Beam characterization of a microfading tester: evaluation of several methods

Abstract

Microfading testing allows to evaluate the sensitivity to light of a specific artwork. Characterization of the illumination spot is important to determine its shape, dimensions, light distribution, and intensity in order to limit and account for possible damage. In this research the advantages and disadvantages of several methods used to determine the beam shape and intensity profiles are described with the aim of providing various options to microfading researchers interested in characterizing their irradiation spots. Conventional and imaging methods were employed and are compared in terms of their accuracy, cost, reliability, and technical features. Conventional methods consisted of an aperture technique using aluminium foil and four different materials namely stainless steel, silicon, muscovite, and Teflon used as sharp edges. The imaging methods consisted of digital photography of illumination spot, direct beam measurement using a CMOS camera, and direct beam measurement using a laser beam profiler. The results show that both conventional and imaging methods provide beam width measurements, which are in satisfactory agreement within experimental error. The two best methods were direct measurement of the beam using a CMOS camera and sharp-edge procedure. MFT illumination beam with a CMOS camera followed by a determination of the beam diameter using a direct method, more specifically one involving a sharp-edge technique.

Full text

Świtetal. Herit Sci (2021) 9:78
https://doi.org/10.1186/s40494-021-00556-7
RESEARCH ARTICLE
Beam characterization ofamicrofading
tester: evaluation ofseveral methods
Paweł Świt1* , Marco Gargano2 and Julio M. del Hoyo‑Meléndez3
Abstract
Microfading testing allows to evaluate the sensitivity to light of a specific artwork. Characterization of the illumination
spot is important to determine its shape, dimensions, light distribution, and intensity in order to limit and account for
possible damage. In this research the advantages and disadvantages of several methods used to determine the beam
shape and intensity profiles are described with the aim of providing various options to microfading researchers inter‑
ested in characterizing their irradiation spots. Conventional and imaging methods were employed and are compared
in terms of their accuracy, cost, reliability, and technical features. Conventional methods consisted of an aperture tech‑
nique using aluminium foil and four different materials namely stainless steel, silicon, muscovite, and Teflon used as
sharp edges. The imaging methods consisted of digital photography of illumination spot, direct beam measurement
using a CMOS camera, and direct beam measurement using a laser beam profiler. The results show that both conven‑
tional and imaging methods provide beam width measurements, which are in satisfactory agreement within experi‑
mental error. The two best methods were direct measurement of the beam using a CMOS camera and sharp‑edge
procedure. MFT illumination beam with a CMOS camera followed by a determination of the beam diameter using a
direct method, more specifically one involving a sharp‑edge technique.
Keywords: Beam profile, Illumination intensity, Sharp‑edge method, Microfading, Spot shape
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Introduction
Microfading testing (MFT) has become widely accepted
by the conservation science community as an adequate
tool for establishing and recommending appropriate gal-
lery lighting conditions that minimize damage to collec-
tions. ese devices offer the opportunity of measuring
the photostability of objects due to their optical setup,
which allows to conduct and quantify accelerated photo-
aging over a spot of approximately 0.5mm. Also, by using
a high sensitivity photodetector it is possible to measure
spectrocolorimetric change before it is perceived by the
human eye. Although a considerable amount of testing
is currently performed with these instruments, there are
still safety concerns in terms of possible damage to the
objects due to the use of a high intensity spot during test-
ing. Nevertheless microfadeometry is widely considered
a non-destructive technique providing the evaluation of
the light sensitivity of objects over the last two decades
[1–8]. is is due to the small spot size employed, which
permits the evaluation of the durability of materials,
while causing no harm to the object. e process is less
time consuming than traditional accelerated light aging
tests and the results can be followed in real time. e
sensitivity of objects to visible light can be determined
by short increments in exposure time and simultaneous
direct lightfastness measurements. is equipment offers
the opportunity of direct testing the sensitivity to light
of objects due to the microscopic size of the area under
investigation. Prior to this equipment’s development it
took weeks to acquire lightfastness data and it was not
possible to obtain information directly from the collec-
tions of unique objects.
Open Access
*Correspondence: pawel.swit@us.edu.pl
1 Institute of Chemistry, Faculty of Science and Technology, University
of Silesia, 40006 Katowice, Poland
Full list of author information is available at the end of the article
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Świtetal. Herit Sci (2021) 9:78
MFT is a well-established technique and many institu-
tions are aware of the multiple advantages of microfad-
ing such as being a unique tool for exhibition planning
[9, 10]. Besides this, it is important to note that there are
many institutions, which are not equipped with MFT
instruments and have to just rely on illuminance meas-
urements. Additionally, some conservators still consider
MFT harmful due to possible photo-oxidation reactions,
heating of the sample, and possible interactions of indi-
vidual chemical components present in the sample area.
