A SWIR hyperspectral imaging system for art history and art conservation

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IX
CONGRESO NACIONAL
DEL COLOR
ALICANTE 2010
A,     J,
    J  
U  A
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UNIVERSIDAD DE ALICANTE
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IX CNC -Libro de Actas-
El IX Congreso Nacional de Color cuenta con el apoyo de las siguientes entidades:
IX CONGRESO NACIONAL DEL COLOR. ALICANTE 2010

IX CONGRESO NACIONAL DEL COLOR. ALICANTE 2010
I X C o n g r e s o N a c i o n a l d e C o l o r
A l i c a n t e ,
29 y 30 de J u ni o, 1 y 2 de J u li o
U n i v e r s id a d d e A l i c a n t e
Departamento de Óptica, Farmacología y Anatomía
Facultad de Ciencias
Instituto Universitario de Física Aplicada a las Ciencias y las Tecnologías (IUFACyT)
Universidad de Alicante
IX CNC -Libro de Actas-

IX CONGRESO NACIONAL DEL COLOR. ALICANTE 2010
COMITÉ ORGANIZADOR
Presidente Francisco M. Martínez Verdú Universidad de Alicante
Vicepresidente I
Vicepresidente II
Secretaria Científica
Secretaria Administrativa
Secretaria Técnica
Tesorero
Vocal
Vocal
Vocal
Vocal
Vocal
Eduardo Gilabert Pérez
Joaquín Campos Acosta
Esther Perales Romero
Olimpia Mas Martínez
Sabrina Dal Pont
Valentín Viqueira Pérez
Elísabet Chorro Calderón
Verónica Marchante
Bárbara Micó Vicent
Elena Marchante
Ernesto R. Baena Murillo
Universidad Politécnica de Valencia
IFA-CSIC
Universidad de Alicante
Universidad de Alicante
Universidad de Alicante
Universidad de Alicante
Universidad de Alicante
Universidad de Alicante
Universidad de Alicante
Universidad de Alicante
Universidad de Alicante
COMITÉ CIENTÍFICO
Natividad Alcón Gargallo
Joaquín Campos Acosta
Pascual Capilla Perea
Ángela García Codoner
Eduardo Gilabert Pérez
José Mª González Cuasante
Francisco José Heredia Mira
Enrique Hita Villaverde
Luís Jiménez del Barco Jaldo
Julio Antonio Lillo Jover
Francisco M. Martínez Verdú
Manuel Melgosa Latorre
Ángel Ignacio Negueruela
Susana Otero Belmar
Jaume Pujol Ramo
Javier Romero Mora
Mª Isabel Suero López
Meritxell Vilaseca Ricart
Instituto de Óptica, Color e Imagen, AIDO
Instituto de Física Aplicada CSIC
Universidad de Valencia
Universidad Politécnica de Valencia
Universidad Politécnica de Valencia
Universidad Complutense de Madrid
Universidad de Sevilla
Universidad de Granada
Universidad de Granada
Universidad Complutense de Madrid
Universidad de Alicante
Universidad de Granada
Universidad de Zaragoza
Instituto de Óptica, Color e Imagen, AIDO
Universidad Politécnica de Cataluña
Universidad de Granada
Universidad de Extremadura
Universidad Politécnica de Cataluña
IX CNC -Libro de Actas-

189
A SWIR HYPERSPECTRAL IMAGING SYSTEM FOR ART HISTORY
AND ART CONSERVATION
Haida Liang
1
, Kafing Keita
1
, Chris Pannell
2
, Jon Ward
3
1
School of Science & Technology, Nottingham Trent University, Nottingham NG11 8NS, UK
2
Gooch & Housego, Orlando, 4632 36
th
Street, Florida 32811, USA
3
Gooch & Housego, Dowlish Ford, Ilminster, Somerset TA19 0PF, UK
http://www.ntu.ac.uk/apps/Profiles/55616-1-9/Dr_Haida_Liang.aspx
Haida.Liang@ntu.ac.uk
Abstract:
PRISMS is a portable hyperspectral imaging system designed for high resolution, in situ, remote
imaging of paintings at inaccessible heights from the ground level. It currently operates from
400nm to 1700nm to provide detailed conservation monitoring, material identification and to
assist art historical investigations for wall paintings. The system can be fitted with a variety of
imagers to operate in the visible, near infrared and short wave infrared at close range and
remotely. This talk will focus on the SWIR (short wave infrared) hyperspectral imager and its
applications in art history and art conservation. The filtering system in the SWIR is based on an
AOTF (acousto-optical tunable filter) system developed by Gooch & Housego that operates
between 900nm and 1700nm which provides fast switching between the spectral bands as well as
full flexibility in terms of central wavelength and bandwidth of the filters. Applications in
imaging of underdrawings (preparatory sketches) beneath oil paintings and identification of paint
materials will be presented.
