Valuable UV Information
Ultraviolet (UV) radiation is invisible to the naked human eye and is generally considered to be that radiation which has a wavelength of less than 400 nanometers (nm) and greater than 100 nm. Ultraviolet radiation is part of the electromagnetic spectrum between visible light (which is above 400 nm) and X rays (which are below 100 nm). Ultraviolet radiation is divided into three bands: UVA (315-400 nanometers), UVB (280-315 nanometers), stronger radiation that is the common cause of sunburn; and UVC (280-100 nanometers), the strongest and potentially most harmful form which is used to sterilize surfaces because it kills bacteria and viruses. There is a much higher proportion of UV energy in daylight radiation (natural sunlight) through glass than in incandescent, halogen, or fluorescent light sources shining through glass. Since UV radiation can cause certain unwanted effects in materials, it's important to filter it through the use of protective UV light fixtures.
Measurement of UV Radiation
The typical light meter used to measure illuminance in foot-candles (fc) will not give a good indication of the amount of UV radiation that is present. A standard device that measures UV radiation is the Crawford UV Monitor manufactured by the Littlemore Scientific Engineering Co. in Oxford, England. UV radiation is measured in microwatts of UV radiation per lumen (µW/lm). This is useful for checking a particular lamp or window because the proportion of UV radiation does not change with the distance from the light source. Traditionally, conservators have recommended that if a museum light source emits more than 75 µW/lm of ultraviolet radiation, it requires a UV absorbing filter. Newer recommendations by the staff at the Canadian Conservation Institute suggests eliminating UV radiation completely, aiming for under 10 µW/lm through the use of window films, UV absorbing light covers, and UV absorbing filters.
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Fabrics and paper become brittle, fade in color, or turn yellow when exposed to sunlight. A good example is that of street maps or magazines left in a car below the rear window. Printed paper articles left there often change noticeably within a few weeks. Those changes are mainly due to exposure to the UV radiation in sunlight. Natural high polymers from cellulose and protein are decomposed by solar radiation with a high percentage of UV light. Cellulose fibers, from which paper is made, are particularly sensitive to UV radiation. In conjunction with water, which is always present in the fibers, and under UV rays, atmospheric oxygen reacts with the water to form hydrogen peroxide. This results in a gradual oxidative breakdown of the cellulose. This breakdown is the main factor causing decomposition of cellulose in light. Consequently, because oxygen and moisture cannot be eliminated, every precaution has to be taken to prevent the exposure of valuable articles of paper or cellulose fiber to UV radiation. Similar reactions can be seen in some pigments causing colors to fade. Photochemical damage (damage due to chemical change by radiation) of a fabric or a painting is related to the wavelength of the radiation striking it. Ultraviolet (UV) radiation causes more damage than the same amount of visible blue radiation, which causes more damage than the same amount of visible yellow radiation, which causes more damage than the same amount of visible red radiation. If a "Probable Relative Damage" (PRD) factor is assigned for different wavelengths, the shortest UV rays would rank highest and the factor would decrease exponentially with increasing wavelength. For example, the PRD factor for UV light of 300 nm would be approximately eight, for UV light of 380 nm, one, and for visible green light of 500 nm, .03. This illustrates the importance of filtering out UV light to protect valuable materials. Photochemical damage can occur in organic materials, textiles, watercolors, paper, oil paintings, leather, and glue just to name a few. Both UV and visible radiation can cause color change and surface deterioration. Therefore, it is not sufficient to remove only the UV radiation; it is also very important to minimize visible radiation. Damage from light is cumulative and permanent; its effect on artifacts cannot be reversed by subsequent storage in darkness.
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In general the same amount of photochemical damage will be produced by a strong light in a short time or a weak light in a long time. If illuminance is measured in foot-candles (fc) and the time in hours, 10 fc on a painting for 5 hours gives it an exposure of 50 foot-candles-hours and 5 fc for 10 hours would give it the same exposure. In reducing damage by light the most effective strategy is clearly to reduce both illuminance and the time of exposure. Note that the Reciprocity Law does not state that twice the exposure will cause twice the amount of fading. Rates of fading commonly decrease with time, until there comes a point when no more fadeable material is left.
How To Reduce UV Damage
To minimize photochemical degradation the amount of natural illumination (sunlight) must be minimized if not eliminated altogether. In addition, the amount of artificial illumination should be minimized to no more than what is necessary for proper viewing and the time of illumination should be minimized as much as possible. To those who are viewing the exhibit the quantity of light in an exhibition space seems to be less important than the balance of illumination levels between the works and the remaining space. In general, 10-25 fc of illumination is probably adequate for the perception of detail in the works of art being shown. Remember, good viewing of exhibits is not determined solely by the amount of light incident upon the piece. Other factors play a role as well - especially the amount of glare present. Visible light levels should be kept at a minimum. Usually 5 fc is recommended for watercolors and textiles; 15 fc is recommended for oil paintings, wood, and leather. Use UV absorbing materials such as UV glass filters, UV absorbing tube guards, UV absorbing, prismatic, acrylic light fixture diffusers, and/or UV absorbing plastic film. It is essential to re-check the adequacy of these filters (especially the plastic filters) periodically since they often deteriorate over a period of years. White paint that contains titanium dioxide is a fairly good absorber of UV radiation. Therefore, white paint might be used to control the amount of UV radiation in a given space.