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Today's sophisticated manufacturers require instant communication, not only across the production floor but between facilities in different cities or even continents. Companies often grow through acquisitions or expand by outsourcing some of their manufacturing to other companies. Others may even transfer production to other facilities of their own-around the corner or 12 time zones away-that may have unused capacity. Maintaining good color control to a master standard is an especially difficult task in these situations.
An additional challenge is that so many different methods can be used to measure color. It seems that every instrument supplier has their own variation of components to obtain X, Y, and Z, or L*a*b* color numbers: sphere geometry vs. 45/0 or 0/45; or tungsten vs. xenon flash vs. LED technology.
While all of these methods can be effective, measurement data obtained on one instrument type will not necessarily match numbers generated by a different instrument type. Even the comparison of numbers between two sphere geometry instruments made by the same manufacturer can be different if the sphere size or measuring port size is different.
Understanding Gloss and TextureIt can be difficult for manufacturers striving for consistent color control to determine what is correct when there are so many options. The first thing that needs to be addressed is the product itself. Is the product manufactured in such a way that the only variable in the appearance of the final product is color, or does the process allow for variations in gloss and texture that will modify the total visual impression of the product? Two products with the exact same pigmentation can look different if one has a higher gloss than the other, or if one has a rougher texture than the other.
The way that light bounces off the surface and reaches the eye of the evaluator impacts both gloss and texture. When light strikes a high-gloss surface, a portion of that light is bounced off of the surface at the specular angle and never really reaches the eye as a component of the color. The visual impression of a high-gloss surface is that it looks darker than one with a lower gloss. The same holds true of surface texture; a smooth surface appears darker than a textured one.
Measurement OptionsTo combat these issues, it is first necessary to standardize the instrument supplier for all locations, and to make sure that the exact same instrument is used in all locations. This eliminates the variability of instrument geometry, the light source used by the instrument, and sphere and port size variables. The chosen instrument should have unquestioned inter-instrument agreement and be robust enough in its manufacture to withstand extremes of temperature variability and less than "clean room" manufacturing conditions.
Also keep in mind that a 45/0 spectrophotometer and an integrating sphere won't provide the same information. A 45/0 (or 0/45) instrument measures the color similar to how our eye sees it. If the gloss on the part is higher than that of the standard, the 45/0 instrument will give color readings to indicate that the part is darker (the L* value will be lower than that of the standard).
Is the part really darker? The eye thinks it is and the instrument will say that it is, but the only way to really know is to also measure the gloss of the part. If the gloss is the same, then yes, the part really is darker and will have to be adjusted for color. If the gloss is higher, however, then the part has to be adjusted for gloss before a true color reading can be obtained (see Figure 1).
With an integrating sphere instrument, all of the light that bounces off of the surface of the sample is measured. The light that bounces off at the specular angle on a glossy surface gets measured as the exact same amount of light that bounces off of a matte surface. The integrating sphere measures the true color of the object but does not necessarily agree with what the eye sees (see Figure 2).
A 45/0 instrument is often used for quality control when manufacturers check a product against a standard made of the same material and manufactured in a well-controlled process. If the color difference is greater than the specification, the most likely cause is probably the color, not the gloss.
An integrating sphere instrument is most often used when trying to match the color of two different materials, regardless of the gloss (e.g., matching grout color to a ceramic tile). The materials are too different to have a gloss match, but it's important for the pigmentation to be the same. The sphere instrument will report a good color match even if the gloss difference is huge, as long as the pigmentation is the same (see Figure 3).
That's not to say that the gloss measurement in either case is not important; gloss is an key element of any color evaluation. In fact, a 60° gloss meter is included on BYK-Gardner spectro-guide handheld spectrophotometers. These instruments, which are available as 45/0 or specular included integrating spheres, measure color and gloss at the same time on the same spot on the sample. If the total color difference (ΔE) of the product is out of the specified range for that product, then the ΔGloss measurement will help determine if there is a pigmentation problem or gloss problem. Likewise, a zero ΔE on the spectro-guide sphere may not visually match the standard. The ΔGloss will indicate what needs to be done to match the standard.
Additional ConsiderationsA number of other factors should be considered when selecting the instrument of choice for multiple locations. Two important parameters are the repeatability of the instrument and the reproducibility of that instrument when compared to others. If the instrument varies by more than .01 ΔE between readings of the same spot, it is difficult to know that the color is correct. Likewise, if two instruments at different locations don't measure within a ΔE of 0.2 to each other, it is impossible to know which unit has the right reading.
With some instruments, the variability between instruments requires that a physical standard be measured at each location. That doesn't work in a global manufacturing environment where the results are needed today, not next week when the physical standard gets to the other location.
Another important feature to consider is the stability of the unit once it is calibrated. LED-based instruments have been known to hold good calibration values for months at a time. Xenon and tungsten technologies burn up a little every time they flash and need much more frequent calibration to compensate for the reduced flash intensity.
Calibration values can also change with ambient conditions, not just time. With some manufacturers, a change in room temperature of even 5°C requires a new calibration to be performed on the instrument. The LED output in the spectro-guide is stable across all temperatures due to a patented technology that compensates for ambient room temperature. Results obtained in the air-conditioned lab and on the production floor will be identical.
Other features to look for when choosing an instrument for quality control include its ability to store a large number of standards in the internal memory; a number of different color difference equations built into the firmware; all of the commonly used illuminant and observer angle combinations; and a number of built-in indices for commonly evaluated properties such as whiteness, yellowness, or metamerism. The unit should be lightweight for comfortable use, and should use standard batteries.
Manufacturers need not compromise quality by moving to a handheld instrument. Many handheld instruments offer the same 10 nm resolution found in high-quality, high-cost benchtop instruments without the accompanying cost or maintenance requirements.
The easy-link software package included with the spectro-guide provides a method for storing and sharing standards, as well as producing a quality control report to share between companies or provide to customers. Instrument manufacturers also offer many software packages available for purchase; some are specific to that manufacturer's instruments, while others include interfaces for many manufacturers and instrument configurations. The cost of the software can rival that of the instrument, so be sure to carefully review the specifications of any purchased software and make sure that it does everything that's required.
The Best ChoiceIn summary, when choosing a solution for color verification between different locations, it's important to choose an instrument that delivers consistent results-on the same instrument and between different instruments, and under all operating conditions. Gloss is an important feature in determining total color, so a combination of gloss and color measurement is more valuable than just a color measurement. Most importantly, make sure that all locations are using the exact same manufacturer, geometry and instrument model.
For additional information, contact BYK-Gardner USA at Rivers Park 2, 9104 Guilford Rd., Columbia, MD 21046; call (301) 483-6500; fax (301) 483-6555; e-mail firstname.lastname@example.org; or visit www.byk.com.