Other researchers have raised concerns about the differ-
ences between the spectral power distribution and inten-
sity of gallery lighting used to illuminate the objects and
those used for MFT tests. e authors believe that all
these considerations could limit the further development
and spread of the MFT technique.
Fortunately, in most cases, thanks also to the relatively
short testing time the techniques allows conservators
and conservation scientists to make surveys of collec-
tions prior to their display in order to determine if there
are critical works having unusually high sensitivity to
light exposure [2, 4, 11]. e instrument also offers the
possibility of performing light-fastness tests on a single
material (e.g. powder pigment) or mock-ups prepared in
the laboratory consisting of combinations of two or more
materials (e.g. pigment/binder system) [12, 13]. Recently
there has been considerable interest in the use of MFT
and the number of cultural institutions currently using
or considering construction or purchase of a MFT device
increases steadily [14]. Although some people have
expressed concern regarding the safety of objects during
and after testing, the technique continues to be widely
employed in institutions around the world. In contrast
to research-based applications where it is important
to accurately determine the power density of the beam,
the safety of the object under investigation is of less con-
cern in MFT as the degree of change is monitored in real
time and the test can be stopped once a threshold level is
reached.
Several authors have pointed out the importance of
working with a safe and stable irradiation spot when per-
forming spectroscopic measurements on objects. Due to
concerns associated with two-photon effect at high irra-
diance, local heating leading to local relative humidity
change, which could cause significant movement in the
Z direction of the area being microfaded, post-exposure
recovery of colour or further colour change after meas-
uring, previous publications have discussed the impor-
tance of conducting safe measurements on objects when
using techniques such as Raman spectroscopy [15], X-ray
fluorescence spectrometry (XRF) [16], optical coherence
tomography (OCT) [17], and laser induced breakdown
spectroscopy (LIBS) [18], among others. e potential
temperature increase at the surface of an object caused
by exposure to light from a microfading device has been
described by Ford [3], Lerwill [19] and Whitmore [1].
ese three studies revealed that the temperature of the
investigated surface may experience an increase up to a
maximum of 5°C. Ford has noted that these changes pre-
clude thermal damage to objects or having an influence
on fading rates and mechanisms [3]. e concern about
initiating two-photon processes has also been expressed
in relation to flash photography [20]. However, the
author concluded that modern flash units are not power-
ful enough to initiate two-photon degradation processes,
as had been suggested in the past. e illuminance deliv-
ered to a white standard can be used to contextualize the
discussion on potential photodegradation. For example,
white reflectance standards, typically used in calibration
of MFT instruments, have a surface with high flat spec-
tral response. It is known that factors such as movement
of the light source, fiber optics, or measuring probes may
increase the uncertainty of measurements, especially
when working at high intensity levels. However, spectral
measurements performed on a similar instrument using
a barium sulphate white tile did not alter for about 40 µm
through focus [21]. e authors also indicated that spec-
tral measurement is more sensitive than the variation in
size of the illuminated spot with probe position since the
alignment of illumination and measurement probes is the
key to correct interpretation of the results.
A well characterized spot is essential in microfading
research since the technique makes use of a high inten-
sity spot that acts on the object’s surface to determine
its stability to light. e shape, dimensions, light distri-
bution and intensity of the spot are important param-
eters to assess and verify non-invasiveness. erefore,
characterizing the irradiation spot is essential to ensure
that the tests are performed in a non-invasive and non-
destructive way. Safety of the irradiated surface is also a
priority for other research applications. For example, a
well-defined focused laser spot is especially important
for materials processing and medical applications [22,
23]. ere are several methods currently used for the
determination of the beam profile and diameter. One of
the most popular is the knife-edge method, where the
power of the laser beam is measured using a photo diode
while moving a sharp edge through the laser spot using
small distance increments [24]. Although this is a rela-
tively slow technique with a resolution of about 1 µm, it
is still widely employed in many research fields. Another
simple and relatively inexpensive method is exposure of
thermal or photopaper to the investigated beam in order
to produce a visible spot, which can later be accurately
measured. A disadvantage of using thermal or photo-
paper is its low dynamic range and the dependance of
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Świtetal. Herit Sci (2021) 9:78
the measured beam profile on the exposure time [25].
Another commonly used conventional way of determin-
ing the beam diameter of a point device is the aperture
method. e beam diameter may be determined for a
Gaussian beam by positioning an aperture in the center
of the beam and measuring the fraction of emitted power
passing through the aperture [26].
Other methods involve the use of digital cameras
to acquire photographic images of the output beams.
