Standoff Raman spectroscopy for architectural interiors from 3-15 m distances

Abstract

Portable and mobile Raman spectroscopy systems are increasingly being adopted in in situ non-invasive examination of artworks given their high specificity in material identification. However, these systems typically operate within centimeter range working distances, making the examination of large architectural interiors such as wall paintings in churches challenging. We demonstrate the first standoff Raman spectroscopy system for in situ investigation of historic architectural interior at distances > 3 m. The 780 nm continuous wave laser-induced standoff Raman system was successfully deployed for the in situ examination of wall paintings, at distances of 3–15 m, under ambient light. It is able to identify most common pigments while maintaining a very low laser intensity to avoid light induced degradation. It is shown to complement our current method of standoff remote surveys of wall paintings using spectral imaging.

Full text

Research Article Vol. 27, No. 22 / 28 October 2019 / Optics Express 31338
Standoff Raman spectroscopy for architectural
interiors from 3-15 m distances
YULI, CHI SHING CHEUNG, SOT IRI A KOGOU, FLORENCE LIGGINS,
AND HAIDA LIA NG*
School of Science & Technology, Nottingham Trent University, Nottingham NG11 8NS, UK
*haida.liang@ntu.ac.uk
Abstract:
Portable and mobile Raman spectroscopy systems are increasingly being adopted in
in situ non-invasive examination of artworks given their high specificity in material identification.
However, these systems typically operate within centimeter range working distances, making
the examination of large architectural interiors such as wall paintings in churches challenging.
We demonstrate the first standoff Raman spectroscopy system for in situ investigation of historic
architectural interior at distances
>
3 m. The 780 nm continuous wave laser-induced standoff
Raman system was successfully deployed for the in situ examination of wall paintings, at distances
of 3–15 m, under ambient light. It is able to identify most common pigments while maintaining
a very low laser intensity to avoid light induced degradation. It is shown to complement our
current method of standoff remote surveys of wall paintings using spectral imaging.
Published by The Optical Society under the terms of the Creative Commons Attribution 4.0 License. Further
distribution of this work must maintain attribution to the author(s) and the published article’s title, journal
citation, and DOI.
1. Introduction
The identification of materials such as pigments plays an important role in the study of wall
paintings in caves, tombs and buildings since they can reveal information about art history, trade
and cultural exchanges in the past, as well as monitoring signs of degradation. However, these
paintings are fragile, which limits the possibilities of sampling or contact measurements, and
therefore necessitates in situ and non-invasive measurements. Some challenges specific to wall
paintings may arise from the remoteness of the sites, the inaccessible height of the paintings
and the difficulty in controlling the environment they are in. Traditionally, the inspection of
upper parts of a wall or ceiling needs scaffolding, which is costly, inconvenient and unstable for
sensitive measurements that require long acquisition time [1].
One of the solutions addressing these issues is a visible/near infrared remote spectral imaging
system, PRISMS, developed in our group with the ability to image wall paintings at sub-millimeter
resolutions from a distance of tens of meters [2]. However, reflectance spectroscopy alone is
sometimes not sufficient to give definitive identification of pigments. Therefore, a standoff
Raman system is needed as a complementary technique.
Raman spectroscopy is capable of giving very specific material identifications by observing
the sharp Raman lines, which provide information about the molecular structure of the material.
With the development of powerful laser sources and sensitive detectors in the last three decades,
it has become one of the commonly used methods in the field of cultural heritage research for
non-invasive material identification (e.g. pigments of paintings), diagnosis of art works (e.g.
the extent of degradation), provenance determination, etc. [3–7]. However, limitations include
the presence of fluorescence that could potentially mask the Raman signal, and the potential
laser-induced degradation that limits the incident laser intensity.
In this paper, we define standoff Raman systems as instruments that can work at distances
greater than 3 m and less than 50 m between instrument and object. The idea of performing
Raman measurements remotely emerged in the 1960s in atmospheric scattering experiments [8,9].
#375455 https://doi.org/10.1364/OE.27.031338
Journal © 2019 Received 15 Aug 2019; revised 4 Oct 2019; accepted 7 Oct 2019; published 14 Oct 2019

