PMG Section 1.4.1 Standards, Guidelines, and Recommendations for Light Levels During Exhibition

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Photographic Materials Conservation Catalog
Exhibition Guidelines for Photographic Materials

Date: July 2004
Contributor to WIKI version: Your name could be here!
Compiler: Stephanie Watkins, 1993-2004
Initiator: Douglas Severson, 1992-1993
Contributors (Alphabetical): Catherine Ackerman, Nancy Ash, Sarah Bertalan, Jean-Louis Bigourdan, Barbara N. Brown, Ed Buffaloe, Carol Crawford, Corinne Dune, Thomas M. Edmondson, Debra Evans, Julia Fenn, Betty Fiske, Gwenola Furic, Judy Greenfield, Doris Hamburg, Marc Harnly, Pamela Hatchfield, Cathy Henderson, Nancy Heugh, Ana Hofmann, Emily Klayman Jacobson, Martin Jürgens, Nora Kennedy, Daria Keynan, Lyn Koehnline, Barbara Lemmen, Holly Maxson, Constance McCabe, John McElhone, Cecile Mear, Jennifer Jae Mentzer, Jesse Munn, Rachel Mustalish, Douglas Nishimura, Leslie Paisley, Sylvie Pénichon, Hugh Phibbs, Dr. Boris Pretzel, Dr. Chandra Reedy, Nancy Reinhold, Andrew Robb, Grant Romer, Kimberly Schenck, Douglas Severson, Tracey Shields, Angela Thompson, Sarah Wagner, Clara von Waldthausen, Dr. Mike Ware, Stephanie Watkins, Dr. Paul Whitmore, Faith Zieske, Edward Zinn.

First edition copyright: 2004. The Photographic Materials Conservation Catalog is a publication of the Photographic Materials Group of the American Institute for Conservation of Historic and Artistic Works. The Photographic Materials Conservation Catalog is published as a convenience for the members of the Photographic Materials Group. Publication does not endorse nor recommend any treatments, methods, or techniques described herein.

Standards, Guidelines, and Recommendations For Light Exposure During Exhibition[edit | edit source]

Light exposure: Levels, duration, and wavelength of light [edit | edit source]

Currently, there is no agreed upon standard for illumination type, illumination level, or duration of exhibition.

History of standards[edit | edit source]

The standards prevalent in the conservation literature and within institutions follow established guidelines for light sensitivity of media (pigments, inks, dyes) and supports (paper, animal skins, textiles, metals) obtained through research in other disciplines. It is accepted opinion that photographic materials are sensitive to light exposure and environmental factors, as are composite objects. Research specifically on photographic materials has increased in the last few decades in the private sector in settings not attached to major manufacturers of photographic materials.

Current guidelines and recommendations for light exposure levels[edit | edit source]

  • Light exposure should be kept to the minimum level that will both safely illuminate the photograph and provide adequate visibility to the viewer. Ultraviolet radiation should be reduced as much as possible. A maximum limit of 75 microwatts per lumen is accepted in exhibit lighting for general collections material. Ultraviolet radiation will vary by light source. The appropriate light source will depend on the material displayed. The type of light, spectral distribution, intensity (illuminance level), duration, and distance of illumination (heat from the source) are all factors to be considered when exhibiting photographic materials. Accuracy of instrumentation is critical to monitoring. It is unrealistic to set any limits that cannot be measured with available equipment.
  • Organizations such as the American National Standards Institute (ANSI) and the International Standards Organization (ISO) regularly issue standards for the display and storage of currently produced photographic materials. As these complex systems become better understood and manufacturing processes become more refined, standards may change. Standards are based on newly produced materials. Despite similar chemistry, aged materials may respond differently to external conditions. It may not be suitable to use a standard based on current photographic materials when displaying historic, aged materials.
  • The National Information Standards Organization (NISO), Committee MM, drafted a standard for the exhibition of library and archives materials (Z39.79-1999) that included photographic materials. The NISO standard recommends a general light exposure of 4.6­ - 9.3 fc (50 -100 Ix) with reduced ultraviolet radiation and limited to 12 weeks exposure per year, with reduced light levels when exhibition areas are not in use. To obtain current standards, please refer to the standards organizations listed in section 1.8 for contact information.

