Thanks to Noni for raising this, and to Henry and others for weighing in.
I've personally never encountered BW3 appearing more sensitive than BW2. In the past I have used a modfied Whitmore design and now a 3000K Fotonowy but there are a few factors at play that plausibly lead to this kind of behavior.
The biggest source of error can be small misalignments or out of focus measurements, both of which can introduce variability in the results and increase the standard deviation across replicates. Picking a spot on a fiber crossover can be challenging with the Fotonowy. In a situation like that, you might get a surprisingly high BW3 value in comparison with a lower-than-expected BW2. I know from experience that the SD for BW1 measurements can often be on the higher end because of the high light sensitivity of the dye in combination with spot selection. Care in selecting areas for assessment with MFT is of the most importance.
Also it's worth looking at the full measurement context- not just the DE2000 values- like investigating reflectance spectra. Additionally, starting Lab* values can result in potential baseline discrepancies. This can help tell if this is a measurement artifact or a true material response.
It's also known that dropping UV (as is common, particularly with LED sources) tends to skew the differences between BW steps away from the 2:1 ratio- in anoxia or in the presence of oxygen. The DE2000 metric further exaggerates this non-linearity, since it weights chromatic shifts differently than DE76.
While I understand the concern about the Blue Wool standards, they're still widely used in LED-based MFT setups (and have been for some time) for internal consistency across labs and as good a qualitative measure of light sensetivity as we can get at the moment. And while yes, we make assumptions about reciprocity when it comes accelerated aging, which has been demostranted to break down for some materials, overall what MFT does allow for is some benchmark testing as part of a comprehensive preventive strategy :)
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Abed Haddad
AIC Vice President (2025-2027)
Assistant Conservation Scientist
The Museum of Modern Art
New York, NY
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Original Message:
Sent: 08-12-2025 14:10
From: Henry Duan
Subject: Issues with measuring BW standards
Hi,
Catherine and Noni,
I do not have any good ref. standard. I heard someone in Europe was trying to find replacement reference materials, and was unable to get into the commercial market; I guess it is too small for mass production with commercial profit.
Alternative might be relying on relative comparison with a material we familiar with its light fading characteristics under museum display conditions. However, like with the Bluewool, we do not know when we shorten the exposure duration from years in display to hours under MFT, the reference material will still behave the same or quite differently, like Bluewool (which faded much slower under MFT at a same light dosage of Mlx.hr.). The only way is to do the same exposure test under a much lower light intensity with much longer exposure, on both the selected reference material and on your testing material. If their relative fading rates keep the same vs. under MFT (at a same total light dosage of Mlx.hr.), then I think the MFT results are reliable. But of course this cannot be done with the actual artwork unless you can reproduce it with the same set of materials. This is less likely for historical artworks, but possible with modern photographic prints, as long as you can find out the original commercial ink set and substrate media.
Henry
Original Message:
Sent: 8/11/2025 7:52:00 PM
From: Noni Zachri
Subject: RE: Issues with measuring BW standards
Dear Henry,
Thank you for your response.
In this instance I used a 3500K LED light source. I also second Catherine's comment in that I'm interested in what you would recommend as a standard when using LEDs if the Blue Wools are not appropriate?
Thanks,
Noni
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Noni Zachri
Paper and Photographs Conservator
Grimwade Conservation Services
North Melbourne
Australia
Original Message:
Sent: 08-11-2025 13:58
From: Catherine Stephens
Subject: Issues with measuring BW standards
Henry
This is an interesting comment. Can you recommend an appropriate standard for use with the Fotonowy instrument/LED source?
Thanks,
Catherine
-- Catherine H. Stephens, Ph.D. (she/her)
The Sally and Michael Gordon Conservation Scientist
Museum of Modern Art (MoMA)
30 W. 54th St., 10th Floor
New York, NY 10019
phone: (212) 708.9821
Original Message:
Sent: 8/11/2025 12:27:00 PM
From: Henry Duan
Subject: RE: Issues with measuring BW standards
Hi, Ms.
Zachri,
What is the light source of your MFT? I assume it is LEDs for Instytut Fotonowy MFT. If so, I suggest avoiding using Bluewool reference, since it is designed for Xe light source (not sure D65 or D50), which include some UV components. But the LED light source does not apply.
In addition, we find that the Bluewool fading rate does not follow reciprocity between exposure duration and light intensity. It is highly sensitive to the exposure time due to the impact of O2 in the air. Since the O2 concentration is a constant, thus exposure duration will determine how much impact is from the light catalyzed dye fading by O2 and how much fading is from direct dye degradation by photons.
In concussion, the bluewool reference cannot be used for accelerated light fading study when you trade a high light intensity with a shorter exposure duration, except under O2-free environment.
Henry Duan
Preservation program
NARA - US
Original Message:
Sent: 8/6/2025 1:44:00 AM
From: Noni Zachri
Subject: Issues with measuring BW standards
Hello,
Has anyone ever encountered an issue where the measured dE values for BW 3 are greater than BW2? Particularly when looking at dE2000 calculations?
I have found that sometimes there is the odd measurement where BW3 appears more sensitive than BW2 when running the standards.
I'm using an Instytut Fotonowy MFT for reference.
Any advice would be greatly appreciated.
Thanks,
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Noni Zachri
Paper and Photographs Conservator
Grimwade Conservation Services
North Melbourne
Australia
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