EVA London Conference ~ 11–13 July 2007
Eva Zányi, Carla Schroer, Mark Mudge, and Alan Chalmers
_____________________________________________________________________
LIGHTING AND BYZANTINE GLASS TESSERAE
Eva Zányi†, Carla Schroer‡, Mark Mudge‡, Alan Chalmers†
†
Warwick Digital Laboratory
University of Warwick
Coventry CV4 7AL
United Kingdom
[email protected], [email protected]
http://www.warwickdigital.org
‡
Cultural Heritage Imaging
San Francisco
USA
[email protected], [email protected]
http://www.c-h-i.org
Abstract – A key component of many Byzantine churches was the mosaics on the curved
walls and ceilings, which included gold and silver glass tesserae. As the viewer or the light
moved within the church, these tesserae sparkled. In this paper we describe how we
captured a Polynomial Texture Map of the apse mosaic at the Angeloktisti Church at Kiti,
Cyprus and used it to investigate how the position of the lighting may have affected the
appearance of the mosaic. Our study showed that the appearance of the mosaics is indeed
significantly different when lit from various directions.
INTRODUCTION
From the outside Byzantine churches look unimposing; without much decoration, no
paint or precious materials. This is very different to the interior, which provided those
inside the space with dramatic visual affects aiming at alleviating and engaging the
viewer to approach God [14]. The architecture used light and shadow to symbolically
represent different sacral hierarchies and direct the attention of the viewer. Therefore
the upper parts of the churches, which represented heaven, were better lit than the lower
parts. In early Byzantium this was achieved with the help of daylight through small
xxxx
Figure 1. Outside of the church of Panagia Angeloktisti at Kiti, Cyprus.
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EVA London Conference ~ 11–13 July 2007
Eva Zányi, Carla Schroer, Mark Mudge, and Alan Chalmers
_____________________________________________________________________
openings in the upper parts of the walls. From middle Byzantium on, the buildings had
less openings letting in natural light and these were replaced by oil lamps and candles
[18]. The positioning of these artificial lights was regulated in great detail in manuals,
so called typicons, in order to underline the difference between divine light and profane
darkness and to let their flickering make precious materials such as the gold and silver
of the icons, mosaics and frescoes, sparkle and draw the viewer into contemplation [1].
BYZANTINE GLASS MOSAICS
The Romans perfected techniques for the design and construction of intricate floor
mosaics, using natural resistant materials. The Byzantines extended these methods to
wall mosaics and were able to now include fragile materials and more precious one such
as the glass gold and silver tesserae [19]. Since so few of the mosaics are left, it has
previously been assumed that they were very expensive, especially those which
contained glass tesserae. However, recently James questioned this assumption and
suggested that the raw material, glass, was not expensive, since Byzantine was close to
desert regions and suppliers [5]. She further suggested that manufacturers of glass
tesserae were spread all over the Byzantine Empire and in fact the setting of the mosaics
was the most expensive aspect since it was very labour intense. All this means that
mosaics were far more widely spread than previously believed and would also explain
why small and politically insignificant churches such as Kiti, which were not situated in
any major centre, were so decorated. The glass tesserae were manufactured in a number
of ways and often coloured. Metallic tesserae such as gold and silver ones were made by
covering a ca 6mm thick glass plate with, for example gold leaf, and then coating it with
a thin layer of transparent or coloured glass. The sandwiched glass plate was then
heated up until the layers fused and subsequently cut into pieces. The surfaces of the
tesserae were slightly uneven and different effects could be achieved depending on if
the glossy or the rougher surface was exposed on the mosaic [6.10].