Golnabi and Haghighatzadeh have obtained reflection
and image transmittance data of an optical beam shap-
ing system based on a plastic fiber-bundle prism-coupled
waveguide by using a light emitting diode (LED) source
for the illumination and performing image analysis with a
charge coupled device (CCD) digital camera [27]. Moreo-
ver, complementary metal oxide semiconductor (CMOS)
cameras have also been employed to perform laser beam
profile measurements [28]. More recent approaches have
involved the use of smartphones [29] and webcams [30]
as low-cost beam profilers. Characterization of the illu-
mination spot is also important in microfading research
since the experiments are conducted on objects some-
times containing extremely sensitive materials. A test
conducted by an unexperienced operator on a very sen-
sitive material may result in a visible small spot (esti-
mated range 0.3–0.5mm) on the object as a result of a
prolonged exposure to microfading illumination. Exam-
ples showing discoloration of colored samples exposed
to excessive exposures can be found in [1, 3, 31]. It is
important to note that these were extreme tests that were
conducted with the aim of producing a clearly detecta-
ble color change for visualization purposes. In contrast,
microfading tests require much smaller exposures, which
can be real-time controlled to prevent such discernable
alterations. In addition to experience, a careful selection
of the testing areas is recommended along with monitor-
ing and documentation of the tested spot using imaging
methods. Liang et al. have measured the profile of the
incident spot of a retroreflective microfading spectrom-
eter using a CCD camera finding that the minor axis of
the spot was approximately 0.46 mm full-width at half
maximum (FWHM) [12]. Whitmore employed a similar
approach to the thermal or photopaper method men-
tioned above to determine a 0.4mm diameter for a test
area using his original instrument [1]. Lerwill determined
a 0.25mm diameter of the incident light, of a custom
built microfadometer, acting upon the investigated area
through a series of observations using a CCD camera
[19]. Pesme etal. developed three portable MFT instru-
ments and compared their performance to that of Whit-
more’s original design [32]. e study revealed that all
instruments operated within safe exposure limits, even
when working with a contact technique. e authors
report estimated illumination beam diameters, which
remained within the 0.5–0.6mm range. It was noted that
the intensity delivered by each of these three instruments
on the tested surface was related to the diameter of the
target spot, which was considered difficult to measure
accurately. e objective of the present study was to
obtain qualitative and quantitative information about the
output beam shape of a custom-built MFT instrument
and discuss the performance and efficiency of several
measuring methods with the aim of providing a range
of alternatives to MFT users interested in characterizing
their illumination spots.
Experimental
Microfading tester (MFT)
e MFT evaluated in this study is a custom-built instru-
ment developed by researchers at the National Museum
in Krakow, the Faculty of Physics and the Faculty of
Chemistry of the Jagiellonian University [31] and it is
based on the original prototype designed by Whitmore
etal. [1]. is instrument consists of a high-power light
source, a 0°/45°geometry optical setup, and a Vis reflec-
tance spectrometer that is used to measure the materials’
responsiveness upon irradiation. e high-power light
source employed is a HPLS 30–04 solid state plasma light
source (LIFI) from orlabs (New Jersey, US), with emis-
sion in the 350–750nm range. e spectral power distri-
bution of the light source is shown in Fig.1. e intensity
of the peak at 545nm was monitored during the direct
measurements performed to characterize the illumina-
tion spot.
An OSL 1-EC high intensity fiber light source (or-
labs, US) with a 380–800 nm emission range was used
in some of the experiments to determine the shape and
size of the collection spot. Both sources were filtered to
remove wavelengths shorter than 400 nm and higher
than 750 nm. e system uses fiber optics to provide
Fig. 1 Spectral power distribution (SPD) of the solid‑state plasma
light source (LIFI) used in microfading testing experiments.
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non-contact measurement and detection. e illumina-
tion fiber is connected to a probe containing three ach-
romatic lenses with focal lengths of 75, 75 and 50mm,
which focus the beam to a spot of about 0.5mm. e col-
lection fiber is connected to a probe holding two achro-
matic lenses with 50 and 75mm focal lengths collecting
the scattered light reflected from the sample at an angle
of 45°. e estimated illuminance measured at the spot
is in the 4.0–6.0 Mlx range. e MFT uses an Ocean
Optics (Florida, US) Jaz UV–Vis spectrometer for regis-
tering spectral data. e Jaz spectrometer is responsive
in the 200–1100 nm range and has an optical resolution
of ~1.5nm (FWHM). In addition, a PM100USB power
and energy meter from orlabs was used to monitor the
stability of the generated beam every day before making
the measurements. e signal drift recorded in mW,
remained within 5% after 15 min of continuous exposure
from any of the two light sources used. After stabilization
of the light source, the spectrometer was calibrated using
a USRS-99-010 white Spectralon® calibration standard
(Labsphere, USA). Visible reflectance spectra were col-
lected using a 100 ms integration time, 10 average scans
and a boxcar width of 20.
Some examples of the MFT during measurements are
presented in Fig.2 to show the interaction between the
illuminated spot and the surface of various objects.
