A typical MFT instrument setup consists of two basic elements:
• A light source—a xenon lamp or a LED source connected by a fiber optic to a
lens which focuses the beam on the surface of the material being tested (distance
between the lens and the objects’ surface ranges from few centimeters to zero—
a contact MFT designs were recently developed [149]).
• The Vis fiber optic spectrometer, which collects the signal using its own lens,
most often used in 0°/45° geometry or sometimes in retrofocus arrangement,
with light paths separated by a beam splitter between illumination and spectra
collection.
For the proper operation of a microfader, a high stability of the light power of the
used light source is essential as its fluctuations would be interpreted as changes in
the tested objects’ color. In case of a xenon lamp, which typically has short-term
flicker and longtime drift of it’s light power, a closed-loop feedback system is
necessary to ensure proper operation of the light source.
When carrying out lightfastness measurements by MFT, in addition to calibration on white and black standards, necessary to convert spectrometer signal into %
reflectance and CIELab color values, an additional calibration for the system light
power is usually performed. For this calibration, a reference material for lightfastness is used, which allows for a direct comparison with the results obtained for
examined historical objects, which immediately indicates the maximum light
exposure allowed for a given object to reach its intended preservation target (i.e.,
maximum color change acceptable for a given timeframe). More details on the most
commonly used lightfastness standards (the Blue Wool Standards) will be given in
the section below.
The MFT technique is currently applied at around 60 museums and dedicated
heritage research centers around the world. Most instrument setups are based on the
design described by Whitmore and have a character of DIY setups. Recently, a
ready-made instrument with software dedicated to conducting MFT tests with the
automated operation of many functions has been developed and became available
on the market.
16.3.1.2 Data Processing and Interpretation
An important criterion in the lightfastness analysis for the heritage materials is
whether the expected color change of the object exceeds the just-noticeable difference (JND) threshold. Art conservation literature recommendations often refer to
this value [150], although its precise determination encounters difficulties due to the
perceptual nonuniformities of the CIELab color space. The microfading tests allow
for many materials to achieve color change values that exceed the JND value as the
dose of illumination used in the tests could be very high—depending on the
instrument design and the light power of the source (usually being between 2 and
20 Mlx) and the time of testing (5–20 min), it could easily reach a dose of light
504
T. Łojewski and B. Łydżba-Kopczyńska
• A light source—a xenon lamp or a LED source connected by a fiber optic to a
lens which focuses the beam on the surface of the material being tested (distance
between the lens and the objects’ surface ranges from few centimeters to zero—
a contact MFT designs were recently developed [149]).
• The Vis fiber optic spectrometer, which collects the signal using its own lens,
most often used in 0°/45° geometry or sometimes in retrofocus arrangement,
with light paths separated by a beam splitter between illumination and spectra
collection.
For the proper operation of a microfader, a high stability of the light power of the
used light source is essential as its fluctuations would be interpreted as changes in
the tested objects’ color. In case of a xenon lamp, which typically has short-term
flicker and longtime drift of it’s light power, a closed-loop feedback system is
necessary to ensure proper operation of the light source.
When carrying out lightfastness measurements by MFT, in addition to calibration on white and black standards, necessary to convert spectrometer signal into %
reflectance and CIELab color values, an additional calibration for the system light
power is usually performed. For this calibration, a reference material for lightfastness is used, which allows for a direct comparison with the results obtained for
examined historical objects, which immediately indicates the maximum light
exposure allowed for a given object to reach its intended preservation target (i.e.,
maximum color change acceptable for a given timeframe). More details on the most
commonly used lightfastness standards (the Blue Wool Standards) will be given in
the section below.
The MFT technique is currently applied at around 60 museums and dedicated
heritage research centers around the world. Most instrument setups are based on the
design described by Whitmore and have a character of DIY setups. Recently, a
ready-made instrument with software dedicated to conducting MFT tests with the
automated operation of many functions has been developed and became available
on the market.
16.3.1.2 Data Processing and Interpretation
An important criterion in the lightfastness analysis for the heritage materials is
whether the expected color change of the object exceeds the just-noticeable difference (JND) threshold. Art conservation literature recommendations often refer to
this value [150], although its precise determination encounters difficulties due to the
perceptual nonuniformities of the CIELab color space. The microfading tests allow
for many materials to achieve color change values that exceed the JND value as the
dose of illumination used in the tests could be very high—depending on the
instrument design and the light power of the source (usually being between 2 and
20 Mlx) and the time of testing (5–20 min), it could easily reach a dose of light
504
T. Łojewski and B. Łydżba-Kopczyńska