Institutional guidelines and recommended standards [edit | edit source]

The following information was gathered by an informal survey beginning in 1995, augmented by compilers to provide references. Please consult individual institutions as necessary for current guidelines and recommended standards. Please note that 1 footcandle (fc) = 10.76 lux (Ix). For ease, footcandles are often multiplied by 10 for lux. Light sources have below 50 microwatts per lumen for these guidelines.

Institution (In house candle publication date) Print Type Footcandle Lux Considerations Duration
National Gallery of Canada --- 5 - 15 53.8 - 161.4 Assessed individually for light sensitivity Loans not to exceed 11 months
Museum of Fine Arts Boston, MA (2000) Most sensitive 5 53.8 Assessed individually for light sensitivity 3 months display, every 5 years, 57 months dark storage
Museum of Fine Arts Boston, MA (2000) --- 5 53.8 Assessed individually for light sensitivity 3 months display, every 3 years: 33 months dark storage
International Museum of Photography at the George Eastman House, NY --- Below 10 Below 107.6 --- ---
Metropolitan Museum of Art, NY Color --- --- --- Color, not more than 3-4 months, every 5-10 years
Metropolitan Museum of Art, NY Albumen 4 43 --- ---
Philadelphia Museum of Art, PA --- 5 53.8 Exceptions up to 10 fc/ 107.6lux on rare occasions ---
National Gallery of Art, WDC (1998) --- Max 5 Max 53.8 --- ---
Cincinnati Museum of Art, OH (1996) 19th c. 5 53.8 Assessed by type of photograph and individual condition 4-7 months per exhibit venue with 5 year dark storage
Cincinnati Museum of Art, OH (1996) Silver gelatin 10 107.6 --- 6-12 months per exhibit venue with 3 years dark storage
Cincinnati Museum of Art, OH (1996) Color 5 53.85 --- 6-12 months per exhibit venue with 3 years dark storage
Art Institute of Chicago, IL (ca 1990s) --- Max 10 Max 107.6 Assessed by type of photograph and individual condition Maximum 12 weeks/ 3 months per exhibit venue
Humanities Research Center, University of Texas at Austin, TX General illumination in gallery 7 - 10 75.32 - 107.6 --- ---
Humanities Research Center, University of Texas at Austin, TX Light falling on specific identified item 5 - 7 53.8 - 75.32 Assessed by type of photograph and individual condition ---
Legion of Honor, San Francisco, CA 19th c., vintage, important 5 53.8 Assessed by type of photograph and individual condition ---
J. Paul Getty Museum of Art, CA --- Max 7 Max 75.32 Lower for sensitive materials. Tungsten light sources used ---
British Library, UK (ca 1990s) Paper based Max 10 Max 107.6 UV removed from light source ---
Victoria and Albert Museum, UK (1998) --- 5 53.8 --- ---
National Trust, UK (1998) --- 5 53.8 --- ---
Amon Carter Museum of American Art Paper and Photographs Max 10 Max 107.6 Assessed by type of photograph and individual condition Generally maximum 6 months; amount of "available exhibition time" is calculated as 1 month of exhibition exposure per year that the item has been in the collection
George Eastman Museum Photographs Max 10 Max 107.6 Assessed by type of photograph and individual condition Maximum 3-6 months exhibition length, followed by dark storage for 3 times the length of exhibition

Artist's Comments [edit | edit source]

Some photographers have commented in writing and directly to gallery and museum personnel regarding preferred light levels for their work: Ansel Adams (1950, 21): "The best gallery illumination, to my way of thinking, is a mixture of daylight and tungsten sources. Straight daylight illumination is perhaps too cold for general interior-display effect. I personally prefer to judge my prints with a light equivalent to that of north skylight (between the hours of 10 a.m. and 4 p.m. [16:00] in summer) with a white-card intensity reading of about 40 on the Weston meter". Cindy Sherman: Deterioration Series chromogenic prints are printed dark with fully saturated colors. Her printer recommends display under bright lighting. When the images fade or deteriorate, Sherman feels the images can be reprinted. William Wegman: Wegman specified that the light levels be 25-30 fc (269-322.8 Ix) for his Cinderella and Little Red Riding Hood Polaroid prints. However, one institution displaying these prints determined that 12-15 fc (129.12-150 Ix) would be adequate lighting for the color saturation of these prints. Contemporary photographers using historic methods (sometimes called "alternative photographic processes") are aware of the discrepancies that different types and levels of illumination can have on the aesthetic look of their photographs. Photographers may request exhibition lighting to be similar to their studio lighting for optimum display.

