Light levels are perhaps the most important issue for exhibition of paper-based collections. If originals must be exhibited, light levels should be no higher than 50 lux for very sensitive materials (e.g., textiles, paper, dyed leather, 19th-century photographs, and color photographs). Moderately sensitive materials (this includes only a few paper-based materials, such as carbon black inks on high-quality paper and modern black and white silver gelatin photographs on baryta paper) can be exhibited at levels no higher than 100 lux. There should be no natural light in the exhibit area, and ultraviolet (UV) light should be no higher than 75 microwatts per lumen.
Because items have different sensitivities and some may be exhibited more frequently than others, it is best to determine the length of exhibition separately for each item. Materials should never be exhibited permanently. In fact, the NISO standard for exhibition sets the maximum length for exhibition at 52 weeks, but for most items recommends a much shorter period. The standard suggests that very light-sensitive collections that are exhibited repeatedly (defined as no more than once every two years) should be on display for no longer than 12 weeks at a time.
The concept of total light exposure (TLE) can be helpful in determining light levels and length of exhibition for specific items. A yearly limit for light exposure would be set for an item, which would be calculated by multiplying the exposure time (e.g., three weeks) by the light level (e.g., 100 lux). Because of the principle of reciprocity, the same yearly TLE could be reached by exhibiting the item for one week at 300 lux. To use this principle effectively, good record keeping of exhibition times and light levels is of paramount importance.
The NISO standard recommends that relative humidity in the display environment (e.g., inside the exhibit case) be kept at a set point between 35 percent and 50 percent with a variation of +/-5 percent, while temperature should not exceed 72 degrees Fahrenheit with a variation of +/-5 degrees. In addition to monitoring the climate in the exhibition space as a whole, climate conditions inside exhibit cases should be monitored to ensure that they are not damaging. A min/max thermohygrometer will give a general indication of conditions. Exhibit cases should not contain lights, because these cause significant changes in temperature and relative humidity within the case. Fiber-optic lighting is preferable because it does not produce heat.
Pollutant levels should be limited to the greatest extent possible. Materials used to construct the exhibit cases should be as chemically stable as possible and should not off-gas damaging pollutants.