The Discovery of a Watermark on the St Cuthbert Gospel using Colour Space Analysis Christina Duffy 1. Introduction Watermarks since their introduction in the thirteenth century have been used as a method of establishing the provenance and origin of paper, identifying mill trademarks and locations, and determining the sizes and intended functions of papers. Watermarks are designs such as a name, initials, or a decorative motif impressed on paper in a similar way to chain and laid lines.1 A watermark design was incorporated by manipulating wire into a recognizable shape and affixing it to the mould. The thickness of the paper is reduced over regions of wire which results in the familiar appearance of watermarks, chain lines, and laid lines when paper is held up to the light. Chain and laid lines are important even if they obscure the watermark design as their relative positions can aid in determining the orientation of the paper when the pages were laid out for printing. Imaging of watermarks has been problematic for curators and scholars with partial marks often found in non-adjacent gutters owing to the ordering and cutting of the folios. Techniques such as tracing, transmitted light and the Dylux method2 have been used in the past. More sophisticated methods such as beta radiation, IR imaging and thermography have improved results, but are still highly dependent on the material and only successful in certain cases. Best results are usually found when the watermarked folio can be accessed from both sides to allow light to pass through from the back, and the image to be photographically captured from the front. Indeed the majority of watermarks are initially observed accidentally by curators or researchers turning the folios and capturing the light which illuminates the watermark. This region can then be imaged later with better techniques to identify the mark more thoroughly, but initial observation is highly unlikely when the folio is adhered to a backing such as another folio or a binding cover. Therefore the probability of observing with the eye a watermark on the inner paste down of a binding is very low. The inner paste down is often a region where author’s notes, signatures or inscriptions are placed, and is an important aspect of the collection item. Even if the folios have been rebound and post-date the main content there is still a great deal of information to be gained from understanding the full history of a collection item. In this paper we explain the basics of colour spaces and introduce software which allows users to convert standard RGB images into other colour spaces. The recent discovery of a watermark on the lower board of the St Cuthbert Gospel inner pastedown is used as a case study to illustrate how image processing can reveal and enhance information. A discussion of the watermark and the implications and potential uses of colour space analysis to curators and scholars are presented. 1 Chain and laid lines are the result of fine mesh wires lying parallel (laid lines) and thicker mesh wires running perpendicular (chain lines) to the long sides of a rectangular mould. 2 The Dylux method for making watermark prints was developed by Thomas L. Gravell. It was used extensively by Gravell in his publications A Catalogue of American Watermarks, 1690-1835 (New York, 1979) and A Catalogue of Foreign Watermarks Found on Paper Used in America, 1700-1835 (New York, 1983) from documents in the Library of Congress. The method placed the watermarked paper on top of a sheet of commercially produced Dylux 503A photosensitive paper which was then exposed to blue daylight fluorescent light. The watermarked paper was removed and the Dylux exposed to UV light to reveal the image of the watermark. This image was fixed by further exposure to blue daylight fluorescent light. 1 eBLJ 2014, Article 2 The Discovery of a Watermark on the St Cuthbert Gospel using Colour Space Analysis 2. Colour spaces Colour is an attribute of visual perception: it is not a physical property. Reflection occurs when light or any other wave bounces off a surface. The light reflectance of a surface is its ability to reflect light. Light reflection makes it possible to see objects that do not produce their own light (light emission). The perceived colour of an object is completely determined by its reflectance spectrum, although objects with different reflectance spectra can have the same perceived colour. Colour can be specified by three parameters in a colour space and there are mathematical relationships that enable the parameters of one colour space to be transformed into another. Alternative ways of describing colour numerically are useful for making certain calculations easier and making colour identification more intuitive such as by describing colours by their hue, saturation and luminance. The colour of an image is most often described in terms of the percentage of red, green and blue components combined. Images such as these exist in RGB colour space, but there are other ways to describe the colour of a pixel using different colour spaces. A single colour space which provides satisfying results for the enhancement of a watermark does not exist. Therefore it is important to convert an RGB image into as many colour spaces as possible to determine where most new information is found.Vandenbroucke 3 proposed to classify the various colour spaces into four categories based on their definitions and properties (fig. 1). Fig. 1. Colour Space Families from Busin, Vandenbroucke and Macaire and Postaire, fig. 1. The proposed categories are the primary spaces, the luminance-chrominance spaces, the perceptual spaces and the statistical independent component spaces. 