Stereomicroscopy
A Zeiss SteREO Discovery V12 stereomicroscope
(Oberkochen, DE), equipped with a Zeiss AxioCam ERc
Fig. 2 Examples of MFT measurements: a technical drawing from the collection of the Coal Mining Museum in Zabrze, Poland; b large‑format
pastel artwork on paper by Stanisław Wyspiański (measurement carried out through glass on a vertical surface); c fragment of the painting “The
Ecstasy of St Francis” by El Greco from the Diocesan Museum in Siedlce, Poland; d area of fresco paintings at the The Church of the Holy Trinity in
Lublin, Poland (measurement on a vertical surface)
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5s camera, operating in reflection mode was used to
observe and acquire images of the materials used for the
direct measurement methods, which included the pin-
hole in aluminum foil and the materials used as sharp
edges. Samples were illuminated using an external LED
source with a color temperature of 6200 K.
Characterization ofMFT illumination spot
Several methods were used to characterize the MFT
illumination spot with the aim of comparing the results
obtained through each technique and discussing their
advantages and limitations. e methods were divided
into two categories, namely direct and optical imaging. In
Table1 are summarized the considered methods speci-
fying beam diameter, reliability in terms of measurement
accuracy (or others parameters) and main features.
Digital photography
As a preliminary step, images of the MFT illumination
beam projected on a piece of white paper were acquired
with a Canon® EOS 40D camera (Tokyo, JP) using a
Canon Macro Lens EF 100 mm.
Aperture method
For the aperture method, a homemade pinhole approach
was used to determine the MFT illumination beam out-
put intensity distribution. is method uses a relatively
smaller size pinhole relative to the expected diameter
of the investigated beam. e pinhole is scanned across
the beam with the aim of accurately locating its center
[26]. e intensity of the beam is measured over the
aperture of the pinhole and a plot of intensity as func-
tion of beam radius is obtained by scanning the pinhole
through the beam. For this purpose, a pinhole setup was
prepared using laboratory aluminum foil with thickness
0.030mm (Witko, PL). A 2 × 2cm piece of aluminum
foil was pierced with the tip of a 25-gauge syringe needle.
e aluminum foil pinhole was attached to a MVS005
(orlabs, US) vertical stage and a PT1 (orlabs, US)
horizontal stage to allow movement using a 0.02mm step
in x- and y- directions. e pinhole was placed between
the illumination and collection probes. e spectrometer
used for these measurements was the USB4000 (Ocean
Optics, US). e integration time was 1s with 10 average
scans and a boxcar width of 20. e SpectraSuite (Ocean
Optics, US) software was used for reflectance spectra
acquisition. A schematic diagram of the setup is shown
in Fig.3.
Sharp edge method
Another direct technique employed in this study was the
sharp edge method. is method has been widely used
for determining the beam quality in laser applications
using a knife-edge approach [26, 33]. It is characterized
by its simplicity and its extensive use over a wide range
of wavelengths. In this method a sharp edge is trans-
lated perpendicular to the direction of propagation of the
beam, while a comparison as line scan power differences
dependent on the position of the sharp edge provide an
accurate measurement of the beam diameter. When
the beam is not covered by the material, the measured
power reaches its maximum value. In contrast, the power
gradually decreases as the material reduces the amount
of light reaching the detector by blocking the beam. e
first step was to prepare a plot of the recorded signal as a
function of the distance travelled by the edge of a specific
material. Next, the first derivative of this plot was calcu-
lated, which in turn allowed to determine the diameter
of the illumination spot. e step used was 0.01 mm.
e measurement setup was similar to the one used for
the pinhole measurement, the only difference was that
instead of a pinhole, the edge of the material was moved
along x- and y- directions using the translation stages to
reduce the amount of light reaching the detector (Fig.3).
Edges obtained using a safety knife blade, a silicon
wafer, muscovite, and Teflon tape were evaluated within
Table 1 Summary of methods used to characterize the illumination spot
Method Category Estimated beam
diameter (µm) Reliability Features of the method
Photography Imaging 1200 Low Easy to use, inexpensive, simple apparatus ‑ camera or smartphone
Aperture
Aluminium foil Direct −Moderate Inexpensive, low accuracy with homemade pinhole
Easy to use, inexpensive, material with a well‑defined edge, soft‑
ware for data analysis and calculation
Sharp edge
Razor blade
Silicon wafer
Muscovite
Teflon tape
590.2
605.9
757.1
610.0
CMOS camera Imaging 702.2 Moderate Easy to use, moderate cost, dedicated CMOS detector
Laser beam profiler Imaging 386.1 High Specialist knowledge, costly, advanced equipment ‑ beam profiler
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this method. e selection of muscovite, silicon and Tef-
lon was based on previous publications in which these
materials have been employed in microscopy [34–36],
spectroscopy [37, 38], and optics [39, 40] applications.
e safety knife blades and the Teflon tape were pur-
chased from a local market in Krakow, Poland. e sili-
con wafer was produced and made available for research
by the Jerzy Haber Institute of Catalysis and Surface
Chemistry, Polish Academy of Sciences (Krakow, PL).