Assessment approaches[edit | edit source]

Assessment by individual item basis [edit | edit source]

Before 1993, the National Archives and Records Administration (NARA), Washington, D.C., conservation staff discussed a ranking system for individual photographic materials based on internal factors of composite photographic processes. The proposal was to develop a "value-list" for each category of sensitivity. Items with large numbers or within pre-designated letter categories (e.g., A = highest risk) would equate to being more sensitive and prone to damage from display conditions. For example, a conservator might wish to limit display of a 19th century hand-colored albumen print in good condition because it is the collection's only pristine copy and has fugitive dyes. Alternatively, one could argue that display is permissible because the image, the binder, and the support are in good condition. The "value-list" approach would give the conservator a rating for each photographic item that curators, historians, and exhibitions staff might easily comprehend. As with checklists for condition surveys of collections, however, it is hard to make complex and varied materials fit into a standardized formula. Material that is most at risk may have few "marks" indicating the extent of the risk, while another item that could be safely exhibited more often might come out of the equation looking as if it were a high-risk material. In addition, intuitive value judgments are necessary in each case.

Assessment by class of material[edit | edit source]

Assessment by type: Proposed Guidelines Library of Congress
Draft guidelines for exposure levels were compiled for the Library of Congress, Washington, D.C, in 1991 and updated in 1996 and 2000 (Wagner with McCabe and Lemmen, with later additions within this chapter by reviewers Brown, Dune, Jacobson, Kennedy, McElhone, Pénichon, Reinhold, and Robb. Printed without additions in Topics in Photographic Preservation, 9/2001.) This alphabetical listing is similar to ones developed for the Smithsonian Institution's collections. In these guidelines, decisions about the true light sensitivity of an object are made on an individual basis. It is presumed that exhibition materials used have passed the Photographic Activity Test (ISO 14523:2000). It is also presumed that the light sources have ultraviolet and infrared content removed, relative humidity maintained around 45 to 50%/ and air contaminants filtered out. Light sources that do not meet these specifications should be used at reduced exposure limits as damage from exposure is cumulative. The sample calculations are based on 10 hours of light exposure per day. 3 fc x 5 months [150 days] = 5 fc x 3 months [90 days] = 4500 fe-hours [48/420 lx-hours] 5 fc x 6 months [180 days] = 10 fc x 3 months [90 days] = 9000 fe-hours [96/840 lx-hours]

EXTRAORDINARILY light sensitive materials
Recommend displaying facsimiles ONLY:

  • autochromes (color) and other early dye processes experimental processes (e.g., unfixed salted paper prints) stabilized silver gelatin prints

VERY light sensitive materials
3000-5000 fc-hrs (32,280-53,800 lx-hrs) 1 year maximum with all ultraviolet light removed from light source. Consider using facsimiles:

  • all hand-colored, deteriorated, or poorly processed prints (fixer stains or smell)
  • albumen, printing-out process (pristine)
  • architectural plans or photo-reproductions (cyanotype [blueprint], brown lines, diazotype, Pellet prints, photostats, Vandyke brown)
  • carbon prints made of non-earth-based pigment or on poor-quality paper
  • cased objects where case, velvet, dyed leather, or silk is exposed (fugitive colors)
  • coated photographs (organic or unknown) color photographs and processes (especially pre-1990) *computer-generated prints (including ink jet dye-based inks or poor paper support)
  • cyanotypes (blueprints) (variable; composition determines light sensitivity extent)
  • dye imbibition (Dye Transfer, color)
  • gum bi/dichromate (unknown paper, suspected poor-quality paper, or light sensitive pigments)
  • hand-colored photographs (all types)
  • modern ink inscriptions (e.g., felt-tip pens, ballpoint, purple manuscript)
  • mounts of colored paper, fabric (especially colored construction paper or dyed materials)
  • platinum, palladium, and mixtures with silver, with pink-or yellow-stained paper
  • Polaroid® and other instant processes
  • resin-coated supports (pre-19S0, since 1950 optical brightening agents may be present)
  • salted paper prints, printing-out process
  • silver dye bleach (color, Cibachrome™ 1963-1991/Ilfochrome™ 1991-present)
  • tinted bases (e.g., blue or pink albumen or baryta layer)
  • transparencies (color, pre-1990)
  • Woodburytype (non-earth-based pigment or poor-quality paper)