Research Article Vol. 27, No. 22 / 28 October 2019 / Optics Express 31339
Currently, most standoff and remote Raman systems have been applied to planetary science and
explosive detection, employing high-power pulsed laser sources [10]. However, laser-induced
degradation must be taken into careful consideration in the field of cultural heritage research.
The intensity and fluence of the laser need to be strictly controlled in Raman measurements to
avoid damaging the artworks.
In this paper, we present the developments of a mobile standoff Raman system for wall paintings
working in the range from 3 to 15 m using a continuous wave (CW) laser at 780 nm, and its first
application in remote pigment identifications of wall paintings.
2. A remote Raman system for architectural interiors
The standoff Raman spectroscopy system for architectural interiors including wall paintings is
designed to satisfy the following requirements: 1) it must be sensitive enough to be able to detect
and identify most common historic artist pigments from a distance of 4 m at a spectral resolution
of
∼
8 cm
−1
in 30 mins; 2) laser intensity must be safe for all pigments; 3) it must be mobile to
allow for in situ measurements; 4) it must be able to operate under daylight conditions, since it is
not always possible to gain access at night; 5) laser excitation beam needs to be co-axial with the
collection optics such that the alignment between the excitation and collection beams is distance
independent; 6) the laser spot should be
<
10 mm in diameter at a distance of 10 m; 7) precise
measurement location must be recorded online.
The schematics of a standoff Raman system developed for working at distances
>
3 m is shown
in Fig. 1(a). The laser beam is reflected by two mirrors placed at 45
°
such that the laser beam
is co-axial with the telescope’s axis. The instrument employs a CW laser source for excitation
at 780 nm (Newport TLB-7113-01, a temperature stabilized external cavity diode laser with
a maximum output power of 90 mW and tuning range from 765 nm to 782 nm), a telescope
(Meade ETX-90, a Maksutov-Cassegrain reflector with 90 mm clear aperture and 1250 mm
focal length), and an Andor Shamrock spectrograph equipped with a high sensitivity Andor iDus
CCD detector thermoelectrically cooled to
−
75
°
C. The output laser beam is collimated with a
slight divergence which results in a spot diameter on the target of
∼
4 mm at 3 m and
∼
8 mm
at 8 m. A bandpass filter centred at 780 nm with a FWHM bandwidth of
∼
3 nm was used to
clean-up the laser beam. The scattered and reflected lights are collected by the telescope. A
PC controlled mechanism moves the primary mirror of the telescope to focus on a target. A
780 nm long-pass filter with a sharp cut-off is placed in front of the 200
µ
m diameter collection
fiber for rejection of the Rayleigh scattered line. A pair of plano-convex lenses together with
the long-pass filter are used to couple the received signals
>
780 nm into the fiber which is then
directed through a mechanical slit of 50
µ
m width into the spectrograph. The entire system is
mounted atop a motorized altitude-azimuth fork, which is controlled by a handset. The cut-off
wavenumber, dictated by the long-pass filter, is
∼
130 cm
−1
. A 770 nm dichroic beamsplitter
can be directly coupled to the rear port of the telescope to reflect light in the visible spectral
range to a guide camera to remotely align the laser in situ and record the position at which the
Raman measurement is taken. Since only the Stokes Raman signal is recorded, the dichroic
would not affect the Raman measurements. The spectral resolution is
∼
9 cm
−1
over the spectral
range recorded with a 1200 lines/mm grating or ∼15 cm−1with a 500 lines/mm grating.
A mercury argon calibration light source (Ocean Optics HG-1) was used for wavelength
calibration of the spectrograph. A second order polynomial was fitted to 10 known spectral lines
between 790 and 870 nm, with a rms residual of
∼
0.03 nm, which translates to an uncertainty of
∼
0.5 cm
−1
in Raman shift. The integration time was chosen to be such that the peak intensity is
<
80% of saturation and less than 15 mins so that the spectra are not overwhelmed by cosmic rays
such that they can be removed by taking a median of 3 spectra. Shorter integration times and
more repeat measurements are taken when there are significant variations in ambient light levels.
Background subtraction was performed to remove the contribution from ambient light using the