Muscovite Mica 53000 was purchased from Kremer Pig-
mente (Aichstetten, DE).
Direct methods models
e mathematical model followed to determine the
diameter of the irradiation beam is the one proposed by
Chapple [41], originally used to calculate the irradiance
of an ideal laser beam that displays a Gaussian profile.
According to Chapple, the irradiance I(x, y) of a laser
beam can be described by the following equation:
where I0 is the peak irradiance at the center of the
beam, x and y are the transverse Cartesian coordinates of
any point with respect to the center of the beam located
at (x0, y0), and r is the 1/e2 beam radius. e irradiance
is replaced by the total power PT, since power and irra-
diance are related by the area, which is a constant fac-
tor. Direct methods will result in gradual increases or
decreases in total power depending on the position of
the attenuation material during a series of step measure-
ments. e derivative of the edge or pinhole data is found
using the equation below to obtain a two-dimensional
Gaussian profile. e derivative at any data point was
I
x,y

=I0exp

−
2

(x−x0)2+y−y02

r2


performed averaging two adjacent data points derivatives
computed using the finite difference method:
the 1/e2 radius was then obtained by fitting a Gaussian
function to the data using OriginPro 2021 (OriginLab,
USA). e model is equally applicable to a beam of light
focused to a small spot.
CMOS camera
A compact DCC1645C-USB 2.0 (orlabs, US) comple-
mentary metal-oxide-semiconductor (CMOS) camera
was used to record images of the illumination beam and
determine its diameter. is camera has a resolution of
1280 × 1024 pixels and an exact sensitive area of 4.61
mm × 3.69 mm. is model offers a pixel size of 3.6 µm
(square) and has a micro lens with a 25° chief ray angle
(CRA) correction. e camera was installed directly
in front of the illumination beam at the same position
where the pinhole or sharp edge materials were origi-
nally placed (Fig. 3). e orCam software (orlabs,
US) was used to provide system control and image acqui-
sition. ImageJ software (National Institutes of Health,
Bethesda, MD) was used for image analysis and perform-
ing measurements of the size and shape of the spot as
well as the intensity of the spot.
CCD camera beam proler
A BC106-Vis CCD Camera Beam Profiler (orlabs, US)
was also used to characterize the illumination spot of the
MFT. is camera operates in the 350–1100nm wave-
length range and has a resolution of 1360 × 1024 pixel.
e aperture size is 8.77 and 6.60 mm for width and
height, respectively. e pixel size was equal to 6.45µm
for both width and height. e filter wheel was rotated to
dP
T
dx
=
1
2
y
i+1−
y
i
xi+1
−
xi
+
y
i−
y
i−1
xi
−
xi−1
Fig. 3 Experimental setup used to determine the diameter of the MFT illumination beam
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set the ND filter with the greatest intensity reduction (40
dB corresponding to 0.0001 transmittance) in front of the
camera aperture to prevent damage to the camera sensor.
e signal intensity for the measurements was 100µW
and the exposure time was 180ms.
Results anddiscussion
e characterization of the illuminated spot is usually
the first step taken by microfading researchers in order to
determine the diameter, intensity, and shape of the illu-
mination spot. Typically, once the spot size and shape are
characterized, a micro-integrating sphere sensor together
with a radiometer are used to determine the intensity of
the illuminated spot; this procedure has been described
elsewhere [1, 4].
In the present work, the first approach used to evalu-
ate the shape and size of the beam was to project the
illumination spot of the custom built MFT on a piece of
white paper as shown in Fig.4a. e working distance of
about 1.0cm typically used during a fading test was also
used for these measurements. e diameter of the beam
was estimated at 1.2 mm. e configuration of micro-
fading tester used for acquiring the images of the spot
is shown in Fig.3b, c. shows a similar measurement car-
ried out on the analysis spot of a commercially available
Oriel 80190 Fading Test System produced by Newport
(California, US). In this case, the light exiting the illumi-
nation probe was focused to a pinhole size by adjusting
the working distance from the edge of the illumination
probe to the surface of a semi-transparent glassine paper
to 1.0 cm. An image of the spot was then acquired using
a Leica MZ-16 microscope equipped with a digital high
resolution color camera. e estimated diameter of the
illuminated spot was 400 μm. Visual inspection of the
image reveals that the center of the spot receives higher
illuminance indicated by color variations that range from
bright white passing through yellow, orange and finally
red, as one moves from the inner part of the spot out
towards its edges.