Light sensitive materials
5000-10,000 fc-hrs (53,500-107,600 lx-hrs)/ year maximum with all ultraviolet light removed from light source:

  • albumen printing out process
  • color processes (post-1900, at higher light for shorter duration)
  • Cibachromes/Ilfochromes™ (at higher light for shorter duration)
  • collodion printing-out process
  • gum bi/dichromate (more sensitive paper, but earth pigments)
  • gelatin developing-out process (includes pre-19S0 resin-coated papers or contemporary papers with fugitive optical brighteners) gelatin printing-out process manuscript ink inscriptions
  • photomechanical prints with sensitive inks or poorer paper (photogravures, collotypes, carbro-carbon prints, etc.) platinum, palladium, and mixed with silver on good-quality paper (less if discolored) salted paper prints, printing out process (faded)

Less light sensitive materials
10,000-20,000 fc-hrs (107,600-215,200 lx-hrs)

  • ambrotypes (positive collodions) without cases (good condition, no coloring)
  • carbon prints (good paper with carbon or earth pigment)
  • daguerreotypes without cases (good condition, no coloring)
  • gelatin-silver developing-out print on fiber-based paper (well-processed black-and white, no hand coloring, no optical brightening agents) gum bi/dichromate (good paper with carbon or earth pigment)
  • matte collodion, printing-out print (good condition, no coloring)
  • photomechanical prints with lightfast inks on good paper without hand coloring (photogravures, collotypes, carbro-carbon prints, etc.)
  • tintypes without cases (good condition, no coloring)
  • Woodburytypes (good-quality paper with carbon or earth pigment)

Centre de Recherches sur la Conservation des Documents Graphiques (CRCGD; Center for Research on the Conservation of Graphic Documents)
Based on his research, Dr. Bertrand Lavédrine (2003, 161, 163) recommends the following light levels and annual durations, assuming that all ultraviolet and infrared radiation have been eliminated from the light source and all display materials are of the highest quality:

Category 1 (Particularly sensitive) not to exceed 1115 fc-hrs (12,000 lux-hrs) annually with a maximum illumination of 4.6 fc (50 lux):

  • chromogenic development
  • color photographs
  • instant photography
  • 191h century photographs

Category 2 (Very sensitive): Not to exceed 3903 fc-hrs (42,000 lux-hrs) annually with a maximum lumination of 7 fc (75 lux)

  • dye-transfer photographs
  • dye-bleach color photographs (Ilfochrome Classic)
  • resin-coated black-and-white photographs

Category 3 (Sensitive): Not to exceed 7807 fc-hrs (84,000 lux-hrs) annually with a maximum lumination of 14 fc (150 lux)

  • baryta paper black-and-white photographs
  • monochrome or pigment color photographs

Objects that are not very sensitive to light are recommended to have a maximum illumination of 28 fc (300 lux).

Assessment by type: Earliest 19th century photographs
Wilhelm (1993, 608 to 609) recommends that "salted paper prints (ca. 1840 to 1855) and albumen prints (ca. 1850 to 1895) never be displayed, even for short periods." Ian and Angela Moor (1992, 201) believe that the following original 19th century materials should not be exhibited until future research identifies "safe methods" (listed alphabetically);

  • calotype (potassium bromide stabilized)
  • chromatype (Hunt)
  • chrysotype (Herschel)
  • cyanotypes (pre-Pellet variant 1877, Herschel)
  • direct paper positives (Fife, Talbot, Bayard)
  • Energiatype (Hunt)
  • Leucotypes
  • photogenic drawings (stabilized sodium chloride-silver chloride, potassium bromide, or potassium iodide);
  • salted paper print (sodium thiosulfate fixed)
  • Talbot's processes without exception.

