Microfade testing in cultural heritage

Abstract

Microfade testing (MFT) is an accelerated aging technique used to test the lightfastness of a material or object directly rather than relying on generalisations or models; it was developed specifically for the cultural heritage sector, unlike many other analytical methods currently used in museums and cultural institutions. Since its development in the 1990s, MFT has helped to guide museums’ lighting policies and the responsible display and loan of cultural heritage objects. MFT is considered to be minimally invasive and is frequently used to test materials that are suspected to be highly light-sensitive, including textiles, works on paper and more complex three-dimensional objects. Tests are conducted directly on the object, allowing the user to observe changes as the test is performed – despite the name of the technique, MFT can be used to detect any colour change, not just fading.

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How does it work?

MFT measures the colour change of an object when exposed to high-intensity light,¹ using stable, high-powered light sources delivered to an object through fibre optic cables. The areas affected are small, less than 0.5 mm across, having negligible visual impact, only a short way along the fading path, which is then extrapolated. Tests may vary in length of time, and the intensity of the light delivered to the object may be changed based on the sensitivity of the object or the anticipated display conditions. Ultraviolet and infrared radiation are filtered out, and testing is in the visible wavelength region of 400–700 nm. Light-induced colour change is monitored in real time using a spectrophotometer (Fig. 1). Reflectance spectra collected during testing help the user visualise at which wavelengths the area tested had changed and how.

Fig. 1 Example of reflectance spectra of a typical MFT on ISO Blue Wool 1 in the Spectral Viewer software, showing the initial (blue line) and final (red line) spectra. Changes in ΔE are shown in the dialogue box at the bottom left.

Colour change is monitored in the CIELAB colour space, a colour model defined by the International Commission on Illumination (CIE), and is expressed by a value known as ΔE (delta E).

Typically, the cultural heritage field uses both delta E 1976 (ΔE_ab) or delta E 2000 (ΔE₀₀). The equation for ΔE_ab is simpler and frequently used, but delta E 2000 is more accurate with respect to perceptible colour change.^2,3 The numerical change in delta E and how it is perceived can be expressed as Just Noticeable Difference (JND) or Just Noticeable Fade (JNF).

What is the effect of the MFT on an object? Generally, a ΔE_ab or ΔE₀₀ of about 1.5 is considered a JND, and is deemed an unacceptable change in the cultural heritage field.⁴ However, the change may be harder to perceive in visually complex works and more easily detected by people with good eyesight in bright light. It is recognised that it is unlikely that a human with good eyesight will detect a ΔE of 1.5 without a reference point, but it is possible that a ΔE of 2 or more could be detected visually.³ Before conducting MFT, there should be a conversation with stakeholders to determine at what value of delta E tests should be stopped, especially if change is occurring quickly.

Categorisation of sensitivity to light

In the cultural heritage field, the light sensitivity of objects is frequently compared to ISO Blue Wool Standards, developed to characterise the lightfastness of textiles.^5,6 For example, lake pigments (colourants obtained by precipitating a dye on an inert substrate) are considered highly light-sensitive and therefore have an ISO Blue Wool rating of #1–#3 (see ISO 105: Textiles – Tests for colour fastness). Materials such as carbon pigments are considered to have low sensitivity and are therefore generally given an ISO Blue Wool #7 or #8.⁷

Blue Wool Standards consist of a set of eight strips of dyed fabric with known sensitivity to light, ranked by sensitivity, which approximates a geometric progression (Fig. 2). Blue Wool Standards #1 to #3 are used as a comparison for the sensitivity of an object. Blue Wool Standards #4 to #8 require ultraviolet radiation to fade, so are not used in MFT practice.⁵

Fig. 2 A Blue Wool Card, with Blue Wool #1 on the left. Image: from the CAMEO website provided by University Products (https://cameo.mfa.org/wiki/File:Blue_scale.jpg).

There is some variation between Blue Wool Standards from the United States, the United Kingdom and Japan. Some are mounted on cards while others come as loose cloth, but all will fade if left exposed to light and should be stored in the dark. The dyes used to make these standards also may differ.¹ Generally, a day of MFT starts by testing Blue Wool Standards #1 through to #3. The measured ΔE of the Blue Wool Standards will help the tester determine if the lamp is running optimally, or if adjustment is needed. A comparison of the rate of fading can then be made day to day. The rate of fading of a standard will eventually slow, and is recorded for tracking purposes.

Equipment variations

The microfade instrument consists of a high-intensity light source, a spectrophotometer, a computer and appropriate software. Some set-ups make use of the built-in software of the spectrophotometer; others use an additional program, Spectral Viewer, developed by the Getty Conservation Institute.⁵ An advantage of the Spectral Viewer software is that it gives the user the light dosage in megalux hours compared with the change in ΔE and may thus make it easier for some users to relate the results to lighting policy. Pre-assembled custom-made MFT instruments are commercially available, or typically an institution or individual may purchase the separate parts and assemble an instrument.

Light sources are required to be high-intensity, with a stable output, and are usually xenon arc lamps or (more recently) light emitting diodes (LEDs),¹ which are more representative of gallery lighting. However, when considering moving an object to another institution that does not use LEDs, or historic houses and display spaces that make use of daylight, xenon sources represent the ‘worst case scenario’ and may be more appropriate. Xenon sources also allow for the removal of the UV filter and testing can be done to simulate a display environment subjected to UV light.

