Multispectral imaging was employed to collect data on the degradation of an 18th century parchment by a series of physical and chemical treatments. Each sample was photographed before and after treatment by a monochrome digital camera with 21 narrow-band filters. A template-matching technique was used to detect the circular holes in each sample and a four-point projective transform to register the 21 images. Colour accuracy was verified by comparison of reconstructed spectra with measurements by spectrophotometer

Giacometti, A

Gibson, A

Macdonald, LW

Terras, M

Weyrich, T

English

UCL Discovery

  Colour Analysis of Degraded Parchment Lindsay MACDONALD, Alejandro GIACOMETTI, Tim WEYRICH, Melissa TERRAS and Adam GIBSON University College London ABSTRACT Multispectral imaging was employed to collect data on the degradation of an 18th century parchment by a series of physical and chemical treatments. Each sample was photographed before and after treatment by a monochrome digital camera with 21 narrow-band filters. A template-matching technique was used to detect the circular holes in each sample and a four-point projective transform to register the 21 images. Colour accuracy was verified by comparison of reconstructed spectra with measurements by spectrophotometer.  1. INTRODUCTION Large collections of parchment documents exist in public and private libraries, archives, and museums in varying degrees of preservation. Parchment is prepared from an animal skin that has been wetted, immersed in lime water, dehaired, scraped, then left to dry under tension on a wooden frame. The stretching has the effect of reorganising the collagen fibre network into a laminate structure. The resulting material is a fairly stiff sheet which is durable and can last for centuries, provided it is kept cool and dry (Clarkson, 1992).  In  this  project,  we  investigated  the processes  of  controlled  document degradation  using  multispectral  imaging. We  obtained  an  18th c e n t u r y  m a n u s c r i p t  de-accessioned  from  the  London Metropolitan Archives (Fig. 1). Although in  good  physical  condition,  this manuscript  was  deemed  to  hold  no historical value, and it was kindly donated for  experimentation.  It  comprised  two large sheets of prepared animal skin, i.e. parchment,  written  in  iron  gall  ink  and highlighted  in  red  ink.  Each  sheet measured approximately 70×70 cm (Fig. 1), and the two had been tightly folded together for storage. The document was an indenture (contract) between Mr John Sherman and Mr Christ Gardiner dated 11th August 1753.  Twenty-three square sections of 8×8 cm were cut from the original manuscript, with each  section  containing  written  text.  Each  sample  was  exposed  to  a  different  external deteriorating  agent  to  produce  a  controlled d e g r a d a t i o n ,  i n c l u d i n g  m e c h a n i c a l  d a m a g e ,  heat, humidity, abrasion and a variety of substances with different chemical properties, such as acid, alkaline, bleach, red wine, tea, human blood and mould. These affected the appearance and condition of the samples in different ways, typical of the actual damage suffered  by  parchment  in  real  archives.  Preliminary  tests  proved  the  viability  of  the procedures for degrading the samples and enabled parameters to be established for each treatment process (Giacometti et al, 2012). Figure 1: Detail of C18th parchment.   2. MULTISPECTRAL IMAGE CAPTURE Multispectral imaging can be used to discriminate between reflectance spectra, and hence to identify and separate different types of inks in a single document, showing whether the document has been edited. The carbon black ink of antiquity consisted of graphite or soot particles  suspended  in  an  organic  binder  and  applied  with  a  stylus.  Such  inks  do  not penetrate the parchment but rest on top, with particles adhering to the micro-structure of the surface. Iron gall ink, also known as gallo-tannin ink, was introduced around the third century AD, prepared with organic material that penetrates more deeply into the substrate and  reacts,  staining  it  black.  Chabries  et  al ( 2 0 0 3 )  n o t e d  t h a t  p a r c h m e n t  r e f l e c t a n c e  increases at longer wavelengths, resulting in greater contrast of text in infrared images. Each of our parchment samples was imaged before and after treatment through a series of  bandpass  filters,  illuminated  by  four  tungsten-halogen  lamps  on  a  photographic copystand.  The  Kodak  Megaplus  1.6i  camera  with  Nikkor  50  mm  f/2  lens  captured monochrome images of 1536x1024 pixels in both the visible and near-infrared spectrum over the range 400–1100 nm. A set of 21 bandpass optical interference filters was used, spaced at 20nm intervals across the visible spectrum and 50nm in the infrared. The spectral transmittance of each filter was measured with an Ocean Optics HR2000+ spectrometer. Because parchment is prone to curl, a 3 mm glass plate with anti-reflective coating was placed over the sample to hold it flat on the baseboard. In total 23x21x2 = 966 images were captured by the Kodak camera for all parchment samples before and after treatment. Four circular holes of diameter 1 mm were drilled in each sample at approximately one third and two thirds of the width and height. These holes are apparent in each image and remained as persistent  features  after  the  samples  were d e g r a d e d ,  b o t h  a s  r e f e r e n c e  p o i n t s  f o r  registration of the multispectral image channels and for comparison of image sets of the samples before and after treatment.        Figure 2: (left) 580nm image of MiniMacbeth target with sampling boxes; (right) reflectance factor vs wavelength for camera (black lines with circles) and spectrophotometer (red lines) for top row. To check the accuracy of reconstruction of reflectance spectra from the multispectral image sets, a MiniMacbeth target was measured with an X-rite i1Pro spectrophotometer at 10nm intervals from 380 to 730 nm. The results were in good correspondence with the corrected  camera  responses  for  all  four  rows o f  t h e  M i n i M a c b e t h  t a r g e t  ( F i g .  