Ware's (1994, 55) conclusion from research of William Henry Fox Talbot photographs was "most of the damage is likely to be done by visible light" although ultraviolet radiation must be removed from any light source whether in storage or display areas. The variability of the material in question does not provide easy quantifiable solutions. "Exhibition under the customary 5 fc (53.8 Ix) would appear to be out of the question; display under very low [1 fc (10.76 Ix)] carefully filtered light, with proximity switching or a viewer activated curtain, would only allow about 30 hours of exhibition before the threshold exposure (limit) was reached" (Ware 1994, 55 to 56). Ware's test image was "fogged." His research suggests that "the threshold exposure lifetime (TEL) for chloride-fixed photogenic drawings is in the order of three hours, and this vulnerability to light may well be panchromatic, that is, sensitive in some measure to all wavelengths of the visible spectrum" (Ware 1994,55-56). Research by Ware (1999) and McElhone (1993) indicates that cyanotypes can be exhibited safely for short periods of time at or under 5 fc (53.8 Ix) using light with all ultraviolet radiation removed and in a deep mat to provide a large air space. The light stability of cyanotype prints is highly variable, as the light stability is dependent, in part, on the original chemistry of each print. However, since these materials will be exhibited, over the objections of conservators, these recommendations should be used as a basic guideline to determine the longevity of display, the illuminance level and design, and temperature and relative humidity levels to be maintained. Another alternative is to make replicas and duplicates. Duplicates with similar processes and at times, better processing, may be an option to display images prone to light fading or just too sensitive to display. (see 1.6.5 Rotation, replacement, duplication, and facsimiles.)

Assessment by type: Cased 19th century photographs
Daguerreotypes and other cased photographs (ambrotypes, also called positive collodions, and tintypes) were historically illuminated from the side by natural light from windows or by lamps on tables. Overhead illumination will descend across the polish lines, which are on a horizontal orientation to the plate, creating a haze of scattered light that can obscure 40% or more of the image. Daguerreotypes are best viewed with "single-source," directional light illumination coming from a 45° angle to the plate surface. This arrangement will allow the reflected light to pass to the side of the viewer; otherwise, only glare or a mirror effect will be seen. Fiber optic light systems are probably the optimum, but excellent results can be had from light piping as well. Other light sources can be very effective, but they may require a great deal of effort to achieve results more easily obtained with a fiber optic source. Uncolored daguerreotypes, ambrotypes, and tintypes should be considered moderately light sensitive, but because of the auxiliary components that comprise the entire object, the standard restrictions regarding ultraviolet should be maintained. Auxiliary components include cases covered with dyed leather or paper, dyed textiles inside the cases, and glass covers. Some cases are made of a more durable thermoplastic, but some have a lacquer finish with coloring and inlaid mother-of-pearl, which can be heat and light sensitive. Light levels need to be high enough to make the daguerreotype image easily viewable, but low enough to pose no undue risk to the textile linings. The singular advantage to fiber optic lighting is the extremely high degree of control over where the light strikes. It is possible to focus a higher light level on the image, perhaps 10-15 fc (107.6-161.4 Ix) and have less light striking the textile covering on the case lid liner. Ambrotypes and tintypes come with many of the same components as do daguerreotypes, and therefore the same caveats apply. Heat and humidity probably pose a greater risk than does actual levels of illumination because these images commonly have at least one layer of a natural resin varnish. However, light levels should not exceed 10-15 fc (l07.7-161.4 Ix). Although it is rare for unvarnished images to be in any condition to be placed on exhibit, some have survived remarkably well.

Assessment by type: 20th century black and white developing-out prints
Hendriks et al. (1991, 438) proposed that black-and-white silver gelatin developing out prints could be "safely exposed at illuminance levels between 500 fc (5380 Ix) and 1000 fc (10,760 Ix) provided there is clean air in the display area and both temperature and relative humidity are controlled and stable." Length of time was not specified. However, the thoroughness of processing, length of exhibit, and previous cumulative exposure history should also be considered in determining light exposure. Many conservators believe that 500 fc (5380 Ix) is an excessive and unnecessary level of light exposure and continue to recommend light levels be kept between 5 and15 fc (53.8 and 161.4 Ix).