Some interesting observations can be made after com-
paring the illumination spot obtained for the custom-
built MFT instrument with the one observed for the
commercial version. e latter was developed by Whit-
more [1] and was commercially available as the Oriel
80190 fading test system. In general, the use of a digital
camera may result in overestimation of the diameter of
the illumination beam relative to the image obtained
using a microscope. After analyzing and measuring the
spot, it was observed that even at the proper working
distance a relatively higher size of the spot was obtained
relative to the other methods employed. e diameter of
the MFT beam acting on the analyzed surface has been
reported to be up to 0.5mm [1, 3, 12, 42]. e 1.2mm
value obtained using digital photography seemed too
large indicating that a different measurement method
was necessary. us, micrographs can be used to deter-
mine the size of the illumination spot since they offer a
better alternative to digital photography in terms of accu-
racy. Following this approach, the pinhole used for deter-
mining the size of the illumination beam of the MFT
instrument was examined with stereomicrophotography.
An example of a microscopic image of the pinhole used is
shown in Fig.5a. Measurements along the x and y axes of
the pinhole were 69.85 and 69.50µm, respectively. Very
accurate measurements are possible when using a com-
bined approach of microscopy and image analysis. In
the example presented in Fig.5a nearly symmetric circle
was obtained after comparing measurements of x- and y-
axes, which have a standard deviation of 0.18µm.
Plots of the intensity of the beam measured over the
aperture of the pinhole along x- and y- axes are pre-
sented in Fig. 5. e red and black profiles obtained
correspond to unfiltered and filtered beam signals. e
filter used was a NE05B 25 mm absorptive ND filter
(orlabs, US) a with an optical density of 0.5. e fil-
ter was used to attenuate the beam in order to obtain
a closer profile to a Gaussian curve. One of the advan-
tages of this technique is that the position of the pin-
hole does not need to be known beforehand as it can
be located during the measurements [43]. Although a
fairly symmetrical hole was pierced on the aluminum
foil (Fig. 5a), the plots showed significant deviation
from a Gaussian curve. e profiles obtained along
the x-axis were similar with and without the use of the
filter. However, after inspecting the profiles obtained
along the y-axis it can be seen that the ND filter
resulted in a different profile showing a decrease in
intensity between 200 and 300µm. Attenuation of the
beam also resulted in a broader beam diameter rela-
tive to the measurement performed without a filter.
e results obtained here show that the determination
can be inaccurate using this direct method. is was
likely due to lack of cleanliness when piercing the alu-
minum foil resulting in microscopic irregularities such
as barbs and other material residues inside the pinhole.
In addition, manually piercing the aluminum foil with
a syringe needle did not provide an accurate optical
aperture. e aluminum foil and the detection system
used were not optimal for observing subtle intensity
differences near the edges of the pinhole. In this con-
figuration, the image of the spot was not uniform,
while attempting to fit a Gaussian function to such an
irregular curve resulted in unreliable results. Although
it seemed like a straightforward approach, a home-
made pinhole using aluminum foil was not an adequate
method to determine the shape and size of the spot.
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Świtetal. Herit Sci (2021) 9:78
While a near top-hat beam shape was observed for
the unfiltered measurement along the y- axis, the use
of a commercially available pinhole with a consistent
diameter and a more stable material is recommended.
is will provide a more accurate measurement of
the diameter of the beam due to the cleanliness of the
material and accuracy of the pinhole.
e sharp edge method was a more reliable alterna-
tive to determine the diameter of the illumination spot. A
series of measurements of power as a function of position
Fig. 4 Digital images of: a illuminated spot of the MFT evaluated in this study; b setup used to acquire the images of the spot and c the spot
delivered by an Oriel 80190 Fading Test System [42]
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Świtetal. Herit Sci (2021) 9:78
of the razor blade were carried out, starting from a posi-
tion where the laser beam was not blocked at all, so the
measured power was equal to the total power. e final
position was chosen at a point where the beam was
totally blocked out and the measured power was negli-
gible and remained constant. To obtain the full profile
of the beam a displacement of the razor blade of about
1.20mm along the x axis was needed. e characteristic
S-shaped curve expected is shown in Fig.6a.
Figure6b shows the first derivative (black line) of the
measured curve reported in Fig.6a. e red line shows
the Gaussian function fitted to the experimental data.
After selecting the appropriate mathematical function, it
was possible to determine the parameters necessary for
the broad interpretation of the measurements carried out
on the illumination spot. ese parameters are full-width
at half-maximum (FWHM), diameter at the baseline (b),
diameter at a power of 13.5% (z), and diameter for the
height of 86.5% (w). e calculated parameters for the
different materials used are summarized in Table2.