2.1 Primary colour spaces (RGB, RGBW, XYZ) Primary colour spaces are based on the trichromatic theory which assumes that it is possible to generate any colour using a combination of the three primary colours in varying proportions. The default colour space for most available image formats is RGB which corresponds to the three primary colour components: red (RRGB), green (GRGB) 3 C L. Busin, N. Vandenbroucke, L. Macaire and J.-G. Postaire, ‘Colour Space Selection for Unsupervised Colour Image Segmentation by Analysis of Connectedness Properties’, International Journal of Robotics and Automation, vol. xx, no. 2 (2005), pp. 70-7. 2 eBLJ 2014, Article 2 The Discovery of a Watermark on the St Cuthbert Gospel using Colour Space Analysis and blue (BRGB), respectively, called tristimulus values. These components match the absorption spectra of the three visual pigments in the eye. The visual colour perceived is the combined effect of one or several of these normalized components. RGBW has four colour components red (RRGBW), green (GRGBW), blue (BRGBW) and white (WRG- 4 BW). The RGBW colour space is a method for more efficiently displaying colours by using an amount of white light to substitute for the part of each of those three channels that are the same. XYZ is the first international colour space and was developed by the CIE5 in 1931. Col- our is described in this 3D space as the luminance component Y (YXYZ) and two additional components X (XXYZ) and Z (ZXYZ). The components are based on results from psycho- physical experiments and are weighted relative to their importance in the human visual sys- tem. A human’s visual response is what allows hidden information to be observed making XYZ a useful colour space for locating hidden information. Other colour space categories are obtained from a linear or non-linear transformation from the primary colour spaces. 2.2 Luminance-chrominance colour spaces (AC1C2, Lab, Luv, YIQ , YUV, YQ1Q2) Luminance-chrominance colour spaces consist of one component representing ‘lightness’ and two components representing ‘colour intensity’. RGB information is converted into luminance and chrominance information. The separation of luminance and chrominance components of a colour space is advantageous over basic colour spaces in compression applications. AC1C2 is an opponent (antagonist) colour space containing one luminance achromatic channel (A) and two opponent chrominance channels (C1, C2). It was derived by an examination of the human visual system and creating axes which pass through the most populated colour region.6 In the Lab colour space light distribution can be optimized and operations in this space are used on digital photographs to enhance particular regions by separating layers. Lab is based on the XYZ space and is non-linear. L describes lightness, and a and b describe the red/green and yellow/blue axes. This space is used as part of a shade-matching tool in dentistry where the lightness, chroma and hue of teeth are matched.7 Orthogonal colour spaces or device-dependent colour spaces are the TV transmission colour spaces. Two colours deemed to have the same CIE colourimetry will only match if they are viewed under the same conditions. The International Colour Consortium (ICC) defined a Colour Management System (CMS) allowing colour information to be compatible between input, output and display devices. Specific devices are assigned colour information profiles that can be mathematically translated to function accurately on any device. YIQ was defined by the National Television Systems Committee (NTSC) and is used in televisions in the United States. In the YIQ space greyscale information is separated from colour information so that the same signal can be used for both colour and black and white TV monitors. Y represents luminance, I hue and Q saturation. 4 WRGBW is the common base shared by R, G, and B i.e. WRGBW = min(RRGBW, GRGBW, BRGBW). 5 The International Commission on Illumination is known as the ‘CIE’ from its French title ‘Commision Internationale de l’Eclairage’. The CIE is an independent, non-profit organization founded in 1913 and is devoted to international cooperation and collaboration in regards to the science and art of light and lighting, colour and vision, photobiology and image technology.