Identifying colour accurately

11 January 2005




Communicating colour How would you describe the colour of this rose? Would you says its yellow, sort of lemon yellow or maybe a bright canary yellow? Your perception and interpretation of colour are highly subjective. Eye fatigue, age and other physiological factors can influence your colour perception. But even without such physical considerations, each observer interprets colour based on personal references. Each person also verbally defines an object's colour differently. As a result, objectively communicating a particular colour to someone without some type of standard is difficult. There must also be a way to compare one colour to the next with accuracy. The solution is a measuring instrument that explicitly identifies a colour. That is, an instrument that differentiates a colour from all others and assigns it a numeric value. Ways to measure colour Today the most commonly used instruments for measuring colour are spectrophotometers. Spectro technology measures reflected or transmitted light at many points on the visual spectrum, which results in a curve. Since the curve of each colour is as unique as a signature or fingerprint, the curve is an excellent tool for identifying, specifying and matching colour. The following information can help you to understand which type of instrument is the best choice for specific applications: Spherical Spherically based instruments have played a major role in formulation systems for nearly 50 years. Most are capable of including the 'specular component' (gloss) while measuring. By opening a small trap door in the sphere, the 'specular component' is excluded from the measurement. In most cases, databases for colour formulation are more accurate when this component is a part of the measurement. Spheres are also the instrument of choice when the sample is textured, rough, or irregular or approaches the brilliance of a first-surface mirror. Tanneries would all be likely to select spheres as the right tool for the job. 0/45 (or 45/0) No instrument 'sees' colour more like the human eye than the 0/45. This is simply because a viewer does everything in his or her power to exclude the 'specular component' (gloss) when judging colour. When we look at pictures in a glossy magazine, we arrange ourselves so that the gloss does not reflect back to the eye. A 0/45 instrument, more effectively than any other, will remove gloss from the measurement and measure the appearance of the sample exactly as the human eye would see it. This type of unit is typically used in printing. Multi-Angle In the past ten or so years, carmakers have experimented with special effect colours. They use special additives such as mica, pearlescent materials, ground up seashells, microscopically coated coloured pigments and interference pigments to produce different colours at different angles of view. Large and expensive goniometers were traditionally used to measure these colours until X-Rite introduced a battery-powered, hand-held, multi-angle instrument. X-Rite portable multi-angle instruments are used by most auto makers and their colourant supply chain, worldwide. Attributes of colour Each colour has its own distinct appearance, based on three elements: hue, chroma and value (lightness). By describing a colour using these three attributes, you can accurately identify a particular colour and distinguish it from any other. Hue When asked to identify the colour of an object, you'll most likely speak first of its hue. Quite simply, hue is how we perceive an object's colour— red, orange, green, blue etc. Presented as a wheel or circle of colour there is a continuum of colour from one hue to the next so that if you were to mix blue and green paints, you would get blue-green. Add yellow to green for yellow-green, and so on. Chroma Chroma describes the vividness or dullness of a colour; in other words, how close the colour is to either gray or the pure hue. For example, think of the appearance of a tomato and a radish. The red of the tomato is vivid, while the radish appears duller. Chroma changes as we move from the centre to the perimeter. Colours in the centre are gray (dull) and become more saturated (vivid) as they move toward the perimeter. Chroma is also known as saturation. Lightness The luminous intensity of a colour — ie, its degree of lightness — is called its value. Colours can be classified as light or dark when comparing their value. For example, when a tomato and a radish are placed side by side, the red of the tomato appears to be much lighter. In contrast, the radish has a darker red value. Scales for measuring colour The Munsell Scale Hue, saturation and lightness demonstrate that visible colour is three-dimensional. These attributes provide three co-ordinates that can be used to 'map' visible colour in a 'colour space'. The early 20th Century artist Albert H Munsell - creator of the Munsell Colour Charts - is credited as a pioneer of intuitive three-dimensional colour space descriptions. There are many different types of colour spaces that are based on or resemble Munsell's designs. Over the last century many different scales of measuring colour have been developed including: * The CIE Colour Systems * CIELAB (L*a*b*) * CIELCH (L*C*H0) * CMC and CIE94 Equation (Further details on these scales are detailed in the Understanding Colour Communications book). To obtain these values, we must understand how they are calculated. There are many different colours as there are many different object surfaces and each object affects light in its own unique way. As stated earlier, our eyes need three things to see colour: a light source, an object and an observer/processor. The same must be true for instruments to see colour. Colour measurement instruments receive colour the same way our eyes do, by gathering and filtering the wavelengths of light reflected from an object. The instrument perceives the reflected light wavelengths as numeric values. These values are recorded as points across the visible spectrum and are called spectral data. Spectral data is represented as a spectral curve. This curve is the colour's fingerprint. Measured spectral data has a couple of advantages in that it is both device and illuminant independent. * Spectral data measures the composition of light reflected from an object before it is interpreted by a viewer or device. * Different light sources appear differently when they are reflected from an object because they contain different amounts of the spectrum at each wavelength. However, the object always absorbs and reflects the same percentage of each wavelength, regardless of amount. Spectral data is a measurement of this percentage. So the two components of colour that change with every viewing condition - the light source and the viewer or device - are 'bypassed' and the ever stable properties of the object's surface are measured instead. Detecting metamerism Another advantage of spectral data is its ability to predict the effects of different light sources on an object's appearance. As mentioned earlier, different light sources have their own compositions of wavelengths, which in turn are affected by the object in different ways. For example have you ever matched a pair of socks and trousers under fluorescent department store lighting and then, later, discovered that they do not match as well under your home's incandescent lighting? This phenomenon is called metamerism. Colour control Colour control - or process control - is