When considering what light levels are acceptable, we need to take into account the brightness or intensity of the light, as well as the length or duration of exposure to the light. If your museum is only open one day per month and in darkness the rest of the time, the items on display will not receive the same level of light as items on display in a museum open seven days a week, which has windows that allow in daylight. This may sound obvious, but our point is that both the intensity of the light (in lux) and the length of exposure are relevant.
The other key point to remember is that objects should not remain on display indefinitely, or for extensive periods of time. Depending on how much light items receive when displayed in your museum, you should consider rotating light sensitive objects and changing displays every three, six or twelve months. Where this is not possible, we recommend you investigate ways to reduce the total amount of light objects receive. There are many ways this can be achieved, and these will vary with specific circumstances. A few examples may incorporate keeping window curtains closed when the museum is closed, using room lights only when visitors are actually present, and using curtains or cover sheets on display cases housing sensitive items, which visitors can lift to view the display and replace when finished.
Very Light Sensitive Materials
For very sensitive materials, it is generally recommended that light levels be as low as possible. The recommended industry standard is 50 lux. In the past, this was considered quite an acceptable level of light for normal viewing. Since the advent of fluorescent lighting, however, the trend has been towards more brightly lit situations. For instance, the recommended light levels for ease of reading at a desk in a library is 500 lux. 50 lux, therefore, is now considered quite a low level of light which, especially where unfiltered daylight is present, can be difficult to achieve. With careful planning to remove glare and sharp light contrasts, however, it can be achieved and objects can be viewed quite comfortably.
Visitors will potentially have difficulty when they enter a dimly lit gallery area direct from outdoors or a brightly lit space. This can cause some discomfort and perhaps a feeling of disorientation, and accidents have been known to occur as a result, so keep this in mind in your planning. Visitors’ eyes will need time to adjust. If available, a corridor and vestibule through which the visitor can pass will assist them to comfortably move from the brightly lit outdoors into the gallery space.
Another factor to consider is the reading requirements of visitors. Small printed labels can be difficult to read in low-lit spaces.
Remember, you need to consider both the intensity of the light and the duration of exposure. Displaying sensitive items for shorter periods of time will reduce the total light to which they are exposed and therefore the light damage they suffer. In theory, this means that if a sensitive object can be displayed at a 50 lux light level for twelve months, an equivalent level of exposure would be a 100 lux light level for six months, that is, double the light for half the time.
Moderately Light Sensitive Materials
For moderately sensitive materials, 200 lux is the generally recommended light level. This can be readily achieved with artificial lighting, careful planning and diffusing of natural light.
Non-light Sensitive Materials
Materials which are not sensitive to light damage do not in themselves need careful control. Viewing of adjacent items, however, can be compromised if these items are lit at higher light levels than the surrounding area. It is important to consider all components of an object when determining suitable lighting conditions. For example, a telescope may be made from brass, which is not affected by light, but the leather components of the casing or focusing ring, are highly sensitive and affect the light levels under which this object should be placed.
As we mentioned above, the UV component of light is not visible to the human eye, it is the component of light with the highest energy and is therefore the most damaging to collections. Just as we try to stop the UV in sunlight from reaching our skin by using sunscreen lotions, so it is strongly recommended that UV be eliminated from the museum environment by placing filters on windows and on light sources, such as fluorescent tubes, that emit UV.
The industry standard for the maximum amount of UV radiation in museums is