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Whites especially can be tricky-according to Jim DeGroff, president and owner of Color-Tec in Clinton, N.J., a small imbalance in the colorants can have a big impact in a white glaze, and the human eye is extremely sensitive to differences in whites and lighter colors.
To maintain a high product quality level and ensure customer satisfaction, ceramic manufacturers should be able to measure and evaluate their product colors during the production process, but this has traditionally posed a challenge. Conventional color measurement instruments are expensive and hard to justify for smaller manufacturing operations. Additionally, they are often sensitive to the dust and temperature fluctuations inherent in a plant environment, making them impractical for online color measurement.
A new generation of color measurement instruments using light-emitting diodes (LEDs) might change all that. These new instruments are both portable and rugged, making them practical for industrial use. And while their operation is high-tech, they're designed to fit into a low-budget operation. With these advancements, color measurement is ready to go mainstream in ceramic manufacturing.
How LED Color Measurement WorksIn a traditional colorimeter, a white light-usually tungsten or xenon-is used to measure the color. As the signal comes back from the sample, it is passed through RGB (red, green, blue) filters. The better the filters, the better the data collected from the measurement. Unfortunately, the filters in many traditional instruments have lacked uniformity, and instrument-to-instrument variation can be common. One solution is to use a group of interferance filters to create an abridged spectrophotometer. Another is to use a diode array to create a portable spectrophotometer. Both of these solutions are available in the marketplace but are expensive and sensitive to plant operating conditions.
In an LED-based colorimeter, the sampling across the visual spectrum is achieved with six different LEDs: violet, blue, green, yellow, orange and red. The LEDs are fired in sequence, and a sensor measures the full spectrum response. That data is gathered and mathematically analyzed the same as with a white light, but with a higher degree of uniformity. The instrument is calibrated the same way that traditional instruments are calibrated, to a white standard and a black standard. The data output meets all the international standards for color difference calculations.
LED-based spectrophotometers, the next level of color measurement, incorporate 12 LEDs and take measurements every 10 nanometers (nm) within the 400 to 700 nm color spectrum, providing a more precise color reading. Based on that spectrophotometric data, the RGB response weighted by an illuminant (such as daylight, incandescent or fluorescent) can be calculated to determine the way a given color looks under different lighting conditions. With spectrophotometry, a manufacturer can mix new colors to match a given color by predicting the way that each colorant absorbs and scatters light. A database is typically created using a series of samples for each of the colors in the palette that will be mixed. Those samples are then measured into the software, and the software calculates at different concentrations how a particular color absorbs and scatters light as the light comes into the matrix of the material.
"The computer uses that data if I show it an unknown color I want it to achieve," says DeGroff. "It measures that color and gives the spectral data, and then it mathematically mixes the colors using this scattered absorbance data and predicts the best mixture of colors that will achieve the color I'm trying to match. It then sorts those results to give the user the best match under the various lighting conditions specified."
Traditional portable colorimeters have only performed colorimetry. But with the LEDs, both colorimetry and spectrophotometry have been brought into a smaller, hand-held, inexpensive package.
Sophisticated, Stand-Alone DevicesLike most color measurement instruments, the LED instruments run on PC-based software. Unlike most instruments, however, the portable LED-based tools are sophisticated enough to run as stand-alone devices.
"You can take measurements independent of the computer, you can take measurements and send the data directly to a computer, or you can save the data on the instrument and then send them to the computer later. So you really have a lot of flexibility," says DeGroff. Using a portable LED instrument, a ceramic engineer can perform color measurements on a particular product run during the manufacturing process, then bring that data back and download it into a computer archive for a historical record of what's happening on the line.
There is also a move toward integrating the LED instruments into online color measurement. The instruments can perform color measurement without contacting the surface of the product being measured, so they can easily be integrated into a production line without slowing down the process. Where the instruments are placed is up to the manufacturer. "Each line is a little different, so each plant needs to determine its own 'best' placement," says DeGroff.