Research Article Vol. 27, No. 22 / 28 October 2019 / Optics Express 31340
Fig. 1.
Standoff Raman spectroscopy: (a) schematics of the standoff Raman instrument
setup using a CW laser at 780 nm co-axial with the receiving optics; (b) the mobile system
deployed in situ at the Blessed Sacrament Chapel in the Cathedral Church of St. Barnabas in
Nottingham.
same integration time as the target spectrum but with the laser off. The background subtracted
spectra were then corrected for the system spectral response by measuring the reflected spectrum
of a Tungsten light source (Ocean Optics DH-2000, with a known continuous smooth spectrum)
off a standard Spectralon white target. A baseline found by smoothing this spectrum using a
moving median window of
∼
45 cm
−1
was then subtracted from the spectral response corrected
spectrum.
Almost all remote Raman systems use pulsed laser sources to maximize the laser intensity
to increase the Raman signal and minimize daylight contribution for outdoor work. However,
these systems are mostly used for the remote detection of minerals on planets, or for the safe
detection from a distance, of minute quantities of explosives, where the effects of laser induced
degradation are not the primary concern as long as the material is not burnt to lose their spectral
signature [11].
In applications to cultural heritage, the effect of any laser induced degradation is to be avoided
to comply with conservation ethics. Increased laser intensity increases the chances of laser
induced degradation effects. For each material, there is an intensity threshold above which laser
induced degradation effects occur. Recent studies have shown that high intensity pulsed lasers
are damaging to all non-transparent materials from slightly scattering to slightly absorbing paint
[14]. For some materials, the intensity damage threshold of pulsed lasers can be higher than that
of a CW laser because pulsed lasers allow heat dissipation between pulses. However, for other
materials such as red lead the damage threshold for CW and pulsed lasers are rather similar [14].
Overall, there is a trade-off between detection efficiency and laser induced degradation. Raman

Research Article Vol. 27, No. 22 / 28 October 2019 / Optics Express 31341
intensity is proportional to the laser intensity and therefore the Raman signal
Scw
induced by a
CW laser is given by
Scw ∝IcwAtcw (1)
where
Icw
is the intensity of the CW laser, Ais the area of the laser beam on the target (assume
that the collection spot is matched to the excitation spot) and
tcw
is the integration time. For a
pulsed laser, the Raman signal SPis given by
Sp∝IpANpδt(2)
where
IP
is the peak intensity of the pulsed laser, Ais the area of the laser beam (or the collection
area when matched) on the target,
NP
is the number of pulses,
δ
tis the pulse duration. The
effective measurement time is then given by
tp=
Np
R
, where Ris the pulse repetition rate. The
laser intensities need to be less than the damage threshold
Ith
For a pulsed laser, this translates to
a pulse energy threshold of
εth =Ith
A
δ
t, which effectively limits the maximum Raman signal
achievable without degrading the material. For the same spot size on the target, to achieve
the same Raman signal using a CW laser and a pulsed laser without causing laser induced
degradation, the ratio of the measurement times is given by
tcw
tp
=
εthR
Pcw
>Rδt(3)
where
Pcw
is the incident power of a CW laser on the target. Compared with a typical ns-pulsed
laser with repetition rate of 10–100 Hz and pulse duration of
∼
5 ns, a CW laser can have a
measurement advantage of up to 7 to 8 orders of magnitude in efficiency, if the damage threshold
is comparable to the intensity of a typical CW laser in remote operation. For example, highly
light sensitive pigments such as cochineal, orpiment and realgar will be in this category [12].
For a pigment of medium sensitivity to laser induced degradation such as red lead, the damage
threshold of
∼106
W cm
−2
[13,14] means that the detection efficiency is about an order of
magnitude lower for an ns-pulsed laser compared with a 50 mW CW laser assuming a spot size
of a few mm in diameter. It is, therefore, better to use a CW laser in the remote Raman system for
the investigation of historic wall paintings. The repetition rates of common ns-pulsed lasers are
simply too low to ensure efficient Raman signal detection without elevating the power above the
damage threshold. Including daylight subtraction procedure, the duty cycle of a CW laser Raman
system is 50%, while the duty cycle of a 10–100 Hz ns-pulsed laser is 5×10−6- 5×10−7%.
A wavelength of 780 nm was chosen as a compromise on overall reduction in fluorescence in
most materials of interest whilst having the benefits of a higher Raman efficiency and a lower
cost Si CCD detector compared to the short wave infrared (SWIR) range (i.e. 1000–2500 nm).
3. Performance evaluation
3.1. Daylight subtraction
A big challenge for in situ Raman measurements for cultural heritage is the presence of variable
ambient daylight during working hours. In buildings like churches and palaces where there
are large windows without curtains, daylight cannot be avoided. Pulsed lasers at high peak
power have been used to both boost their efficiency to make up for their low duty cycle and to
make daylight contribution negligible in comparison to the high laser power through a gated
detection synchronized to the laser pulse duration [15]. This cannot be applied to paintings as the
high intensity would pose a threat to heritage objects. CW laser has also been applied for long
range outdoor remote measurements, however the experiment was conducted at night to avoid
daylight [16]. An automated online daylight subtraction program was developed in this project to
address the issue of, on one hand, the need to lower the laser intensity to avoid damage whilst