Keywords:
hyperspectral imaging system, AOTF, remote imaging, multispectral imaging,
colour, near infrared, short wave infrared, wall painting, pigment identification
INTRODUCTION
Scientific imaging has been an integral part of ‘technical examination’ ever since the
beginning of conservation science. In the last 15 years, research in the application of imaging
science to art conservation has flourished as a result of the achievements in other fields where
non-invasive imaging is in high demand.
Multispectral and hyperspectral imaging are non-invasive imaging techniques capable of
recording the spectral reflectance per pixel of an object with accuracy comparable to a
spectrometer but with the added advantage of being able to capture the spectral reflectance of
millions of points simultaneously. The spectral information obtained for a painting can be used
for non-invasive pigment identification, for the monitoring and examination of any deterioration
and for the accurate colour rendering in any given illumination. Multispectral imaging in the
visible spectral range has been applied to art conservation for at least 15 years, however, it is only
recently that it has gone from being research led to being in demand by conservators [1].
Multispectral/hyperspectral imaging is flexible and can be used for accurate colour, infrared and
fluorescence imaging.
Based on experience with high resolution, precision colour and spectral imaging of easel
paintings in studios, we have developed a portable colour and multispectral camera system that is
light-weight, flexible and without any cumbersome mechanical structure for in situ high
resolution colour and spectral imaging of wall paintings [2, 3]. This is the first instrument to be
able to image paintings at inaccessible heights in situ from ground level to produce not only high
resolution colour images but also multispectral images. The recording, monitoring and non-
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190
invasive examination of wall paintings in grotto sites, tombs and buildings are particularly
important since these paintings are extremely vulnerable. Traditionally, imaging of wall paintings
at high resolution requires either scaffolding or some heavy and cumbersome mechanical
structure to lift the camera to the upper parts of a wall or ceiling. We present here the latest
developments on the instrument focusing on the SWIR imager.
INSTRUMENTS AND METHODS
PRISMS (Portable Remote Imaging System for Multispectral Scanning) is designed for the
in situ, high resolution imaging of wall paintings or any painting hang at heights that is difficult
to access without any mechanical structure such as scaffoldings [1, 2]. PRISMS is portable and
can operate from the ground level and achieve sub-mm resolutions of up to 25 pixels/mm. It
consists of a small telescope, a VIS/NIR (400-900 nm) multispectral imaging system, a SWIR
(short wave infrared: 900-1700 nm) hyperspectral imaging system. The VIS/NIR multispectral
imager is a simple, low budget instrument consisting of a filter wheel with 10 filters of 40nm
bandwidth and a CCD camera. The SWIR hyperspectral imager is based on an AOTF (Acouto-
Optic Tunable Filter) system that is software controlled to provide wavelength switching and
bandwidth selection. To improve the flexibility of PRISMS, it is upgraded to take advantage of
the modular design which consists of a filter system (filter wheel or AOTF), a digital camera (Si
or InGaAs detector depending on the wavelength of interest) and focusing optics (various lens for
close range imaging or small reflector telescopes for remote imaging) mounted on either an
Altitude/Azimuth telescope mount or X-Y linear translation stages. The system is automatically
controlled by a laptop to change filters, to focus, to move the pointing direction and to capture
images. Resolutions of at least 5 pixels per mm are achieved for working distances of 1m to 25m,
and sub-millimetre resolutions can be achieved for distances below 100m. For both close range
and long distance imaging, the whole system stays at ground level during operation. The system
can be dismantled and packed in a suitcase with the heaviest component weighing 10kg.
In the following section, we evaluation the capabilities of the AOTF based SWIR imager of
PRISMS.
RESULTS
AOTF allows rapid and flexible choice of the central wavelength as well as the bandwidth
and transmittance of the filters. The AOTF is designed such that one of the diffracted (filtered)
rays is optimised to give the best image quality. In order to separate the optimised diffracted ray
from the zero order beam, it is necessary to allow the beams to travel a sufficient distance before
the detector. The beam is folded to minimize the size of the imager. Examples of measured filter
response produced from the AOTF system are shown in Fig. 1.
The images obtained were calibrated using the usual calibration procedure for
multispectral imaging which involves corrections for the thermal noise of the digital camera (dark
correction), corrections for the inhomogeneity of the illumination and the pixel-to-pixel gain
variation of the digital camera (flatfield correction), and finally corrections for the difference in
throughput between the spectral bands (spectral correction). For dark correction, dark frames are
taken with the lens/telescope cap on for the same amount of exposure time as the image to be
corrected. Flatfield frames can be taken with a matt white or grey card at the same position as the
target to be imaged through the same filter. An image of the target is calibrated by subtracting the
dark frame and dividing by the normalised (i.e. divide the frame by the average pixel intensity),
dark corrected flatfield frame. Spectral calibration is achieved by imaging a spectral standard
(e.g. a Labsphere Spectralon white standard) through all the filter channels. The final calibrated
image cube gives the spectral reflectance at each pixel. There is no need for any special spectral
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reconstruction algorithm to be applied as the spectral reflectance of paint is relatively smooth and
a cubic spline interpolation is sufficient to recover the visible spectrum for medium spectral
resolutions of 50nm (or higher resolution). Given the flexibility of the AOTF, it is efficient and
convenient to image at low resolution first and to re-image a part of the spectrum with suspected
narrow spectral features at higher spectral resolution (e.g. Fig. 2).