Types of MFT

MFT instruments come in a variety of arrangements. The Whitmore set-up (Fig. 3) is widespread, where a head with fibre optic cables at fixed angles is affixed to a rail. The light source is delivered at 90° or 0° to the object, and the reflectance data are collected at a 45° angle (Fig. 4). The head moves along the x-, y- and z-axes to enable accurate focusing. This type of MFT unit is adjustable and the head can be mounted on a tripod for work on different types of objects, or ones that are in a gallery or storage space. Variations of MFT instruments include those that are not mounted on rails and instead make use of tripods for ease of transportation and use with a variety of objects. More recently, retroreflective units, delivering light and collecting reflectance spectra at the same angle (but not always orthogonal to the surface of the object), have been developed. These units are lighter and more portable than Whitmore units. While not common, MFT instruments that rest gently on an object and deliver light from a variety of angles have also been developed.

Fig. 3 A Whitmore MFT instrument at The Virginia Museum of Fine Arts. Left to right: power supply, spectrophotometer, lamp in housing, laptop and the test head mounted to a rail. Photograph by C. Hardman-Peavy.

Fig. 4 The head of a Whitmore MFT instrument. Light is delivered to the object via the red fibre optic cable in the centre, and the spectrophotometer collects reflectance information via the fibre optic cable on the right. A pen camera is mounted on the left side. Photograph by C. Hardman-Peavy.

The cost of MFT instruments can be prohibitively expensive, so many newer set-ups are aimed at reducing the price and improving portability.

Setting up a test

Most types of cultural heritage objects can be tested, with flat objects being easier to test than those with complex shapes (Fig. 5). Difficulties may arise in positioning the head of the tester perpendicular to the object. When testing textiles, care must be taken to ensure that the test spot is on the high point of a fibre, rather than in a valley of the weave. The object must be stationary during testing.

Fig. 5 MFT on an early 19th-century photograph from the V&A collection. © Victoria and Albert Museum.

If the object to be tested is suspected of being highly light-sensitive, the light output for an initial test can be reduced by inserting a neutral-density filter in the optical path. If the colour change is rapid, a test at full intensity may not be necessary. Other users will skip this step and simply stop the test before an unacceptable change is reached.

Mixed colourants (red and yellow pigments to make orange, for example) may have different rates of fading based on the light sensitivity of each component. If the material is applied thinly, the substrate may influence the MFT results: for example, you may see bleaching of paper in addition to fading of a colourant. Testing an area of exposed paper could provide more information regarding its influence on the behaviour of a colourant. The operator may want to test multiple spots of a colour, as well as the substrate, to get a better idea of the fading behaviour of an object.

Restrictions and considerations

Before testing an object, it is useful to have an idea of the restrictions MFT is subject to. Some knowledge of the behaviour of the materials when exposed to visible light is recommended. For example, both cyanotypes and iron gall ink will fade when exposed to light but recover to some extent when placed in the dark. Some media will darken rather than fade when exposed to visible light. Ideally, MFT should be carried out in conjunction with real-time colour monitoring using a spectrophotometer, which requires time and staff resources and may not be realistic. However, published comparative studies suggest that MFT is sufficient to identify sensitive objects and assess the risk of damage due to light exposure.⁵

Other considerations include glazed objects, where obtaining a good focus (and thus accurate data collection) can be difficult. Vibration can cause disturbances; units mounted on tripods may be susceptible to vibrations from people walking nearby and the instrument’s laptop should be on a separate surface to prevent vibrations. Also, testing upright stretched canvas or textiles may be difficult due to movement of the object because of air handling units or vibrations from the floor.

This Technical Brief was prepared by Colette Hardman-Peavy (Virginia Museum of Fine Arts) on behalf of the Heritage Science Expert Working Group of the Analytical Methods Committee and approved by the AMC on 26th June, 2025.

References

1. V. L. Beltran, Advancing Microfading Tester Practice: A Report from an Experts Meeting Organized by the Getty Conservation Institute, Getty Conservation Institute, Los Angeles, 2019, https://www.getty.edu/conservation/publications_resources/pdf_publications/advancing_mft_practice.html, accessed 20 June 2025.
2. Wikipedia, 19 April, 2025, Color Difference, https://en.wikipedia.org/wiki/Color_difference#cite_note-CIE76_JND-12, accessed 20 June 2025.
3. D. Saunders, Museum Lighting: A Guide for Conservators and Curators, Getty Trust Publications, Los Angeles, 2020.
4. B. Pretzel, Now you see it, now you don’t: lighting decisions for the Ardabil carpet based on the probability of visual perception and rates of fading, ICOM Committee for Conservation 15th Triennial Meeting New Delhi, India, 22–26 September, 2008, https://www.icom-cc-publications-online.org/1934/Now-you-see-it-now-you-dont--lighting-decisions-for-the-Ardabil-carpet-based-on-the-probability-of-visual-perception-and-rates-of-fading, accessed 20 June 2025.
5. V. L. Beltran, P. Christel, S. K. Freeman and M. Benson, Microfading Tester: Light Sensitivity Assessment and Role in Lighting Policy, Guidelines, Getty Conservation Institute, Los Angeles, 2021, https://www.getty.edu/conservation/publications_resources/pdf_publications/microfading_tester.html, accessed 20 June 2025.
6. CAMEO Materials Database, 8 May, 2022, Blue Wool Standard, https://cameo.mfa.org/wiki/Blue_Wool_Standard, accessed 20 June 2025.
7. J. Druzik, Oriel Microfading Tester (MFT): A Brief Description, in Textile Specialty Group Postprints, AIC Annual Meeting, Milwaukee, WI, 2010, vol. 20.

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