Research Article Vol. 27, No. 22 / 28 October 2019 / Optics Express 31342
maintaining a high Raman detection efficiency by using a CW laser, and on the other hand, the
need to compensate for a variable background caused by daylight which can change dramatically
in seconds. By modulating the laser beam on and off, the system can quickly respond to sunlight
intensity changes (Fig. 2).
Fig. 2.
Daylight subtraction spectra. (a) comparison between the measurements of an
orpiment in animal glue sample at 3.3 m distance placed in front of the window using
the standoff Raman system with the 780 nm CW laser on and off; the absorption bands
correspond to H
2
O absorptions from the atmosphere; (b) comparison of daylight subtracted
Raman spectrum with that taken in total darkness.
3.2. Detection of common artist pigments
A collection of 53 common historic artist pigments were used to prepare reference paint samples
in linseed oil, egg tempera or animal glue [17,18]. The pigment compositions were chemically
analyzed with the main composition confirmed. Details of the analysis along with the pigment
composition are given in [17]. The level of fluorescence between the different binding media are
not significantly different probably because they are not as aged as those found in historic paints.
The 53 common pigments in various binding media were tested with the 780 nm standoff
Raman. The standoff Raman setup was able to detect most pigments at 4 m within 1 min and
nearly all of them within 30 min except for the copper pigments (e.g. most of the green pigments),
the cadmium pigments, yellow ochres, most of the yellow organic pigments and most of the red
organic pigments. Typical spectra of pigments with a range of Raman scattering efficiency are
presented in Fig. 3.
With the exception of yellow ochre, yellow pigments are in general difficult to identify based
on spectral reflectance alone [18]. The standoff Raman system has been successful in identifying
nearly all the inorganic yellow pigments as well as the yellow organic pigment gamboge, therefore
complementing remote spectral imaging analysis. For red pigments, vermilion, realgar, red
lead, and chrome red are easily detectable as they are known to have strong Raman signals [19].
The only inorganic red pigment not detected was cadmium red. Most red and yellow organic
pigments have low Raman scattering efficiency and their spectra are affected by fluorescence
which increases the photon noise and hence reduces the signal to noise ratio [19,20]. Similarly, it
is difficult to detect Raman signals from cadmium pigments because of the increased photon
noise due to strong laser induced fluorescence of cadmium sulfide at this wavelength [21]. The
intrinsic low Raman scattering efficiency and strong absorption at 780 nm of the copper pigments
makes it difficult to detect them [19], despite the increase in Raman efficiency due to resonance.

