in reducing overall light and UV levels, the former methods are preferred. It is obvious when curtains, blinds and shutters have deteriorated to the point that they are no longer effective in reducing light intensities and need to be replaced, but it is often more difficult to determine when UV absorbing films are no longer performing to their original specifications. Neutral density films can also be applied to windows. These will reduce the overall light transmission and, as long as they are applied to all windows in a particular space, will create the impression that the windows are all clear.
As the intensity of light is reduced with distance from a light source, place the most light-sensitive objects furthest from it.
The same result may be achieved by bouncing light off a reflecting surface to create a diffuse effect. The intensity of the light is reduced because the path length of the light from the source to the object is increased, some of the incident light is absorbed and some is scattered by the reflecting surface. Depending on the nature of the reflecting surface, this is an excellent way of not only reducing light intensity but also of reducing UV levels in the reflected light and creating interesting lighting effects. When daylight is reflected from white walls, usually containing titanium dioxide pigments, approximately 80 % of the UV radiation is absorbed during each reflection.
Options to reduce the impact of UV radiation on objects include the use of low UV fluorescent tubes, installation of UV absorbing covers or sleeves over light sources and the use of UV absorbing perspex or UV absorbing films in display case construction. The use of a diffuser over a fluorescent tube for instance, will usually reduce the UV radiation to acceptable values. This is because most diffusers have UV absorbing chemicals in them to prevent their deterioration when in use.
Light exposure can also be reduced by adopting appropriate management strategies. Material that is particularly light sensitive may be alternated between display and storage. To cut light exposure by half in an historic house for example, only open half of the rooms for six months of the year and the other half for the following six months. Similarly, if a diary or register is displayed open, then rotate the exposed pages.
Another alternative is to use lights that are activated when someone enters the collection area room and which remain on for a limited period (automatic light switching).
Note that the importance of an object as well as its sensitivity to light damage must be considered carefully before remedial measures are taken. Occasionally, maximum light levels are insufficient to show artefact details adequately. This problem may arise when general light levels in the vicinity are too high, making the artefact at say 50 or 200 lux, relatively dark. Decreasing general light levels in the vicinity of the artefact will assist in overcoming this problem.
If the light on an artefact must be increased above recommended levels then reduce the display time proportionately. For example if the recommended light level is 50 lux and the actual level is 200 lux then put the object on display for only three months per year and keep it in dark storage for the other nine months.
If appropriate light levels cannot be established then make copies of particularly sensitive or significant photographs, prints and similar objects. The copy may then be put on display and the original safely stored.
Artificial Light Sources
There are many factors to be taken into account when considering the use of artificial light sources. These include:
- the light source itself;
- how the light gets from the source to the object; and
- maintenance and control systems.
When choosing a light source, factors to be considered include the ability of the source to render colours accurately, its colour temperature and its UV output. The standard measure for colour rendering is the colour rendering index (CRI) with daylight being given a CRI of 100. At present, tungsten halogen lights, with a CRI of 99 and some (not all) fluorescent lights with CRIs in the 90s are the first choices for museum lighting systems.
Note that light sources should not be in enclosed spaces with objects. Heat generated by the ballast of fluorescent lights, by tungsten lights themselves or by the motors of fibre optic systems can build up to damaging levels.
Light emitting diode (LED) technology is developing rapidly and it is envisaged that these light sources will become much more widely used in museum displays and in general spaces. LEDs can be combined to give a full colour spectrum, are UV free, use less energy than traditional halogen sources and because of their greater longevity, reduce on-going maintenance and costs.
While a colour temperature of 3000 K has been recommended for museum light sources, the human visual system responds to illumination in a discriminatory fashion. It has been demonstrated for instance, that the colour temperature of light sources preferred by observers varies according to the dominant colours in the object being viewed. Because of this it is strongly recommended that the same type of light source be used in areas that can be viewed simultaneously.
The most usual way of getting light from the source to the object is via a reflector, with most light sources sitting within a reflector in a light fitting. The type of reflector will affect the shape of the beam and will also control any stray light. Reflectors should be cleaned regularly.
Consult lighting specialists if more sophisticated delivery systems, including those using diffusing or ribbed glass lenses and fibre optics are considered for lighting displays. Glass fibre optic systems have some particular advantages in museum lighting, allowing the distribution of light from one lamp over a greater area, the placement of the lamp at a distance from the lit object and a natural reduction in the UV content of the light along the way. They are not a cure-all however, simply another tool to be considered.
Lighting systems should be designed so that maintenance and control are facilitated with fittings suitable for extended use and for easy lamp changes. Centralised controls and dimming facilities can allow for better overall control over light levels and the transition between differently lit areas. It is worth noting however, that strong dimming of tungsten halogen lights reduces their CRI.
As there are a wide variety of light sources available and lighting technology is continually developing, consultation with manufacturers and suppliers is recommended so that the latest and most appropriate information can be obtained.