Internet Controls Color Measurement
Dr. Christine Connolly
Achieving agreement among spectrophotometers is important to many industries that deal with color, including paints, textiles, plastics, paper and pharmaceuticals. If a paint manufacturer's color-measuring instrument is miscalibrated, a customer may reject the entire batch of paint.
Spectrophotometers are very good at measuring color differences among batches of paint and assessing acceptability, but a customer may not be able to afford to wait for a target sample to be transported to the manufacturer's site. It is preferable to e-mail a color measurement, and it is essential under these circumstances that the customer's and manufacturer's spectrophotometers agree.
Remote calibration brings separate color-measurement clusters of instruments of different types into agreement.
Originally, researchers developed color-control technologies to oversee a collection of instruments in the same laboratory or test facility, but a much larger-scale system is required to address the proliferation of the supply chain into globally distributed manufacturing. Now Internet-capable systems are working to establish agreement among spectrophotometers. Just as timekeepers everywhere are linked to Greenwich Mean Time, the Internet offers the potential to set up a universal color-measurement standard.
GretagMacbeth AG of Regensdorf, Switzerland, has introduced an application called NetProfiler that remotely calibrates, adjusts and certifies color-measuring instruments, enabling companies to keep their instruments in agreement and to compare the color of products or components manufactured in different places. Benjamin Moore & Co. of Montvale, N.J., a manufacturer of paints, stains and industrial coatings, is using the Enterprise Color Management system throughout its North American manufacturing facilities to ensure color consistency. Moore purchased a set of Color-Eye 2145 spectrophotometers from GretagMacbeth for this project, but NetProfiler works with most popular brands of equipment.
Instruments may change their measurements over time because of mechanical or electronic fluctuations or because of the aging of the light source. The solution typically is to calibrate them every few hours against a white reference tile. The instrument manufacturer supplies the tile and its official measurement values, which are traceable to a central standard. In the UK, this is a reference spectrophotometer at the National Physical Laboratory in Teddington; GretagMacbeth's instruments are traceable to the National Institute of Standards and Technology in the US. Instruments that are traceable to one standard should agree on their measurement of white. However, because of wavelength inaccuracies, this does not necessarily mean that they will agree over a range of colors.
To check color performance, a set of colorimetric tiles was developed that displayed excellent stability to light and temperature and that was easy to clean. The British Ceramic Research Association, now known as Ceram Research Ltd. of Stoke-on-Trent, UK, made the original version of these tiles commercially available in 1969. NetProfiler uses a more recent set of these color calibration tiles. The software automatically tests the instrument on the tiles and builds a performance profile to correct subsequent measurements. This is done once a month to counteract changes in the instrument, and the tiles are returned to the NetProfiler standards laboratory to be checked every 12 months.
Achieving agreement among instruments, however, is difficult. In 1997, the National Physical Laboratory published the results of an exercise that compared the measurements by several national standards laboratories and by 24 industrial centers. The study revealed that national standards laboratories sometimes disagree in their measurements by up to two CIELab units and that industrial color measurements can disagree by even more. This did not inspire confidence in instrumental methods, given that the human eye can perceive a color difference of 0.5 units. The laboratory recommended the development of techniques using a set of calibrated standards for the detection and correction of inaccuracies, with the aim of achieving agreement within 0.5 CIELab units.
Since then, the National Physical Laboratory has developed the Measurement Enhancement and Traceability system. This prototype system features a collection of calibrated artifacts such as ceramic tiles, neutral density filters and wave-number standards that were designed to check the zero position, scaling, linearity and wavelength positions of instruments. A set of algorithms that corrects these aspects of performance achieves a level of agreement better than 0.5 CIELab units among instruments from different manufacturers for color, white and gray materials. The laboratory currently is developing the system as an Internet calibration service called iColour and is looking for partnerships with potential users.
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