Research Article Vol. 27, No. 22 / 28 October 2019 / Optics Express 31343
Fig. 3.
Typical Raman spectra of pigments: (a) Prussian blue in animal glue and oil,
integration time: 10 min; (b) zinc white in oil, integration time: 10 min; (c) orpiment in
animal glue with and without varnish on top, integration time: 10 s; (d) gamboge in animal
glue, integration time: 30 min.
4. Standoff in situ investigation of architectural interiors
The Cathedral Church of St. Barnabas in Nottingham is a fine example of Gothic Revival
architecture in England. It was consecrated as a Church in 1844 and as a Cathedral in 1852.
The architect was Augustus Welby Pugin (1812–1852), who also designed the decoration of
the Houses of Parliament in London. Within the Cathedral, the Blessed Sacrament Chapel has
been described as a “prayer book in stone”. It is known that the Blessed Sacrament Chapel
was restored in 1933 and 1974 [22]. However, how much work had been done in the original
decoration or the later restoration remains unclear.
4.1. Complementary methods
The in-house developed standoff spectral system for automated scanning of wall paintings and
architectural interiors, PRISMS [2], was used for the initial imaging of the murals. PRISMS
images from 400–880 nm using a PC controlled filter wheel with 10 filters, where 9 filters are
centered from 400 nm to 800 nm every 50 nm, each with a bandwidth of 40 nm, and one filter is
centered at 880 nm with a bandwidth of 70 nm; a Jenoptik MF
cool
CCD camera and a Meade
ETX90 telescope. The system is placed on an alt-az telescope mount with PC controlled drives.
Both focusing and scanning are automated. A Tungsten light with uniform projected illumination
using a telescopic system was used for remote illumination.
An ASD LabSpec spectrometer (350–2500 nm) was used for high spectral resolution fiber optic
reflectance spectroscopy measurements of the reference samples and for in situ measurements in
the chapel at accessible heights to complement the remote measurements. The spectral resolution
is 3 nm in the UV/VIS regime and 10 nm in the SWIR.

Research Article Vol. 27, No. 22 / 28 October 2019 / Optics Express 31344
X-ray fluorescence spectroscopy (XRF): A handheld Niton XL3t XRF Analyzer was used for in
situ measurements at accessible heights in the chapel to complement the reflectance spectroscopy
and Raman measurements, since it measures the elemental content of the materials. It consists of
an Au anode, with maximum voltage and current at 50 kV and 200
µ
A respectively. It is capable
of detecting elements with atomic number Z >14 without helium purge.
4.2. Analysis of painting materials
The SWIR reflectance spectra of various parts of the mural at accessible heights showed that the
binding medium is consistent with oil because of the characteristic lines at
∼
2303 nm and
∼
2346
nm [23].
Different areas of the mural at various distances were studied from the same position on the
ground using the standoff Raman system (Fig. 1(b)). The pigments used were the focus of our
investigation since they could potentially reveal the time period when the paintings were created.
Figure 4shows the data obtained from a red area of the mural. Firstly, reflectance spectra were
acquired using the PRISMS remote spectral imaging system (Fig. 4(b)) which shows that the
Fig. 4.
Standoff investigation of a red area in the chapel: (a) color image of part of a mural
next to a stained glass window (b) reflectance spectra collected with the remote spectral
imaging system PRISMS of the red area (black filled squares) indicated by the red arrow
in (a) compared with PRISMS spectra of a reference sample of vermilion (red dots) and
cadmium red (blue triangle) oil paints; the inset shows the derivative of the reflectance
spectra of cadmium red (peak at 610 nm), and two vermilion oil samples from two different
sources (peaks at 600 nm and 607 nm); (c) the raw standoff Raman spectrum of the same
red area (red), and the background spectrum collected over the same integration time with
the laser off (blue) showing typical absorption bands of the solar spectrum (the band around
675 cm
−1
corresponds to H
2
O absorption lines
∼
823 nm from the atmosphere; the lines
around 1056 and 1117 cm
−1
corresponds to Ca II lines at
∼
850 and
∼
854 nm from the solar
spectrum) ; (d) the processed spectrum after subtraction of daylight.

