Valinda S. Carroll2008

Collection Care:Environmental Factors

Part II: Light

Effects of Light

Light is necessary for exhibits, but excess exposure can lead to many problems

Fading

Yellowing

Weakening and breakage of fibers and polymers

Light Monitoring Tools

Blue Wool standard

Light meter

UV meter

Light data logger

Blue wool

Advantages•Blue wool swatches provide a qualitative assessment of fading.

•They are compact and portable enough to use in frames or exhibit cases.

•A spectrophotometer or colorimeter may be used to detect blue dye fading before it becomes visible to the human eye.

Blue wool

Disadvantages

•Blue wool standard swatches only show fading; they cannot indicate future fading or other types of damage.

•The extent and rate of fading vary depending on the spectrum of the light source. Blue wool standards were designed for use with sunlight, not artificial lights.

Light MeterAdvantages•Can measure output from windows and lamps•Can measure incident light falling on surface of object•Portable•Can be read instantly•Can show lux or foot-candles (1 foot-candle~ 10 lux)

Light MeterDisadvantages•Does not generate permanent record•Requires batteries

Ultraviolet radiation

The upper limit of 75 microwatts per lumen is maximum recommended level of ultraviolet radiation (UV), based on conventional incandescent light bulbs. Typical halogen and fluorescent lights produce high UV levels, so they must be filtered. Sunlight generates high levels of ultraviolet and visible light, so windows must be covered with shutters, window treatments, or filtering film. Fiber optic exhibit lighting usually contains a built-in UV filter.

Ultraviolet radiation is expressed relative to the overall light level in lux. The maximum recommended amount of UV is 0.375 microwatts per square centimeter, measured at a light level of 50 lux (about 5 foot-candles).

Ultraviolet Meter

Ultraviolet (UV) meter showing 9 microwatts per square centimeter, a rather high level.

UV-filtering materials

Ultraviolet filtering materials can be used in windows, light fixtures, exhibit cases, and/or frames

•Acrylic (Plexiglas UF, Acrylite OP, TrueVue Optium)•Polycarbonate (Lexan)•Laminated Glass (Schott Amiran)•Filtering window films•Fluorescent tube sleeves

Exposure Limits

Light exposure limits vary according to the duration of the exposure. The reciprocity rule dictates that a brief exposure at a high level is the same as a longer exposure at a low light level. This exposure is expressed in lux-hours or foot-candle-hours.

Some light-induced chemical reactions may continue in the dark (free radical reactions). This is why high light levels of short duration can cause more damage to light-sensitive items than low light levels of longer duration.

Mitigation Strategies

Store collections in closed boxes and cabinets, rather than open shelves

Close shutters (in historic buildings), shades, or blinds

Use UV-filtering glazing in frames and exhibit cases and UV-filtering sleeves on lights

Hang curtains in front of vulnerable artwork or drape cloth over exhibit cases when museum is closed

Use motion detector switches for gallery lighting

End of Part II

Proceed to Part III