Guidelines for Measuring Colours
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Colour measurement is an essential part of the production of high quality materials and coatings in the automotive industry. This article gives a number of useful hints on how to get it right.
Automotive suppliers are faced with the challenge of measuring colours accurately and meeting their customers’ requirements reliably. Manufacturers of measuring instruments have developed a range of tools to help in this process which allow everyone to learn the language of colour and use it for quality assurance purposes. In the same way as a craftsman needs the right tools, automotive suppliers should aim first of all to understand what their customers want to measure and what level of accuracy is required. They must ask the following questions:
Which colour scale does the customer use?
Which spectral resolution is needed?
Instruments that produce a very fine resolution of the visible spectrum are more expensive than those with a coarse resolution. For example, a colorimeter will measure one of the standard values referred to above with less accuracy, while a spectrophotometer will give all the standard colour values and a complete reflectance curve to a very high level of precision.
Which system does the customer use to measure colour deviations and how tight are the tolerances?
Common formulas for colour deviations include CIE DE*, DE CMC and DE 2000. It is important to choose the correct tolerance system so that products which do not meet the customer’s specifications are not delivered and so that the delivery of perfect products is not delayed.
How smooth is the surface which is being measured?
Is the surface smooth, like a painted car part, or rougher, like a pressed plastic component? The quality of the surface (gloss or matt) and its texture (woven or with a leather grain, for example) have a significant influence on the appearance of the colour of a sample part. In addition, special effect metallic or pearlescent pigments are often used, which appear to be a different colour depending on the angle from which they are viewed. Therefore, it is important to choose a colour measuring device with the appropriate measurement geometry in order to measure samples correctly.
What lighting conditions are needed?
Many colours give rise to the well-known problem of metamerism. Two colour samples can look the same under artificial light but quite different in natural daylight or vice versa. Customers generally choose a standard type of lighting which is most similar to the lighting in the showrooms where their products will be sold. Common lighting types include D65, A, C, F7 and F11.
Using spectrophotometers with diffuse lighting
Spectrophotometers have three common types of measurement geometries for measuring the colours of paints and coatings precisely: diffuse/8° (so-called sphere geometry), 0°/45° or 45°/0° and multi-angle. A sphere spectrophotometer uses a white-coated hollow sphere to measure the colour and gloss of surfaces independently of one another. This can be an important factor for painted and coated surfaces.
Handheld sphere spectrophotometers are generally easy to use and cost-effective and allow for fast, precise measurements of a variety of different materials, including coatings, plastics and textiles. However, manufacturers often use 0°/45°or 45°/0° spectrophotometers which more accurately reflect the behaviour of the human eye with regard to the texture and gloss of a sample. Most 0°/45° spectrophotometers must come into physical contact with the surface before they can measure it. However, the latest generation of 0°/45° spectrophotometers can take measurements from a distance without touching the product itself. This non-contact technology enables manufacturers to measure wet paints and materials with sensitive surfaces as part of their process quality assurance activities.
Choosing the right measurement angle for materials and coatings that vary depending on the angle of view
Vehicle manufacturers and their suppliers use multi-angle spectrophotometers to measure special effect paints and coatings which contain aluminium or mica pigments. The demand for special effect paints and coatings of this kind has increased rapidly over the last 10 years, because customers like the changing colours and gloss effects when they look at the product from different angles.
Advanced measurement systems give precise results
Spectrophotometers are among the most sophisticated instruments used to measure colours and other visual properties. Other devices, such as colorimeters and densitometers, provide less comprehensive or less accurate data at a lower cost. Improved instruments often give more precise data and a targeted analysis of the data using a professional software package can produce effective results.
More advanced software programs make it possible to link the objective numeric data of a colour sample with the variable data in the production process. This can include, for example, the formulation for a specific paint, the application process for a coating or process parameters such as temperature and pressure. Analysing colorimetric measurements often allows processes to be optimised, which saves time and money and brings very high-quality results.