MEGALUX HOURS EXPLAINED

Photographers understand light exposure. When capturing images with a camera, they know exposure is not just about time. Technically, exposure is the product of light intensity and time. Light exposure applies the same way to works of art on display.

Here’s an example. Vincent Van Gogh’s “Starry Night” was acquired by the Museum of Modern Art (MOMA) in 1941. The painting has thus been on display for nearly 80 years! Let’s assume for purposes of discussion that it has been shown to the public for 8 hours per day, six days per week, and the gallery lights are otherwise off.

Time alone doesn’t describe the painting’s accumulated exposure to light. We also need an estimate of the average illumination level in order to evaluate the accumulated light exposure to which Starry Night has been subjected while on long term display in the museum gallery.

Let’s assume a 50 lux illumination level on the painting, a value often recommended by museum conservators for the safe viewing of artwork on display.

• Daily light exposure on Starry Night:
  50 lux x 8 hours per day = 400 lux-hours.
• Yearly light exposure:
  400 lux-hours x 6 days per week x 52 weeks per year = 124,800 lux-hours.
• Years on display since MOMA acquired the work:
  2019-1941 = 78 years.
• Estimated Total Light Exposure since the painting’s acquisition:
  124,800 lux-hours per year x 78 years = 9,734,400 lux-hours = 9.73 Megalux hours!

Now, what would happen to Starry Night if it were displayed in the lobby of a modern building, one with large skylights and/or liberal use of architectural glass? Light levels in very bright indoor areas easily can reach 2000 lux or greater for sustained intervals of several hours per day.  Starry Night might then be illuminated at 2000/50 = 40x greater light intensity (compared to our MOMA example) and for at least as long a daily light/dark cycle. It might be great lighting for a great viewing experience, but the painting would then reach the same 9.73 Megalux hours (or more of equivalent museum gallery exposure) in only 78/40 = 1.95 years on display! Light intensity during display, not just time on display, is the “elephant in the room” when it comes to protecting paintings, prints, and photographs from fading and discoloration, and an essential quantifier in the science of light fading.

If we were to continue forward from today with 2000 lux illumination for as much time as Starry Night has already been on display at MOMA,  Starry Night will have received a whopping 389 Megalux hours additional light exposure over the next 78 years, far more than enough exposure to cause severe fading in paintings, as well as prints and photographs.

That’s how exposure works! Light fading outcomes for artwork on display are all about accrued light exposure, not just accrued time on display!

To carry the camera exposure versus artwork-on-display analogy one step further, the higher the camera’s film or electronic sensor sensitivity to light, the less total exposure is required to create a properly exposed image. Photographers know this as ISO speed. The more light-sensitive the artwork, the less light exposure is required to observe noticeable fading and/or discoloration. Van Gogh painted Starry Night in 1889. We don’t know precisely what its color values were when it was new, so we don’t know how much fading has already occurred to Starry Night, but its beauty is still greatly admired today. Thus, in today’s market where artists often use the term “archival” to help sell their work, Van Gogh could have claimed a “130 years” light fastness rating for his painting of Starry Night, and time has now proved him right. Yet this result must be tempered by the fact that MOMA has been taking exceptional care not to display the painting at arbitrary light levels for nearly 80 of those 130 years. The exposure limits where noticeable fading is observed can easily accumulate 10x, 100x, even 1000x more quickly in a bright home, office, or store window compared to a dimly lit room or typical museum gallery! Light fastness ratings expressed merely as “years on display” thus tell an incomplete and misleading story, because the range of light levels encountered in real world indoor environments is too great to assign one value for a “one-size-fits-all” display life prediction.

Aardenburg Imaging reports in Megalux hours because Megalux hours are the technically correct way to rate the light fade resistance of paintings, prints, and photographs.

This fact brings us to one final important consideration. Appropriate testing endpoint criteria must also be considered when judging the merits of any light fastness rating. The Aardenburg Conservation Display Ratings (CDR) report the Megalux hour limits, both Lower CDR and Upper CDR, which artwork can withstand and still remain in excellent condition, i.e., where viewers observe “little or no noticeable fading.” It’s easy for anyone, expert and non-expert alike, to agree on the condition of artwork which remains in excellent condition with little or no noticeable fading and/or discoloration, because it still looks essentially like new. However, manufacturers today tend overly to rely on light fastness tests of their products that use more liberal testing endpoint criteria, subjectively described as “easily noticeable fading.” Such claims lead to further debate over how much fading is too much. When a manufacturer offers fading data at a rated limit of easily noticeable and perhaps even objectionable fading, the overall condition of the artwork becomes much harder to describe.

Aardenburg’s more stringent Conservation Display Ratings are thus intended to serve as expert guidance for collectors, curators, conservators, and artists who seek to ensure the highest standards in handling, storage, and display of works of art in their care. The artist’s original intent is thus preserved.

Respectfully,

Mark McCormick-Goodhart
Director, Aardenburg Imaging & Archives