Assessment by type: 20th century color photographs
Wilhelm (1993, 577) maintains that contemporary color prints "should be displayed with adequate illumination. There is no 'minimum' illumination level at which color print fading does not occur." He recommends approximately 30 fc (322.8 Ix) of incandescent or glass-filtered quartz halogen illumination for archives, galleries, and museums and 45 fc (484.2 Ix) for commercial or private home illumination for contemporary color photographs. The length of exhibition is not defined. Hendriks et al. (1991,440) agree, stating, "in order to perceive colors in photographs on display, an illuminance of 30 fc (322.8lx) is recommended." They recommend shortening the duration of exhibitions to compensate for higher light levels. Wilhelm (1993, 61-100, 230) further notes that because of reciprocity failure, not all color photographic dyes fade at the same rates. His accelerated aging results indicate that for some color photographic dyes, displays of shorter duration at higher intensity are less damaging than displays of longer duration at lower light levels. This research is contrary to the current understanding of how light damages materials and may be an indication of the limitations of data gathered from accelerated aging testing coupled with the complexity of understanding light radiation. Conservators disagreeing with Wilhelm and Hendrik's opinions cite conservation's ethical goals to protect materials for as long as possible through insightful balance of the known damaging effects of display. If the material will fade or be damaged by display, additional precautions, instead of abandoning of precautions, would be a more suitable action for preservation of these materials. However, others have noted that the intent of Wilhelm and Hendriks's statements was misinterpreted. Displaying photographic material at the most minimal light levels when appreciation of the medium demands a higher light level is a disservice to the artist's intent and frustrating to the viewer. If higher light levels are used, consideration should be given to reducing the number of hours these materials are on display and using a lighting source with the least damaging light spectra output for that photographic process. Wilhelm (1993, 230) suggests color balance should be considered as much as potential fading and density changes in determining appropriate lighting levels. Some institutions are collecting two copies, one intended for display, another for preservation. When the display copy is "exhausted," the question remains whether a duplicate will be made for continued display or, as some curators have suggested, the second preserved print will be available for display. This issue of suitable light levels and suitable length of display of color photographic materials remains controversial, and the discussion continues.

Assessment by fading of monitoring standard

Blue Wool Standard
Blue Wool Standard BSI006 (1978) or International Organization for Standardization (ISO) standards for sensitivity to light was utilized at the Montreal Museum of Fine Arts (MMFA), Canada, to assess suitability of materials for exhibition and to clarify and quantify the institution's exhibition policy. An "important advantage is that [this system] can be used to communicate concepts of fading to non-specialists, and tends to be more convincing than vague warnings. Effective communication of the rationale behind protective policies is essential if cooperation or support in their implementation is to be forthcoming. Exposure recommendations are based on regular yearly use, with the recommendation that exhibitions never exceed twenty weeks, regardless of accumulated storage time. It is assumed that ultraviolet radiation (UV) has been filtered out or is excluded from the light source. The figures in the policy have been based on a 42 hour exposure week with an intensity of 10 fc (107.6 Ix) for categories 2 and 3 works, and 7.5 fc (80.7 Ix) for category 1 works." Individual works were assessed into three categories (Colby 1993, 4 to 5) See next page:

  • Category #1 Sensitivity levels ISO 1, 2, or 3 (sensitive) 1115 fe-hour / year (12,000 lx-hour/ year) approximately four weeks exposure limit at 7.5 fc predicts a "just noticeable fade" in 100 years
  • Category #2 Sensitivity levels ISO 4, 5, or 6 (intermediate) 3,903 fe-hour/year (42,000 lx-hour/year) approximately ten weeks exposure limit at 10 fc predicts a "just noticeable fade" in 250 years
  • Category #3 Sensitivity levels ISO 7, 8 or above (durable) 7,807 fe-hours/year (84,000 lx-hours/year) approximately 20 weeks exposure limit at 10 fc predicts a "just noticeable fade" in 3500 years