Research Article Vol. 27, No. 22 / 28 October 2019 / Optics Express 31345
reflectance spectrum of the red region is similar to either vermilion or cadmium red. Vermilion
was one of the most common red pigments until the 20
th
C when cadmium red, a less toxic
synthetic pigment, became commercially available in 1919 [24]. Both pigments share similar
reflectance spectral features that are within the natural variations due to particle size and trace
impurity differences (Fig. 4(b)), which makes it impossible to distinguish between them solely
by spectral reflectance. In red areas all over the chapel, such as the wall near the window (3.6
m), the wall behind the altar (11 m) and the central beam across the chapel (9.2 m), Raman
signals characterizing vermilion were detected. Two strong peaks at 252 cm
−1
and 343 cm
−1
respectively, assigned to Hg–S stretching vibrational modes [20], were observed along with
laser-induced fluorescence. It is worth noting that several absorption lines in the solar spectrum
were recorded when measuring the red area near the stained glass window (Fig. 4(c)), which
were fully removed after the subtraction of the ambient background. This is a good example that
demonstrates the ability of this standoff Raman system to remotely detect and analyze pigments
under the influence of daylight.
Portable XRF measurements of various areas at accessible height did not find cadmium and
therefore no evidence for cadmium red and hence no evidence so far for post 1919 material on
the mural.
Right next to one of the red areas near the window, a white area 4 m from the standoff Raman
system was investigated (Fig. 4(a)). Again, reflectance spectroscopy cannot identify the white
pigments. A weak peak at 991
±
0.5 cm
−1
was detected by the standoff Raman system (Fig. 5(a))
in the white area. This line can be attributed to the symmetric stretching mode (
ν1
) of the S–O
bond in BaSO
4
[25]. It could be constant white (barium sulfate, BaSO
4
) or lithopone (a mixture
of barium sulfate and zinc sulfide, BaSO
4
and ZnS). Zinc sulfide is difficult to detect with Raman
spectroscopy due to its low Raman efficiency [26]. The main peak of ZnS at 348 cm
−1
was
not detected in the lithopone reference sample. XRF measurements of various white areas at
accessible heights found Ba, Zn and trace Pb (Fig. 5(b)), which would support the identification
with lithopone. However, BaSO
4
is often used as a paint extender for zinc white (ZnO). Zinc
white is a common white pigment used since the 19
th
century, and it is easily detectable by
Raman spectroscopy with its main peak at 438 cm
−1
(Fig. 3(b)), but this was not detected here.
Therefore, the most likely identification of the white pigment is lithopone.
Fig. 5.
Standoff investigation of a white area in the chapel (shown by the white arrow in
Fig. 4(a)): (a) Standoff Raman measurement of a white area (blue) at a distance of 4 m
compared with a sample of lithopone powder measured from the same distance in the lab
(red); the baseline subtracted spectra are smoothed with a moving window of 6 cm
−1
. (b)
XRF measurement of a similar white area at accessible height.

Research Article Vol. 27, No. 22 / 28 October 2019 / Optics Express 31346
Lithopone was manufactured on a commercial scale starting in 1874 (the year it was patented
in England) but by the 1930s it was superseded by titanium white [27]. The detection of lithopone
would suggest that the current painting scheme in the chapel was after the consecration of the
church in 1844 but possibly before the 1933 restoration. There were records of alterations to the
main parts of the cathedral made in the early 20
th
C but whether that included any alterations to
the chapel is not known.
5. Conclusions
A standoff Raman spectroscopy system using a CW laser source at 780 nm is demonstrated to
be able to operate from a distance of 3–15 m to identify most common historic artist pigments.
The laser intensity of the standoff Raman system presented here is
∼
5 orders of magnitude lower
than a micro-Raman system under standard operation for artworks. The risk of laser induced
degradation is, therefore, negligible. The use of a CW laser presents significant advantages in
measurement efficiency over a pulsed laser when it is paramount to operate at low laser intensity
to ensure safe operation for all pigments.
The 780 nm standoff Raman system is shown to be able to operate under the influence of
daylight through background subtraction by modulating the laser source through on/off cycles.
Standoff Raman spectroscopy is demonstrated to complement our current visible/near infrared
remote spectral imaging using PRISMS.
The standoff Raman system was successfully deployed in the Cathedral Church of St Barnabas
in Nottingham to identify the pigments used in the Blessed Sacrament Chapel in order to date
the painting scheme and hence verify its attribution to the celebrated architect Augustus Welby
Pugin.
The application can be extended from murals to general architectural materials, such as stones,
to determine the types of stones or to monitor the deterioration of buildings under the influence
of the environment.
Funding
Nottingham Trent University; Heritage Lottery Fund (OH-18-02277); Engineering and Phys-
ical Sciences Research Council (EP/E016227/1); Natural Environment Research Council
(NE/R014868/1).
Acknowledgments
The corporation and support from the Cathedral Church of St Barnabas, Nottingham during
the in situ measurement campaign is gratefully acknowledged. We are grateful to Ana Souto
of Nottingham Trent University, Fr Philip McBrien of the Diocesan Art and Architecture
Commission for the information on the architectural background of the chapel and cathedral, and
Patrick Atkinson of Nottingham Trent University for assistance with the first field trip. We are
grateful to the Scientific Department of the National Gallery in London for the reference paint
samples in oil and egg tempera.
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