Figure 1. Measured filter transmission using the AOTF system. LEFT: A set of filters over the full spectral range of
900-1700nm with the narrowest bandwidth of ~10nm; RIGHT: A set of filters with bandwidth of 150nm.
Figure 2 shows the spectral reflectance of azurite mixed with linseed oil measured with
PRISMS using the VIS/NIR filter wheel system and the SWIR AOTF system using the 150nm
bandwidth filter set given in Figure 1. The flexibility in the choice of central wavelength and
bandwidth with the AOTF means that the spectrum could be measured at different sampling
resolutions when necessary. For example, a higher sampling rate and narrower band filters can be
used to better resolve the absorption line around 1500nm in the azurite spectrum.
Figure 2. LEFT: Spectral reflectance of azurite measured with PRISMS using the VIS/NIR imager with 10 filters from
400nm to 880nm and the SWIR imager with the AOTF tuned to 5 filters centred at 1020nm to 1620nm with bandwidth
of 150nm. The open circle gives the data points and the blue line is a cubic spline interpolation from the data. RIGHT:
SWIR spectra of azurite obtained with AOTF tuned to 150nm bandwidth filters (open circle) and AOTF tuned to 10nm
bandwidth filters for high spectral resolution imaging of the absorption feature (crosses); the red spectral line
corresponds to a spectrum of azurite (different sample) measured with a Polychromix DTS1700 spectrometer.
It has long been recognised that most paint are more transparent in the infrared than in the
visible and hence infrared imaging can reveal the preparatory sketches (underdrawings) under the
paint layers. Figure 3 shows an example of SWIR images of azurite and Prussian blue paint over
underdrawings made of a variety of material from charcoal to red ochre. It is interesting to note
that the narrow band (1545-1695nm) SWIR image in Fig. 3 reveals more of the underdrawings
than a broadband (900-1700nm) SWIR image (middle image in Fig. 3). The SWIR hyperspectral
imaging system has the potential of providing better images of underdrawing than a broadband
InGaAs camera.
1000 1100 1200 1300 1400 1500 1600
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transmission (%)
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60.0 MHz
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Figure 3. LEFT: A colour image of a test panel with two vertical bands of paint (azurite in oil on the left and Prussian
blue in oil on the right) over a set of horizontal drawings using a variety of drawing material commonly used for
preparatory sketches for paintings. MIDDLE: A SWIR image of the test panel over the full spectral range of 900-
1700nm. RIGHT: A SWIR image of the test panel centred at 1620nm over a narrower bandwidth of 150nm.
CONCLUSIONS
The new AOTF based SWIR imager is capable of rapid and flexible spectral imaging that
enhances the pigment identification capabilities of PRISMS as well as providing better images of
underdrawings. PRISMS is the first instrument to be able to image both small objects such as
manuscripts at close range and paintings at inaccessible heights in situ from ground level at high
resolution to produce not only colour but hyperspectral images over the UV/VIS/NIR and the
SWIR spectral range. The possibility of in situ spectral imaging without the need for cherry
pickers or scaffolding means long term savings and increased ease/frequency of monitoring. The
portability of the system means flexibility and easy transportation such that it can be taken to
remote sites to image large paintings in situ from ground level. In museums, this means the
possibility of imaging at high resolution very large paintings. The remote high resolution spectral
imaging system provides conservators with an invaluable tool for examining and monitoring the
conditions of paintings.
ACKNOWLEDGEMENTS
This work is supported by a grant from the UK Engineering and Physical Sciences
Research Council (EP/E016227/1). We would like to acknowledge contribution from Simon
Godber and David Parker of Nottingham Trent University and Bill Zhang, Nathan Henderson and
other colleagues from Gooch & Housego plc. The test paint panels were prepared by Sophie
Martin-Simpson of Nottingham Trent University under the guidance of Marika Spring at the
National Gallery in London.
REFERENCES
[1] Fischer, C., and Kakoulli, J. “Multispectral and hyperspectral imaging technologies in conservation: current
research and potential applications”, Reviews in Conservation, 7, 3-16 (2006).
[2] Liang H, Keita K., Vajzovic T., “PRISMS: A portable multispectral imaging system for remote in situ
examination of wall paintings”, O3A: Optics for Arts, Architecture, and Archaeology, Proc. SPIE, Vol. 6618,
661815 (2007).
[3] Liang H., Keita K, Vajzovic T., Zhang Q., “PRISMS: remote high resolution in situ multispectral imaging of
wall paintings”, ICOM-CC Triennial Conference, Delhi, Vol I, p353-358, Allied Publishing (2008).
IX CONGRESO NACIONAL DEL COLOR. ALICANTE 2010