colour & masterbatch | Measurement
Above: The Datacolor 45 unit provides circumferential illumination for repeatable measurement of textured surfaces
(VSI) in the US recently published a new standard using CIE L*, a*, b* values (ASTM D7856) to update current standards based on the Hunter scale (ASTM D6864 for solid colours and D7251 for variegated colours). It is now in the process of transitioning its colour retention certifi cation programmes.
“CIE L*,a*,b* colour space is considered to better
Below: X-Rite’s Ci6x spectro- photometer is designed for accurate colour measurement in production environments
represent human colour perception and offers some advantages for the process of evaluating acceptability of colour change,” says Dave Johnston, senior technical director at the VSI. “The Hunter-based systems required different types of siding colours (such as “medium blue” or “light green”) to be separated into distinct colour regions, each with its own boundaries and equation for evaluating colour change. The CIE L*, a*, b* system allowed us to fi nd an equation that worked for all colours, both solid and variegated, and eliminate separate colour regions. This will greatly simplify the process of setting up and evaluating studies of colour change due to weathering.” Some in colour measurement circles advocate the use of newer measurement algorithms such as CMC, CIE94 or DE2000, but there has been only a slow movement towards their use. “The industry is extremely slow to move because it has a large amount of historical data and is
reluctant to move to a new measure- ment system with different pass/fail tolerances,” says Ron Beck, R&D manager at Americhem.
Ensuring consistency In addition to using the
same units for reporting, instruments must be calibrated and qualifi ed for accurate communica- tion of colour and to ensure data is consistent from one
28 INJECTION WORLD | April 2015
instrument to another at multiple production plants around the world or throughout the supply chain. Calibration is standardised using CIE 15:2004 “Colorim- etry” and auxiliary ASTM/ISO and JIS colorimetric methods, says HunterLab’s Leggett. “It is important to verify the repeatability of your measurement made using your SOP. You should communicate your SOP measure- ment method to your customers and work with them on standardising best practices. This includes performing round-robin comparative measurements with multiple sites and customers that includes typical samples and a performance qualifi cation (PQ) qualifying tile of a colour similar to your products,” he says. Instrument profi ling software is regularly used to correlate a group of spectrophotometers to ensure consistent readings among the
group. “Although all spectrophotom- eters read colour accurately when they
leave the factory, there are variations among manufac- turers,” says CyberChrome’s Becker. “Instruments also drift and change. Profi ling is used to correlate the readings of a fl eet of instruments that are used together in the real world”.
In addition to calibrating and profi ling, a disciplined
programme of instrument monitoring is needed to ensure that an individual instrument’s colour readings are correct, Becker says. CyberChrome introduced its Instrument Performance software, available as a stand-alone program or as part of its OnColor suite of software, to check and monitor the performance of a spectrophotometer to see that it is measuring colour consistently and accurately. “The test uses a set of long-term stable ceramic tiles that are measured on a weekly or monthly basis,” she says. “The software tracks the readings to assess the consistency of the measurements and saves the data into a database so that it can track and plot the variations in the readings. These trend charts and plots help diagnose when it’s time to re-profi le the instrument or if it needs factory service and re-calibration.” Even with tools to help ensure good colour commu-
nication, there is room for physical improvement. “There needs to be improvement in colour communica- tion as it relates to correlation between objects, spectrophotometer measurements, printed materials and colours displayed on a computer screen,” says Americhem’s Beck. The profi ling of monitors, printers and spectrophotometers has helped improve the correlation in colour communication, although spectro- photometer profi ling software must create standard profi les that can be used by all colour software like printer and monitor profi les.
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