Measurements and calculations performed along x-
and y- directions provided a 2D representation of the
MFT illumination spot. e b parameter was considered
the most representative value and therefore, it was used
as the approximate size of the spot. Average b values of
590.2, 605.9, 757.1, and 610.0µm were obtained by using
a razor blade, Teflon tape, silicon wafer, and muscovite,
Fig. 5 Beam profile using the aperture method with aluminium foil:
a measurement carried out along the y‑ axis and b determination
performed along the x‑axis. Red and black lines correspond to
unfiltered and filtered beams, respectively
Fig. 6 Example of measurements taken using the sharp edge
method razor blade along the x axis: a plot of optical power versus
position of the blade and b calculated diameter of the spot (black
line) along with fitted Gaussian function (red line)
Table 2 Beam diameters in µm obtained using various materials
with the sharp‑edge method
Material Direction of
step
FWHM w b z
Razor blade x 172.3 97.1 537.1 292.9
y 223.9 126.2 643.2 382.8
Teflon tape x 214.3 121.8 606.6 364.4
y 214.8 136.3 605.1 360.5
Silicon wafer x 153.6 86.6 459.5 261.3
y 209.3 117.9 1054.7 348.2
Muscovite x 205.1 115.6 607.4 350.4
y 228.5 128.8 612.6 281.9
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respectively. Some scatter in the data is observed depend-
ing on the method used. e estimated average size of
the spot was 640.8±78.0µm. An initial inspection of the
data suggest that the spot has an elliptical shape, espe-
cially for the results obtained with the silicon wafer. In
contrast, the differences obtained with the Teflon tape
and muscovite are almost negligible indicating that the
spot is circular. e calculations made from measure-
ments conducted using a silicon wafer showed higher
dispersion relative to the three remaining materials.
Although this material showed a very sharp edge when
observed under the microscope, the results indicate that
it has poor consistency.
In the next stage, the CMOS camera was used to char-
acterize both the illumination and measuring spot. e
sensor was placed on the path of the MFT illumination
beam and with suitable attenuation images of the spot
were recorded. Since the sensor resolution and the size
of a single pixel were known, it was possible to determine
the size of the spot on the basis of the image recorded.
e estimated spot diameter was 702.2 ± 3.6 μm
(Fig.7a). Profiles of the illumination spot in 2D and 3D
obtained are presented in Fig.7b, c, respectively.
Image analysis shows that the MFT illumination beam
has a top-hat shape, which indicates that the irradiated
area receives a uniform amount of energy throughout
the entire area analyzed. A MFT beam previously char-
acterized by Liang etal. also exhibited a top-hat profile
along the minor axis and near top-hat shape along the
major axis [12]. An evaluation of the MFT optical setup
revealed a dependence of the measured signal on the
working distance. For this reason, the FWHM was also
determined for the collection spot as well as for the com-
mon area (illumination and collection) along the two
main axes to determine the width and length of the two
spots
A second light source was used to pass light through
the collection fiber in order to determine the size and
shape of the collection spot. Figure 8 shows the loca-
tion of individual spots depending on the distance of
Fig. 7 MFT illumination beam: a CMOS detector image; b 2D profile of the spot; c 3D profile
Fig. 8 Influence of the distance of the probe from the surface of the
tested system on the shape and size of the lighting and measuring
spot
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Świtetal. Herit Sci (2021) 9:78
the probe from the surface of the tested object. ree
MFT optical setup positions are shown, namely lower
than optimal, optimal, and higher than optimal. It can be
observed that the shape of the collection spot becomes
distorted as one gets too close or too far from the optimal
position. As one approximates the optimal position, the
collection spot starts taking an elliptical shape up to the
point of optimal alignment. As expected for this meas-
urement configuration, the optimal optics alignment
position is reached when illumination and collection
lines meet at 0 and 45°, respectively, relative to the ana-
lyzed surface. At this working distance, the illumination
spot remains within the collection spot
Further analysis of the illumination and collection areas
was conducted using the laser beam profiler. is instru-
ment is typically used in optics and physics laboratories
to characterize laser beams. Examples of measurements
for illumination and collection spots using this technique
are presented in Fig. 9. Figure 9a, b show the images
obtained for the illumination spot in 2D and 3D repre-
sentations, respectively. e illumination beam exhibits
uniform distribution of energy over the tested area and
has a round nearly top-hat shape (yellow plots) as pre-
viously shown by the CMOS camera. e red lines cor-
respond to Gaussian fits of the data. e main part of
illumination energy is concentrated along the main top-
hat peaks and weaker wavelets of about 50 µm around
the main peak can be recognized. e 2D and 3D images
obtained for the collection spot are shown in Fig.9c, d,
respectively. e oval shape of this spot becomes evident
after inspecting the image, which shows presence of a
major and a minor axis. e energy profile of the collec-
tion spot along with its 3D plot confirm its top-hat beam
shape.