SELECTION OF THE SITE
Only few wall mosaics are left from the early Byzantium, since most were destroyed
during the 300 years of Arab expansion and invasions, the iconoclasm period of
Byzantine history of the 8th and 9th centuries and also because of natural disasters such
as earthquakes and fires. The three extensive apse mosaics on Cyprus dating from the
6th and 7th century are consequently very unique. They show the Virgin Mary and Child
and were placed in quite small and remote churches, which did not belong to the Pope
or any other financially strong ruler [17]. The three churches are: the church of Panagia
Kyra at Livadia, the church of Panagia Kanakaria at Lythrankomi [8] and the church of
Panagia Angeloktisti at Kiti. From these three, the ones in Livadia and Lithrankomi are,
since 1974, in the occupied Turkish section of Cyprus and therefore difficult to access
[16]. Apart from the accessibility problem, these two were very badly damaged after the
occupation. How much is left of the mosaic in Livadia is in fact not known. Kiti, close
to Larnaca on the Greek part of Cyprus, has, however, a well preserved, unrestored apse
mosaic and was thus chosen for our study. The mosaic comprises the Virgin Mary
holding the Child with the Archangel Gabriel on the right and the Archangel Michael on
the left [3.16], Figure 2. The mosaic is lit today from slightly below by spot lights, and
thus a large part of the mosaic is not well lit. The church is still in full use with regular
orthodox masses taking place and several visits by large tourist groups and school
children per day.
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EVA London Conference ~ 11–13 July 2007
Eva Zányi, Carla Schroer, Mark Mudge, and Alan Chalmers
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REFLECTION TRANSFORMATION IMAGING (RTI)
RTI is a term coined by Malzbender and Gelb of Hewlett Packard Labs. RTI
captures the “real world” reflectance characteristics of a subject. A simple, robust, and
forgiving way to capture RTI information is the use of Polynomial Texture Mapping
(PTM). PTMs store surface reflection information with each image pixel. Malzbender et
al., inventors of PTM [9], presented a mathematical model describing luminance
information for each pixel in an image in terms of a function representing the direction
of incident illumination. The illumination direction function is approximated in the form
of a biquadratic polynomial whose six coefficients are stored along with the colour
information of each pixel. This surface reflection information describes the subject’s
surface normals. This normal information indicates the directional vector’s
perpendicular to the subject’s surface at each location recorded by the corresponding
image pixel. Consequently, PTMs are 2D images containing true 3D information. PTMs
are also able to record approximations of other reflection-related properties including
surface inter-reflection, subsurface scattering, and self-shadowing. PTMs can
communicate useful shape information using purely image based transformations
without full photometric stereo or other reconstruction from the surface normals using
3D geometry in Cartesian space.
The normals of a surface describe its shape and are used by computer graphics
lighting models to determine surface reflection properties. In 3D virtual reality
representations, normals are used by lighting models to calculate how light rays will
reflect off the surface of virtual 3D geometry. The normal information present in RTIs
allows them to use similar 3D lighting techniques. The software used to view RTIs
employs these 3D lighting models.
RTI images are interactive. Their dynamic interplay of light and shadow works with
the human visual system to communicate a powerful perception of the object’s shape
[2.7] While RTIs can be used to communicate the effects of different illumination
directions on a surface, they can also transform surface normal information to enhance
the perception of surface features. This enables RTIs to not only disclose surface
characteristics not visible in any of its constituent source photographs, but also reveal
information not readily discernable by direct physical examination. This characteristic
of RTI has been dramatically demonstrated by its recent use in revealing the nature and
use of the Antikythera Mechanism, a 2nd century geared astronomical computation
device [4].
A key characteristic of RTIs captured with the PTM method is that complete surface
normal information can be acquired from highly shiny, specular materials such as gold
without data loss associated with clipping due to specular highlights. PTMs have been
demonstrated to effectively capture highly reflective surfaces without data loss due to
shadows or specular highlights during the documentation of gold and silver coins as
well as highly reflective stone tools [13]. This property has been used to great advantage
in our project with the Byzantine mosaic and gold leafed icons at Kiti. The apse mosaic
contains numerous tesserae, essentially glass gold and silver leaf ‘sandwiches’. The
icons contain gold leaf and tempera painting techniques intended to produce reflective
effects. Gold, silver and glass are notoriously difficult to capture through either using
photography or active 3D range scanning, due to their highly specular nature. Our
successful capture of these subjects and relighting with the simulated illumination
conditions for which they were designed underscore the usefulness of PTM based RTI
techniques for these classes of cultural heritage subjects.