critical to achieving a consistent, quality colour throughout an entire batch, across different shifts or between different batches of materials. Any system of measurement requires a repeatable set of standard scales. Colour tolerances Verification between colour specifications and actual colour results is achieved by using tolerances that are based on numeric colour measurement data. Colour tolerancing involves comparing the measurements of several colour samples (the colour output) to the data of a known colour standard (the specification or input). Then, the 'closeness' of the samples to the standard is determined. If a sample's measured data is not close enough to the desired standard value, it is unacceptable and adjustments to the process or equipment may be required. The amount of closeness between two colours can be calculated using a variety of colour tolerancing methods. CMC tolerancing was developed by the Colour Measurement Committee of the Society of Dyers and Colourists in Great Britain and became public domain in 1988. The CMC calculation mathematically defines an ellipsoid around the standard colour with semi-axis corresponding to hue, chroma and lightness. The ellipsoid represents the volume of acceptable colour and automatically varies in size and shape depending on the position of the colour in colour space. The figure at the foot of the page shows the variation of the ellipsoids throughout colour space. The ellipsoids in the orange area of the colour space are longer and narrower that the broader and rounder ones in the green area. This size and shape of the ellipsoids also change as the colour varies in chroma and/or lightness. Although no colour tolerancing system is perfect, the CMC equation represents a 95% agreement with visual colour differences as our eyes see them. Integrated colour -throughout the supply chain The instrumentation and communication of colour data is as important as the colour data itself. Throughout the supply chain, different suppliers may use different processes and equipment for colour formulation and quality assurance, making compatibility an essential component. X-Rite products are designed for integration and compatibility throughout the supply chain. For example a large installation may use integrated, networked colour formulation and quality assurance software, such as X-RiteColourMaster, and several X-Rite sphere instruments throughout the shop. A small supplier with X-Rite QA-Master I installed on a single computer and one SP62 spectrophotometer will be compatible with the larger installation. Colour control is required in a wide variety of applications, in varied scopes. This is why X-Rite offers the following process solutions: Colour formulation and quality assurance From basic quality assurance functions to the most sophisticated colour formulation needs, X-RiteColour Master software, combined with X-Rite instruments, provides the ultimate flexibility to scale software packages to unique needs now and over time. Multiple math engines can easily and accurately formulate opaque, translucent and transparent colours at fixed loads or with minimised pigment usage. With all databases operating from the same structure in a network installation, managing colour standards and measurements makes X-RiteColor Master the most efficient software for enterprise and supply chain processes. Colorimeter Colorimeters are not spectrophotometers. Colorimeters are tristimulus (three-filtered) devices that make use of red, green, and blue filters that emulate the response of the human eye to light and colour. In some quality control applications, these tools represent the lowest cost answer. Colorimeters cannot compensate for metamerism (a shift in the appearance of a sample due to the light used to illuminate the surface). As colorimeters use a single type of light (such as incandescent or pulsed xenon) and because they do not record the spectral reflectance of the media, they cannot predict this shift. Spectrophotometers can compensate for this shift, making spectrophotometers a superior choice for accurate, repeatable colour measurement. Special effect and pearlescent paint The X-Rite MA68II spectrophotometer offers a full range of angular viewing (15° to 110°) for accurate evaluation of the changes exhibited in metallic, pearlescent and special effect paint finishes. The unique dynamic rotational sampling (DRS) technology utilises a simple, robust optical system which provides simultaneous measurement of all angles.The MA68II interfaces with X-RiteColor Master software for complete colour quality control applications. Sphere and 0/45 Instruments X-Rite offers a wide range of sphere and 0/45 spectrophotometers in portable and countertop models that offer superb inter-instrument agreement and repeatability. These instruments are easy to use and can be set up for streamlined, automated capture of colour data. Non-contact colour measurement The X-Rite TeleFlash system provides online colour measurement and evaluation of colour deviation to the running production line. TeleFlash can accurately measure the colour of products that are textured, finely patterned or glossy, such as extruded vinyl, bulk goods, coil coatings, synthetic films, paints (wet and dry), coated leather, carpeting, granules, food pigments, paper, powders, glass, ceramics, metal, minerals and plaster. TeleFlash offers a measuring distance of up to five feet, tolerating small variations in the measuring distance from system to sample. The system's thermochromism compensation allows for colour measurement without the time usually required for cooling and stabilising. Multi-user, network installations and portable data The networkability of X-Rite software makes it easy to communicate data and share standards across an enterprise. This ease translates into efficiency which has a direct effect on profitability. For applications without networked computers, X-Rite Colour-Mail can be used for fast, easy communication of colour data via standard e-mail. ColorMail can be a seamless part of X-RiteColor Master software. Calibrated, on-screen colour X-Rite offers the only colour formulation and quality assurance software to use the International Colour Consortium's (ICC) standard device profiles for on-screen colour. This means that colours will be consistently displayed on different computers, so long as ICC profiles are used. Use X-Rite monitor optimizers and auto-scan densitometers for complete colour calibration and control on computers, printers and scanners. Applications Spectrophotometry's applications are seemingly boundless. A typical user will make colour measurements many times a day, comparing production to clearly defined standards. Spectrophotometry assisted colour measurement can be useful in areas such as: * Corporate logo standardisation * Colour testing of dyes and inks * Colour control of coated leather * Control of printed colours on packaging material and labels * Colour control of plastics and textiles throughout the development and manufacturing process * Finished products like printed cans, clothing, shoes, automobile components, plastic components of all types To obtain free copies of 'Understanding Colour Communications' please contact X-Rite on +44 1625 871100 or email [email protected] with your name and address details. To find out more about colour communications or to attend one of X-rite's training days, telephone the above number.



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