(Book and Paper) A New Tool for Managing Cumulative Light Exposure by Jordan Ferraro


This presentation addressed the need for a nuanced, object-centered approach to light dosage policies. The speaker began by outlining a previously existing light exposure policy for the U.S. Army museums. They had divided exhibit artifacts into three very broad classes of objects: highly sensitive, sensitive, and insensitive. The categories corresponded to recommended light levels for display. Gallery lighting was set at 50 lux for highly light sensitive items, 150 lux for sensitive items, and 300 lux for insensitive items. The policy also dictated that highly sensitive objects should rotate more often than other categories. The Army had implemented a blanket mandate that excluded all original works on paper from exhibits. The policy set a single light dosage specification for all textiles, regardless of fiber type, dyes, and other factors that could be considered in exhibition decisions.


The only light dosage measurement was the blue wool card. While this is a low-cost, low-tech methodology that can be effective, the blue wool cards were assigned to exhibit cases, not individual artifacts. This meant that the same object could be rotated on and off of exhibit repeatedly, yet there was no way to track the light exposure for that object.

The presenter then described the proactive approach she took to communicate light exposure to other museum stakeholders. Staff members needed light exposure data to inform collections stewardship decisions. It was essential to create a system for tracking exposure for artifacts over many exhibits. By continuing the use of blue wool cards and adapting their use to match the categories of light sensitive objects, the conservator leveraged existing knowledge to make the abstract numbers more concrete. The exposure to “just noticeable fade” is a widely used standard for communicating color change. The term “Light Life” was used to convey this concept in a manner that was easy for all stakeholders to understand.


A new protocol was established for calculating the Light Life of the artifacts on exhibit. By taking light meter readings (sampled at the brightest spot on the artifact) and calculating gallery lighting hours per week over the course of a year, the conservator could project the extent of light exposure for the planned duration of an exhibit. A stop light code (green-yellow-red) was devised to convey that an artifact was good to go (green), endangered (yellow), or exhausted (red). When an exhibit was projected to reach one half of the lux-hours to just noticeable fade (1/2 of its Light Life), then it was placed in the yellow category. If fading was already observed, then the object was placed in the red category. An artifact on display that was projected to exceed its Light Life during the current exhibit would also be placed in the red. This color coding is currently being used to set the rotation schedule.

The implications for the collection have been that (previously excluded) paper and textiles have been added to exhibits with clearly defined lux-hour exposure limits up front. Curators have worked harder to locate different artifacts, rather than reusing items whose Light Lives have been exhausted. Exhibit rotations are more closely aligned with collection stewardship than they were under the old policies.


During the question and answer period, a few areas were clarified. The current system for the Army is very low-tech. They are not using light loggers in the galleries, so lux-hours are only calculated based on the gallery operating hours, not the light exposure during special events, gallery maintenance, and other times that might add to cumulative light exposure duration. The light exposure recommendations are based on literature values and estimates, because they are using neither a microfadeometer nor a spectrophotometer to glean actual color change information from artifacts. There is nothing that would stop a museum from bolstering its recommendations with quantitative data, if the museum had access to a microfadeometer and/or a spectrophotometer to make measurements of actual color changes. The positive aspect of a low-tech approach is that it could be used in small galleries and historic houses with limited staffing and tiny budgets.

A second area of concern is that this method only includes fading. With works on paper, yellowing and darkening are associated with cellulose degradation. The blue wool swatches are also poorly correlated with the fading of optical brighteners, which are fugitive dyes found in many modern materials. Color changes such as darkening or yellowing from either cellulose degradation or loss of brighteners can be detected with a spectrophotometer, but they do not relate to blue wool standards.

The process described in this project is a necessary first step in communicating information about fading, but the cumulative light exposure risks for organic materials on exhibit are not limited to fading. It would be nice to see a follow-up that addressed some of these other risks.


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