An image of the common area of illumination and col-
lection captured with the CMOS camera is shown in
Fig. 9e. e images obtained with the CMOS camera
and the laser beam profiler were further analyzed using
ImageJ to obtain a representation of the common area
irradiated and measured using MFT. e shape and size
of the illumination spot was a circle with a diameter of
386µm, while the diameter along x- and y- axes of the
collection spot were 445 and 629 µm, respectively. A
schematic representation of the two spots at the optimal
working distance is shown in Fig.9f. A similar analysis of
the tested area was conducted by Lerwill etal. [21]. e
authors carried out a similar procedure to verify confo-
cality of the optical setup by focusing both beams onto
a CCD sensor. In this way, they were able to indicate the
sampling area of the collection probe and identify the
region where fading takes place. e results from the pre-
sent study are in agreement with those obtained by Ler-
will and co-workers, where the maximum signal detected
by the spectrometer is associated with a reproducible
spot size and low variation in the calculated fading.
Conclusion
is study has evaluated the performance of various
conventional and imaging methods for the characteri-
zation of MFT illumination and measurement spots.
e advantages and disadvantages of several methods
used to determine the beam shape and intensity profiles
have been described. Although a preliminary view of
the appearance and size of the illumination beam can be
obtained using digital photography, this approach alone
cannot be used to determine the actual shape and the size
of the spot. A homemade pinhole was tested for the aper-
ture method, but no reliable results are reported mainly
due to error associated with manual piercing of the alu-
minum foil. For this reason, the use of a machine-made
commercial pinhole is recommended over a homemade
one. In general, sharp-edge methods enable calculation
of the size of the illumination and collection spots. For
this purpose, a material with a well-defined edge is essen-
tial, otherwise higher dispersion of the beam could be
observed due to the use of an irregular edge or a material
with low beam attenuation power. e results obtained
with the four materials used in the current study show
good agreement. erefore, the use of readily available
materials, such as a razor blade or Teflon tape, is recom-
mended. An imaging approach using a CMOS camera or
a laser beam profiler should be used as a complementary
method to the direct determination method. e laser
beam profiler provided an accurate way of measuring
and characterizing the illumination and collection spots
of the MFT. However, one disadvantage is its relatively
higher cost when compared to other techniques used in
this study. An adequate characterization of illumination
and collection spots is important since during a MFT
measurement the surface of the analyzed object has an
evident influence on the measurement and an object
containing extremely sensitive materials may experience
a microscopic but noticeable change as a result of test-
ing. e accurate characterization of both beams allows
for a better understanding of the tested system and
enhanced knowledge about the interaction between light
and the surface under investigation. From our experi-
ence, it would be advisable to first acquire images of the
MFT illumination beam with a CMOS camera followed
by a determination of the beam diameter using a direct
method, more specifically one involving a sharp-edge
technique. Although characterization of the illumination
spot is a time-consuming task, the authors recommend
conducting and documenting these measurements sub-
ject to frequency of use of the instrument and any signifi-
cant hardware changes.
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Świtetal. Herit Sci (2021) 9:78
Fig. 9. Laser beam profiler measurements: a 2D image of the illumination spot showing energies along x‑ and y‑ axes; b 3D view of the
illumination spot; c 2D image of the collection spot showing energies along x‑ and y‑ axes; d 3D view of the collection spot; e image of the
common area of illumination and collection captured with the CMOS camera; f estimated sizes and shapes of illumination spot (yellow) and
collection spot (grey)
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Świtetal. Herit Sci (2021) 9:78
Abbreviations
CCD: Charge coupled device; CMOS: Complementary metal oxide semicon‑
ductor; FWHM: Full width at half maximum; MFT: Microfading testing; LED:
Light emitting diode.
Acknowledgements
The authors are very grateful to Professor Krzysztof Dzierżęga from the Faculty
of Physics, Astronomy and Applied Computer Science of the Jagiellonian
University in Krakow for allowing us to borrow the beam profiler used in our
study.
Authors’ contributions
PŚ: conceptualization, methodology, validation, investigation, writing original
draft. MG: methodology, validation, writing original draft. JdH: conceptualiza‑
tion, methodology, project leader, writing–original draft. All authors read and
approved the final manuscript.
Funding
This publication is financed by the Research Excellence Initiative of the Univer‑
sity of Silesia in Katowice, Poland.
Availability of data and materials
The datasets used and/or analyzed during the current study are available from
the corresponding author on reasonable request.
Declarations
Competing interests
The authors declare no conflict of interest.
Author details
1 Institute of Chemistry, Faculty of Science and Technology, University of Silesia,
40006 Katowice, Poland. 2 Department of Physics, University of Milan, Via
Celoria, 16, 20133 Milan, Italy. 3 Laboratory of Analysis and Non‑Destructive
Investigation of Heritage Objects, National Museum in Krakow, 31109 Krakow,
Poland.
Received: 26 April 2021 Accepted: 25 June 2021
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