White gold alloys:

Dippal Manchanda Birmingham Assay Office 0 Birmingham B 0 SB 0 England Email: 0 0 Steven Henderson Metallurgical Department , Cookson Precious Metals Ltd, Birmingham B1 3NZ, England A numerical grading system has been established for white gold alloys using the ASTM Yellowness Index D1925, an existing colour quality standard. The Index is calculated from the CIE (International Commission of Illumination) Tri-stimulus values, X, Y and Z, and the scale is linear, so that as the number decreases, the alloy becomes whiter. This Index provides values that have good correlation with visual assessments and permits easy differentiation of colour without knowledge of an alloys composition. The use of a spectrophotometer instrument provided a quick means of sample colour measurement, with high precision and accuracy. - What is Colour? Colour is an occurrence that results from the interaction between light energy, an object and an observer, Figure 1. These three factors are collectively referred to as the Observer Situation (1). This situation influences how colour is perceived. Light Source: Light can be described in terms of the energy it emits at each of the wavelengths in the visible spectrum e.g. a yellowed light emits a greater amount of light between 560 - 590 nanometres. Object: An object will reflect some wavelengths of light better than others (absorbing these others). This will change the observers perception of the objects colour e.g. a blue object appears blue because it reflects more blue wavelengths of light, while absorbing the green and red wavelengths of light. This is commonly referred to as Spectral Reflectance. Observer: One observers perception of colour can differ from another observers due to variation and deficiencies in human colour perception. Colour is only one attribute of Appearance, which is described by 2 key categories; Chromatic Attributes: Characteristics relating to colour Hue: The property by which we differentiate one colour from another e.g. red from blue Value: The lightness of a colour when viewed in daylight and marked 0 (black) to 10 (white) Chroma: The degree of departure of a colour from a grey of the same value. Colours with a low chroma are weak, while colours with a strong chroma are saturated. Geometric Attributes: Characteristics describing how an object modifies the reflected light Gloss: The attribute of an objects surface that accounts for lustrous or mirror-like reflection in conjunction with specular reflection (see below) Haze: Is the object transparent or translucent to light. If it is and the light is dispersed on passing through the object, this is described as a haze effect Texture: Is the objects surface flat and smooth. This will help control light reflection Shape: Is the objects surface flat or curved. The shape will also effect the light reflection Viewing Angle: Altering the angle at which the object surface is viewed will effect the colours observed Surround: The colour of the surroundings a sample is viewed in can influence colour perception as colours of medium value and chroma will appear to change in the direction of lighter, brighter or darker, less saturated colours surrounding them. These categories both effect the observers visual perception of an object. Metallic surfaces, e.g. gold, have a unique colour in their specular reflectance, which is the colour of the metal. Specular reflection occurs when a small fraction of the reflected light (1 - 10%) from the object surface remains unchanged and appears as a white highlight to an observer (a mirror-like reflection). 2.1 Defining Colour As the typical human eye can discern seven to ten million colours, we require an ordered method to relay colour information in a useful way. Several mathematical systems exist that can now define colour, providing specific data points or co-ordinates that can describe any colour. The Munsell Colour Order System (2) was the first system to communicate data in a way that could be readily understood. This system uses the chromatic attributes Hue, Value and Chroma (see above), to specify colours and highlight the relationship between the colours. Other systems have since been developed and include CIE Colour Space and CIELab. 2.1.1 The CIE System The International Commission on Illumination, commonly abbreviated to CIE, developed a system that took in to account the interaction between the elements shown in The Observer Situation, Figure 1, that they believed established the colour of an object. This system subsequently defined a number of key points such as: Standard illumination for colour comparison and the conditions for a Standard Observer Calculation of the Tristimulus values (X, Y and Z), which describes the response to a specific colour by the human visual system. However, these values were not designed to be a practical means for describing an objects colour but rather a means to determine if two colours having the same tristimulus values matched (using a standard illuminant and observer). Converting the Tristimulus values to the more easily understood Chromaticity co-ordinates (x, y and Y), that are often represented on a graph (referred to as a colour space) The chromaticity co-ordinates, x and y, referring to hue and chroma, are a conversion of the tristimulus values and were represented on the CIE 1931 Chromaticity Diagram. The third dimension (projecting out from the paper) is tristimulus Y, or Luminosity, which represents the brightness of the colour. This diagram mapped out the full range of colours that were perceived by the human visual system. Colours near the centre were considered weak (less saturated) and approaching neutrality (colours of white, black and grey). As a colour moved towards the edge of the diagram, the saturation level increased. Refinement of this work resulted in the CIE 1976 Uniform Colour Space (3), Figure 2. The shape of the colour space has changed from the 1931 diagram, resulting in equal spatial distances on the graph equating to equal visual colour differences. This method involved converting the tristimulus values (X, Y and Z) to an alternative set of chromaticity co-ordinates, u' and v'. u' = 4 X / ( X + 15 Y + 3 Z ) v' = 9 Y / ( X + 15 Y + 3 Z ) 2.1.2 The CIELab System An alternative, popular method for defining colours is the CIELab Colour System. This system is based on the premise of there being three different types of colour receptor in the eye (Red, Blue and Green). When these receptors are excited, the brain interprets the three sets of signals as follows; light or dark, red or green and yellow or blue (4), Figure 3. The coordinates selected to represent these signals are L*: Brightness (black if the value is 0, 100 if the colour is white) a*: Red colouring if the value is positive, green colouring if the value is negative b*: Yellow colouring if the value is positive, blue colouring if the value is negative The magnitude of the val (...truncated)