2 ) .  T h e  camera values at 400 nm were not reliable because the very low power of the lamp in this waveband required excessively long integration times for image capture, with consequent low  signal-to-noise  ratio.  The  CIE  L*a*b*  values  were  calculated  for  each  reflectance spectrum, using the CIE 2° standard observer and illuminant D65, and the mean colour difference over the 24 patches was  E*ab = 3.4. The maximum error was 6.5 for the fourth patch in the second row (purple).   3. IMAGE REGISTRATION AND ANALYSIS Because of differences in refractive index of the camera lens for different wavelengths of light, the geometry of the successive images in the multispectral sequence varied from one image to the next, so that no two were in perfect register. Also the treatment in some cases radically altered the parchment, causing substantial distortion to its geometric structure. To make a composite image of all the wavebands it was necessary to put them all into accurate register, using the four holes drilled in each parchment sample as the anchor points for a projective geometric transform (MacDonald  et al, 2013).  The image of the sample before treatment taken through the 600 nm filter was chosen as the reference and all other images, both before and after treatment, were registered to it. A template matching procedure enabled the centroid of each of the four holes in each sample to be located accurately (Fig. 3 left). Given an approximate starting position, the algorithm incremented the diameter of a circular anti-aliased template in units of 0.1 pixel over the range 12 to 16 pixels (1.0 to 1.3 mm), and the template was successively translated along the x and y axes. At each coordinate position a cross-correlation was performed between the  template  and  the  corresponding  image  section.  The  accuracy  of  the  registration procedure is shown in the false-colour image composite (Fig. 3 right). The transformed image (red channel) is closely in register with the reference image (blue channel), so that they appear everywhere as a unified magenta.          Figure  3:  (left)  Correlation  scores  from  template m a t c h i n g  o n  o n e  o f  t h e  r e g i s t r a t i o n  h o l e s ,  visualised as a surface on a pixel grid, showing the peak score at the hole centre, and secondary peaks caused by the ink strokes. Scores < 0 are represented by black dots, and scores > 0 by blue dots. The red cross shows the row and column indices of the peak score. (right) False-colour composite of reference 600 nm channel of sample before treatment (blue), 850 nm channel of sample after treatment (green), and the latter after registration (red). When all of the image channels for the sample before and after treatment had been accurately  registered,  it  was  possible  to  compare  the  reflectance  spectra  at  any  pixel position. Fig. 4 shows the effect of the treatment by blood on the parchment and iron gall ink. The spectra are plotted for all nine pixels in a 3x3 pixel region around the selected coordinates, together with the mean in green (before) and red (after). The blood clearly affected the colour of the parchment, reducing its reflectance factor from about 0.5 to less than  0.1  in  the  short  wavelengths,  rising  to  about  0.4  at  long  and  NIR  wavelengths, producing a dark red colour. The ink spectrum was apparently not much changed except at wavelengths greater than 850 nm.        Figure 4: Reflectance spectra before and after treatment for (left) parchment and (right) ink. A  typical  problem  where  a  degraded  parchment  is  subjected  to  analysis  is  that  its original state (both text and material) is not known, and so assumptions must be made in guiding  the  digital  restoration.  Our  dataset  of  the  parchment  samples  before  treatment provides a ‘ground truth’ for analysis of the effectiveness of image processing algorithms in restoring images of the degraded samples. Our dataset of the samples after treatment shows  exactly  the  effect  of  the  physical  changes  on  the  reflectance.  Analysis  of multispectral images of scraped or mechanically damaged samples enables us to identify even faint traces of ink. It also enables the discrimination of inks similar in appearance, and the recovery of writing from darkly stained parchment and from charred and burned fragments. Because the reflectance spectrum can be estimated from the multispectral image set, the appearance of the parchment under any  illumination  source o f  k n o w n  s p e c t r a l  power distribution can also be predicted. ACKNOWLEDGEMENTS This research was funded by EPSRC in the UK under Grant number EP/F01208X/1. We thank the London Metropolitan Archive for providing the original parchment.  REFERENCES Chabries, D.M., Booras, S.W., Bearman, G. 2003. Imaging the past: Recent applications of multispectral imaging technology to deciphering manuscripts, Antiquity 77(296), 359. Clarkson,  C.  1992.  Rediscovering  Parchment:  the  nature  of  the  beast,  The  Paper Conservator 16(1) 5-26. Giacometti, A., Campagnolo, A., MacDonald, L., Mahony, S., Terras, M., Robson, S., Weyrich, T., Gibson, A., 2012. Documenting Parchment Degradation via Multispectral Imaging, Proc. BCS Conf. on Electronic Imaging and the Visual Arts (EVA), London. MacDonald, L., Giacometti, A., Campagnolo, A., Robson, S., Weyrich, T., Terras, M. and Gibson,  A.,  2013.  Multispectral  Imaging  of  Degraded  Parchment,  Proc.  Fourth Computational Color Imaging Workshop (CCIW), Chiba, Japan, March 2013, 143-157.  Address: Lindsay W MacDonald, Department of Medical Physics, University College London, Gower Street, London WC1E 6BT, UK E-mails: ucfslwm@live.ucl.ac.uk, alejandro.giacometti.09@ucl.ac.uk 

Documenting Parchment Degradation via Multispectral Imaging,

Imaging the past: Recent applications of multispectral imaging technology to deciphering manuscripts,

Multispectral Imaging of Degraded Parchment,

Rediscovering Parchment: the nature of the beast,

Colour analysis of degraded parchment

Colour analysis of degraded parchment

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