The display lighting policy at the Victoria and Albert Museum, London, is based on the British Blue Wool Standards and perceptible change concept introduced by Stefan Michalski in 1997. Initially, the policy had two categories, sensitive (approximately ISO 4 and below) and durable (ISO 5 and above). The two categories primarily separated material by color versus black-and-white media and condition of support. An exception was the "zero tolerance" items such as photogenic drawings. Items in the sensitive category had a 20% display time at 50 lux. In 2001, the policy proposed four categories called vulnerable (formerly "zero tolerance"), sensitive, durable, and permanent. Materials in the vulnerable category include photogenic drawings and a maximum of 50 lux is recommended. Materials in the sensitive category include color photographs and lighting at ISO 4 and below is recommendation. Materials in the durable category include black-and-white fiber-based photographs and lighting at ISO 5 and above are recommended. Materials in the permanent category include stone and metal items that can be displayed at up to 27.9 fc (300 Ix). (Ashley-Smith 2002). An historic association exists for the use of the Blue Wool Standard in conservation. Many dyes used in late 20th -early 21st century printers (e.g. inkjet) have a basis in the textile industry where the Blue Wool Standard is still used. Commercial ink manufacturers incorporated the use of the Blue Wool Standard when adopting ink technology. However, blue wool patches have relative light fading rates, which vary depending on the light source as indicated by research conducted by Dr. Robert Feller at various institutions, by Henry Wilhelm, and at the Image Permanence Institute.

Light sensitive polymer dye
Many types of photographic materials will fade and can be otherwise damaged before the most sensitive sample of the Blue Wool Standard scales will fade. Therefore, Dr. Bertrand Lavédrine, director of the Centre de Recherches sur la Conservation des Documents Graphiques (CRCGD; Center for Research on the Conservation of Graphic Documents), developed a more sensitive gauge designed to be a cost-effective monitoring method. It is paper coated with a blue-dyed polymer that will shift in color from blue to violet to pink until fading to white at approximately 9293.68 fc (100 kilolux hours) (Lavedrine 1998).

LiDo (light dosimeter)
Based on Lavédrine's work, LiDo (light dosimeter) is intended to be similar "to the current blue wool standards ISO 105-B01, but more sensitive than the present Blue Wool Standard One. The product will give an early warning to the potential of light induced damage," (Romich, 2003) so may be useful in monitoring the light sensitivity of photographic prints. One prototype, LiDo (sensitivity one), has been successfully tested. Another prototype, liDo (sensitivity four) is still in development. The target release for the dosimeter is 2004. LiDo development is a collaborative effort between Fraunhofer-Institut fur Silicatforschung (FhG/ISC), Centre de Recherches sur la Conservation des Documents Graphiques (CRCGD), Insitutio di Ricerca sulle Gnde Electromagnetiche "Nello Carrara", and the Victoria and Albert Museum.

Microfading Tester (Oriel® Fading Test System)
Paul Whitmore, with assistance from his colleagues Xan Pan and Catherine Bailie of the Carnegie Mellon Research Institute, Research Center on the Materials of the Artist and the Conservator, developed what they called a microfading tester, currently marketed as the "Oriel® fading test system" by Spectra-Physics. This instrument samples a small area (0.4 mm) with intense light. Reflectance spectra data are gathered, and a computer software program provides real time visuals (charts and graphs) as fading occurs. With information gathered in less than five minutes, the software can also extrapolate the data to predict long-term rates of fading. The test area and amount of fading are generally imperceptible to the unaided eye. As the light level is high, true reciprocity of some materials, such as chromogenic prints, cannot be accurately predicted with this method. The range of the microfading tester is 400-700 nm devoid of ultraviolet radiation that is the cause of much fading. The Carnegie Mellon research team is working on outfitting the machine with a light source that includes ultraviolet radiation (Whitmore et al. 1999). The largest factor limiting this machine and software at present is its cost of US$17,500. Accurate prediction of fading from light exposure in exhibition would necessitate the use of fiber optics with similar light spectra as the microfading tester. It is desirable when lighting photographs to eliminate ultraviolet radiation. Therefore, the current range of the tester may be suitable for assessing fading risks for materials intended for display. The accuracy of the information specific to the unique photograph tested may well warrant the minute fading caused by this machine. Even if the machine can predict only an approximate range for some materials, the alternative has been for individuals to estimate a range based on previous observation of materials of like kind or produce time-consuming density and color monitoring data also minutely light damaging. The size of the photograph relevant to the 0.4 mm test area should also be considered.

Assessment by density changes using monitoring equipment
The conservation community has not yet agreed upon one monitoring standard for photographic materials. There can be confusion in trying to compare data from various monitoring machines and methods. In addition, monitoring is time consuming (see section on monitoring equipment).

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