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EVA London Conference ~ 11–13 July 2007
Eva Zányi, Carla Schroer, Mark Mudge, and Alan Chalmers
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The preceding attributes of RTIs and PTMs as well as their use in cultural heritage
documentation projects, the natural sciences, and law enforcement has been detailed
extensively elsewhere, including [3.4.9.11.12.13.20].
Capturing RTIs using PTMs
To capture single-viewpoint PTM images, the subject is photographed from a fixed
camera position. Multiple photos are shot, each illuminated from a different light
position. If the positions of the lights are known, the photo sequence can be
mathematically synthesized into a single PTM image.
The light positions can be known before the photographic session by using domes or
templates that position illumination sources at pre-measured locations. There is a family
of dome designs, based on the prototype built by Malzbender, which can capture a
series of RTI photographs of a subject under automatic electronic control [15]. These
devices, when used in conjunction with image sequence processing software, are very
efficient when documenting many objects of similar size.
Dome equipment is subject to several limitations which have, until recently,
restricted the use of RTIs of all types and specifically PTMs to small objects in
laboratory contexts. The diameter of RTI subjects is limited to approximately 1/3 of the
dome diameter. As dome equipment is currently custom fabricated and the substantial
cost of these instruments increases with size, RTI acquisition of larger objects or
architectural features was impractical. Transport and configuration within the subject’s
environmental context also becomes difficult or impossible as size increases.
Highlight RTI (HRTI) is a simpler, lower cost, and more flexible method of PTM
acquisition developed through a collaboration between CHI and HP Labs [11].
Eliminating the requirement for prior knowledge of the illumination positions, HRTI
permits capture of the light position as the photo is shot. Highlight RTI recovers light
positions from the highlights produced on one or two shiny black spheres placed in the
image composition by the photographer. After the capture sequence is completed,
software detects the highlights on the sphere(s) and determines the light position. Once
the light positions are known, the spheres can be cropped from the image sequence prior
to the final RTI synthesis. Highlight RTI have been made of objects in a wide range of
sizes from two square centimeters to multiple square meters. The highlight RTI method
was selected for this project because of the desire to capture various size objects,
including the large apse mosaic.
HRTI images are captured using a process affectionately known as the ‘Egyptian
Method’. Using the Egyptian Method, an illumination radius is selected, based on the
diameter of the subject. A string is measured to the radius distance. One end of the
string is tied to the light source and the other end is held near to but not touching the
subject at the location corresponding to the center of the composed image. For each
light position photographed, the subject end of the string is positioned and the light
distance is determined. The subject end of the string is then moved out of the camera’s
field of view and the photo taken. This process is repeated until a representative
hemispheric sample of light directions is acquired around the subject.
Capturing the apse mosaic
Capturing the apse mosaic at Kiti posed several challenges. The presence in the area
around the apse of light sensitive objects, including, tempura on wood icons, frescos,
and fabrics on furnishings, mandated a low photonic damage lighting system. The
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EVA London Conference ~ 11–13 July 2007
Eva Zányi, Carla Schroer, Mark Mudge, and Alan Chalmers
_____________________________________________________________________
apse’s height off the floor, five meters at the top and over three meters at its base,
offered logistical challenges. The enclosure of the sanctuary directly below the apse by
a high, ornate grating both segregated the sacred space from the main volume of the
church and constrained our working area. Within the working area, the locations of the
alter, freestanding crucifixes, ritual objects, furnishings for practical support of ritual
activities such as multiple daily masses, and our own documentary equipment made
positioning of cameras, lights, colour checker charts, and reflection capturing black
balls a creative problem solving exercise.
Figure 2. Capturing the PTM of the apse mosaic.
