Light Guidelines for Museum Display Cases from Wangda Showcases Limited

Light Guidelines for Museum Display Cases

What Are the Concerns Regarding Light for Museum Display Cases?

Of all the hazards to which items on display are subjected, exposure to light is probably the most serious in the majority of situations. Fortunately, this is a problem that you can control, often with little effort and no money. For example, turning the lights off when no one is in the display area is easy to do, costs nothing, and actually saves money.

Light damages most of the materials from which cultural items are made.
The most obvious damage is the fading of dyes, pigments, and manuscript inks. Less noticeable but equally serious is the degradation of materials, especially textiles, paper, fibers, and feathers, which is greatly accelerated by light exposure.
Any exposure to light, even for a brief time, is damaging, and the damage cannot be reversed. For this reason, exposure to light should be limited, and most items should not be displayed permanently. Display should be for the shortest possible time and at the lowest reasonable light levels. Note that some lights, such as incandescent bulbs, generate heat and should be kept at a distance from
displayed items and out of exhibit cases. Also, items should not be displayed where the sun shines directly on them, even if for only a short time and even if the windows through which the sun shines are covered with an ultraviolet-filtering plastic.

What Are Acceptable Light Levels?

Light levels are measured in two different types of units: lux and footcandles (one footcandle equals approximately 11 lux). For many years, generally accepted recommendations limited light levels for very sensitive materials to no more than 50 lux, and for moderately sensitive materials to 150 to 250 lux, although opinions on these levels varied. In recent years these recommendations have been debated, taking into consideration aesthetic concerns and varying rates of light fading for different materials. Also, it has been recognized that older viewers need higher light levels to discern details than do younger viewers. Ultraviolet (UV) light causes damage more quickly, and all light sources should be filtered to remove UV. This light is measured in units called microwatts per lumen. In general, if a light source emits more than 10 microwatts per lumen, it requires a filter.

Suggested Light Levels

The following guidelines serve as a general rule for items that are on display. There are, however, exceptions. Also, opinion varies regarding appropriate light levels for different materials. Consult a preservation professional if you have any questions.

Materials that are very sensitive to light include textiles, paper, dyed quills, fibers, feathers, fur, and most dyes, pigments, and manuscript inks.Suggested maximum light level for these materials is 50 lux. These materials are found in such items as garments, baskets, drawings, documents, bags, and most everything that has color applied to it.

Materials that are moderately sensitive to light include wood, parchment,leather, bone, ivory, horn, and oil paintings.Suggested maximum light level for these materials is 150–250 lux.These materials are found in such items as utensils, drums, decorations, bookbindings, and some weapons.


Materials that are generally nonsensitive to light include unpainted ceramics, glass, metal, and stone.Usually these materials do not have a suggested maximum light level unless they have a light-sensitive material added to them.These materials are found in such items as utensils, bowls, vessels, and spear points.Suggested maximum ultraviolet (UV) light level for all materials is 10 microwatts per lumen.All exposure to light is damaging for most materials. Staying within these light levels will slow the damage but will not prevent it.

For How Long Should Items Be Displayed?

Even if items are displayed at acceptable light levels, fading, embrittlement, and deterioration will eventually occur if items are displayed for too long. How do you know when this will happen? This is difficult, if not impossible, to determine in advance, so you will need to consider all the relevant factors and make a judgment. Every museum must decide for itself what the maximum display times and total light exposure limits should be for its items based on several factors.


How Do You Decide on Limits?

Factors to consider include the amount of time that lights are turned on in the display area, the light levels in the display area, the light sensitivity of the materials in the item being displayed (calculations should be based on the most sensitive material in an item, not the least), the physical condition of the item, an item’s history of prior display, the desired lifespan of the item, the significance of aesthetic concerns (the importance of seeing details, which requires higher light levels), and the audience (an older audience requires higher light levels to see items well). Begin your consideration by looking at the items you have on display at present. See if you can identify any fading that may already have occurred by turning them over to determine if they are darker or brighter where not exposed to light. Note that the amount of light shining on one item in a display area will not necessarily be the same for other items in the display area. The amount of light on each item should be measured, and the position of the items on display adjusted accordingly.


What Are Lux Hours?

Some museums track total light exposure in terms of lux hours, which take into account both the intensity of exposure and the number of hours of exposure. The number of lux hours is obtained by multiplying the light levels (in lux) of the light shining on an item by the number of hours the item is exposed to this level of light.

The more intense or bright the light, the shorter the display times should be. Limited exposure to a high-intensity light will produce the same amount of damage as long exposure to a low-intensity light. For example, if the exposure time is kept the same but the intensity of illumination—the light level—is halved, the resulting damage will be halved (100 lux 50 hours = 5000 lux hours, whereas 50 lux 50 hours = 2500 lux hours). This relationship, referred to as the law of reciprocity, is helpful in deciding on light levels and the length of display time. Some museums have settled on an annual light exposures ranging from150,000 lux hours for very sensitive materials to 450,000 lux hours.


How Do You Measure Light Levels?

The easiest and most accurate way to measure light levels is with a light meter. Note that not all meters measure UV, and you will want to obtain one that does if possible. Light meters are expensive, ranging from several hundred to several thousand dollars, and not many museums have them. The ones that do will sometimes loan them by mail. Also, some regional conservation centers have light meters available for loan by mail. If you cannot obtain a light meter, you can measure non-UV light using a 35mm single-lens reflex camera with a built-in light meter. For instructions on how to do this contact a preservation professional.

Two inexpensive tools are available that will enable you to estimate the possible damage that could occur in your display area. One is a Blue Wool standards card available from conservation suppliers. These cards can be cut into vertical strips to increase the number of them for use. Cover half of the card lengthwise with a light-blocking material, and then place it in a display case or in the display area. Periodically remove the light-blocking material and compare the two halves of the card to see the amount of fading that has resulted from the light. This will give you a general indication of how serious your light exposure problem is.

How Can You Minimize Light Damage to an Item?

Decide on an acceptable exposure time and light level for an item, and do not exceed them. If you have reached the limits for a particular item, one solution practiced in many museums is to take that one item off display and replace it with another. This practice of rotation is a commonly employed strategy for limiting the damage to an item while maintaining the integrity of the display. It requires, however, that other similar items suitable for the display are available and that staff have the time to carry out the rotation (selection and preparation of the replacement item, modification of display label text, removal and cleaning of the rotated item, record keeping to track this change). Rotation works best when exposure histories in lux hours are kept for each item so you know when to rotate one item on display with another. These histories are referred to as lux logs by some museums.

Another method of minimizing light damage is to illuminate an item on display only when a visitor comes to see it. Lights can be controlled by motion sensors to go on when the visitor’s presence triggers the sensors. Alternatively, the visitor can push a button to activate a light when he or she wants to see the item. The simplest and least expensive way to minimize light damage is to cover display cases with a light-blocking cloth that the visitor lifts when he or she wants to view items in the case.

Finally, camera flashes are no longer considered a light problem unless you expect specific items on display to be photographed frequently, in which case you may want to prohibit flash photography. Extended use of photographic or video lighting can cause damage, so you may want to restrict this, explaining to visitors that this restriction is due to the light sensitivity of the items.