Light Guidelines for Museum Display Cases-Wangda Showcases Limited

15 Feb

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


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