In an isolated environment, mosaic tesserae are very resistant to photonic damage,
and standard flashes or other photographic lights could have been used to document
them responsibly. In the apse location, the proximity of light sensitive materials meant
that responsible cultural heritage practice required another approach. Our solution used
a 250 watt xenon arc lamp light source designed to power a fibre optic swimming pool
illumination system. Xenon sources emit visible light as well as large amounts of
photonically damaging ultraviolet (UV) and infrared (IR) light wavelengths. While a
variety of light transmitting fibres and guides are available to carry this light, the least
expensive and most widely used material is PMMA acrylic cable. PMMA acrylic acts
as a band pass filter, excluding both UV and IR light and passing only visible
wavelengths between 400 and 750 nm. We used a bundle of this fibre to filter our light
source.
The height of the apse required modification of equipment and technique, Figure 2.
For the light source, the acrylic fibre bundle was attached to a telescoping monopod
with the end of the fibre bundle curved 90 degrees from the axis of the monopod, much
as a photographer’s light sits atop a light stand. To position the light at locations
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EVA London Conference ~ 11–13 July 2007
Eva Zányi, Carla Schroer, Mark Mudge, and Alan Chalmers
_____________________________________________________________________
necessary to sample illumination directions higher than three meters from the floor, the
person holding the light stood on a four meter step ladder, acrobatically extending the
light. Much of this acrobatics was caused by the limited locations in the sanctuary with
sufficient free floor space for the ladder. The subject end of the Egyptian Method string
was attached to a long pole, a broom handle owned by the church. The string end of the
pole was cushioned with bubble wrap in case it accidentally touched the mosaic.
Fortunately, this precaution proved unnecessary. Following standard procedure, the
string end was held near the apse mosaic and the light person moved to the next
location, the broom handle and string were moved out of the way and the image taken.
Regardless of these circumstances, 79 light positions were acquired. This image
sequence was satisfactory to build high quality RTI images and provide the data
required for the historical lighting studies.
This kind of illumination configuration can be assembled with local equipment in
many places around the world, because this equipment is popular for recreational,
industrial, and commercial applications. A Cypriot lighting contractor, Andreas
Demetriou, loaned the equipment at no charge to our project. We used a dual camera
setup with a Canon 5D and a Canon 1DS, both with 50mm fixed lenses. An RTI can be
created from a single camera position. However, we chose to use a stereo pair, such that
additional 3D data could be generated at a later time. Radio triggers were used to fire
the cameras simultaneously. A black billiard ball for the game of snooker was tapped
and placed on a boom arm, attached to a ball head on a tripod.
CONCLUSIONS
Lighting was a key factor in the design and layout of Byzantine churches. In
particular, the rich mosaics with their abundance of gold and silver tesserae played a
major role in the Byzantine worshippers’ overall experience of the church. As Figure 3
clearly shows, the gold and silver ensured that the perception of the mosaic was very
different depending on the lighting and where the viewer was standing. This deliberate
effect is far less noticeable in the churches today, due to the significant alterations to the
Figure 3. How the direction of lighting affects the perception of the Child in the mosaic.
buildings over the centuries, and the use of modern lighting. By capturing the PTMs of
the mosaic we are able to explore in a controlled manner what sort of perceptual effects
are possible depending on the angle of the light.
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EVA London Conference ~ 11–13 July 2007
Eva Zányi, Carla Schroer, Mark Mudge, and Alan Chalmers
_____________________________________________________________________
Future work will incorporate these PTMs into a detailed computer reconstruction of
the Angeloktisti church as it would have appeared in Byzantine times. In addition we
will consider the nature of the light, including natural light and the candlelight, and how
this may have affected the perception of the mosaics.
ACKNOWLEDGEMENTS
This project was funded by the UK EPSRC grant EP7E024998/1 for which we are
very grateful. A project like this requires significant additional support and effort. We
would like to thank the Cypriot Department of Antiquities for giving us permission,
Ioannis Eliades of the Byzantine Museum and Art Galleries for all his help, the Church
at Kiti for permission and their friendly support on site, Yiorgos Chrysanthou,
University of Cyprus for his support, Andreas Foulias, the author of [3] for his detailed
explanation of the mosaic and Professor Rainer Warland, University of Freiburg for all
his advice, especially as regards to the most appropriate literature.
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