TSG Chapter III. Environmental Concerns for Textiles - Section C. Light
Originally drafted by Sarah C. Stevens. Contributions from: Mary Ballard, Lucy Commoner, Shirley Ellis, Robin Hanson, Irene Karsten, Richard Kerschner, Teresa Knutson, Anne Murray, Zoe Annis Perkins, Patty Silence, Jan Vuori.
Editors: Kathy Francis, Mary Kaldany, Nancy Love, Nancy Pollak, Deborah Lee Trupin. Copy Editor/Layout Consultant: Jessica S. Brown. Original content date: April 2, 1998.
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Definition: Light is electromagnetic radiation in the wavelength range including infrared, visible, ultraviolet, and x-rays, and traveling in a vacuum with a speed of about 186.281 miles/second (299.790 kilometers/second). (Webster's 3rd Ed, 1308)
Factors to consider
- Types of light: Properties, problems, and benefits
- Natural light
- Visible light: most people’s eyes perceive visible light at wavelengths between 400 and 700 nanometers. Visible light is damaging to textiles, but some is required to see textiles.
- Ultraviolet (UV) light or radiation occurs at 300–400 nanometers.
- UV radiation is the most damaging part of the radiation spectrum for textiles and other organic materials.
- UV radiation has some diagnostic and treatment uses for textile conservators.
- Controlled exposure to UV radiation has been used as a bleaching method by conservators.
- Examination with UV radiation can reveal fluorescence of optical brighteners and certain dyes and pigments, which aids material identification, or of microorganisms.
- Infrared (IR) light or radiation: 760–780 to 1,000,000 nanometers
- IR radiation can damage textiles and other organic materials. The heat that it produces can have a negative impact on RH and can accelerate chemical reactions.
- IR light can be useful in examination and documentation of textiles.
- Artificial light
- Incandescent: low UV, but high IR (heat), relatively costly to use, relatively short bulb life, may have poor color rendering index (CRI) and variable color as the bulb ages.
- Fluorescent: low IR, but high UV, relatively inexpensive to use, relatively long lifespan.
- Quartz-halogen: high IR, relatively high UV, typically high intensity, relatively costly to use, relatively long bulb life, with a lifespan about twice as long as normal incandescent light.
- Fiber optic: Source can be filtered to eliminate UV, no IR, remove the heat source from the vicinity of the textile, expensive to purchase and install, long lifespan.
- LED: Can be manufactured to produce minimal or no UV, minimal IR, are moderately expensive initially, long bulb life, use very little energy. LEDs are a new and developing technology for use in exhibit cases; issues to consider include color rendering and bulb output over the lifespan.
- Effect of light on textile preservation
- For many collections, light exposure is one of the most damaging environmental factors.
- Light damage is a combination of exposure time, wavelength, and intensity.
- Damage from light is irreversible.
- Damage from light is cumulative.
- While light in the UV range causes more damage than that in the visible range, all light is damaging to textiles. Light in the blue range (400–500 nanometers) is also particularly damaging.
- Light damage can have many forms, both visible and not.
- Fading of dyes
- Discoloration (yellowing/browning) of fabrics and yarns
- Embrittlement and loss of strength (silk and nylon are the most susceptible to this type of damage)
- Some finishing or dyeing processes make fabrics more sensitive to light damage.
- Effect of light on viewing textiles
- Need to balance amount of light needed to see object versus damage the light will cause to the textile object over time.
- Each type or source of light has different qualities and color rendering accuracy (e.g., cool or warm), so selection of lighting type is also an aesthetic decision.
- Although interior lighting in cases may improve the visibility of textiles, heat-producing interior lighting (including fluorescent lights) can raise the interior temperature of the case unless properly vented.
- Heat from light source can create hot spots with fluctuating temperature and RH. (See above, III.A. Temperature and Relative Humidity.)
Monitoring light levels
Light meters measure light levels at the moment they are used. They can also be used to set light levels and then to check constancy of those levels.
Light meters need to be calibrated regularly. If multiple meters are being used (such as by a lighting designer and a conservator), they should be checked against each other.
Meters are also available to measure UV and IR.
- Devices that measure or record light exposure and/or effects of light exposure over time
- Light dosimeters (commonly called lightfastness cards): small cards placed beside artifacts on display.
- They are made with a photosensitive dye/polymer mixture that progressively changes color with increased light exposure, thus they are highly sensitive to light, permitting use as an early warning system for light-sensitive materials.
- Blue wool standards
- A set of blue wool fabrics specially dyed to give a range of lightfastness from low (no. 1) to high (no. 8); each higher numbered standard is twice as lightfast as the preceding one.
- Not as sensitive as light dosimeters: BWS no. 1 requires a light dose of about 100 lux-hours for noticeable fading.
- May be placed in display settings or used for experimental research.
- Data loggers: Electronic monitoring cells that record cumulative visible or UV light dose (lux-hours) at the particular location.
- Colorimeter/spectrophotometer: instrument used to measure color
- Measurements taken before and after light exposure can indicate changes in color from fading or discoloration
- Proper calibration and use of instrument and proper documentation of readings is crucial to obtain meaningful data.
Limit exposure by controlling cumulative exposure dose, or lux hours (intensity multiplied by duration in hours).
- Maintain light levels at 5–10 foot candles (50–100 lux).
- Avoid direct sunlight.
- Use shades, filters, or awnings on windows to reduce amount of incoming light.
- Keep exhibit areas dark except when they are open to the public.
- Limit the length of time that any textile is on display; develop a rotation schedule for long-term gallery installations.
- Use visitor-activated lighting or curtains for particularly light-sensitive objects.
- Keep storage areas dark except when they are in use.
- Limit amount of UV exposure for textiles on exhibit.
- Eliminate UV radiation with filtration or reduce by using only reflected light bounced off another surface, which has a much lower UV content.
- Use UV absorbing sleeves on fluorescent bulbs or purchase bulbs with built-in UV filtration.
- Use UV absorbing films, acrylic sheets, or shades for windows, cases, and/or frames.
- Establish a replacement schedule for UV filtering/absorbing materials.
Choose a lighting system that will cause the least damage.
- Light cases from outside to reduce heat build-up inside cases.
- If lights must be placed in cases, place them in a “light attic,” separated from the textiles, and vent the case.
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