Spectrophotometers: Cheaper, Better, Faster. The latest generation of color-analysis tools brings lab quality to the shop floor.

Transcription

1 Spectrophotometers: Cheaper, Better, Faster The latest generation of color-analysis tools brings lab quality to the shop floor. by Kenneth Phillips Know & Go 26 Quality Digest/September QDpg26-31(ColorAnalysis).ind /20/07 5:18:35 PM

2 As consumers worldwide become more sophisticated and demanding in their tastes, manufacturers find themselves in need of more exacting and cost-effective ways to measure color and make sure that their products meet evertighter specifications. Any manufacturer of consumer products interior trim for cars, vinyl siding for homes, decorative stone for landscaping, or interior wall paints, for example should implement stringent quality controls to ensure that the first run matches required color specifications, and that each successive lot is consistent. To do otherwise risks significant scrap and rework, delayed shipments, lot recalls, and loss of profits, if not valuable business relationships and contracts. By borrowing technology from other industries, color-instrument manufacturers have risen to the occasion by inventing robust in-line spectrophotometers for the shop floor that measure colors accurately, and at a fraction of the cost of older technologies. As a result, manufacturers can now obtain reliable data from high-speed production lines that use lean manufacturing techniques. Manufacturers of color-sensitive products can now obtain in-line spectrophotometers for less than $20,000 that: Provide real-time or continuous data that match lab-quality accuracy Measure colors without contacting the surfaces that they re measuring Can read colors in a shop-floor environment with fluctuating temperatures, vibration, high humidity, and variable lighting conditions Don t require frequent calibration Have the ability to read products with varied textures Confined to the lab To achieve equivalent color accuracy, companies previously used specialized, expensive (in excess of $100,000) spectrophotometers for in-line inspections, or committed to limited laboratory sampling throughout production. The latter required stopping production equipment to take a physical sample; destroying a salable workpiece so a sample could be obtained for the lab test; spending time conducting the test, including travel to and from the lab and the time involved in the test itself; and frequent (usually daily) calibration. Manufacturers that use lab-confined spectrophotometers often sample only at the beginning, middle, and end of production runs, because of the effort and expense involved. In many of these applications, lab testing serves as the lowest possible denominator of quality control identifying out-of-specification products before they re shipped. On the other hand, in-line spectrophotometers have made it much easier for companies to implement lean manufacturing initiatives, such as statistical process control, data gathering for continuous improvement and first-time quality, and mistake-proofing. Colorimeters vs. spectrophotometers Spectrophotometers aren t the answer for every colormeasurement application. In cases where precise color measurement is not required, other technologies offer more cost-effective quality control. For instance, colorimeters are simpler and less-expensive instruments that use red, green, and blue (RGB) filters to simulate the response of the human eye to light and color. Colorimeters are effective for sorting and for quick in-line checks on less-exacting jobs. To compare the resolution of a colorimeter with a spectrophotometer, a good visual analogy is this: A colorimeter measures on a scale of inches, while a spectrophotometer measures on a scale of one-sixteenth of an inch. There are many manufacturing applications where a colorimeter is perfectly adequate to the task at hand, and a spectrophotometer would be overkill. Still, many new applications are being found for this new generation of affordable spectrophotometers that can be used even in relatively rugged factory conditions. Any company looking to gain a competitive advantage by improving color measurement with shop-friendly spectrophotometers or other technologies should first ask: What exactly is the specification that my customers want and, more important, what level of quality are they willing to pay for? In-line spectrophotometers Consider the customer To arrive at a practical answer to this question, manufacturers first need to consider the color measurement standards of their customers. This point may seem obvious, but many companies speed past this basic question only to discover later that they don t have appropriate quality control equipment. Although it s beyond the scope of this article to describe the science of color measurement in depth, answering a few key considerations can help a manufacturer determine the best equipment for the job: Which color scale does the customer use? Instruments essentially assign numerical values to the three basic elements of color: hue, chroma, and value. There are three common standards that communicate a particular color in the vast universe of possibilities: CIE Xyz, CIE L*a*b*, and CIE L*C*h. What level of spectral resolution does the customer require RGB instruments that have made it much easier for companies to implement lean manufacturing initiatives Spectrophotometers identify colors by assigning numerical values to the three properties of color hue, value and chroma, as depicted by physical samples in this Munsell Color Tree. Quality Digest/September QDpg26-31(ColorAnalysis).ind /20/07 5:18:38 PM

3 can give only a relative nonstandard value; colorimeters that give one of the abovementioned standard values with moderate accuracy; or 31-point spectrophotometers that give all the standard color values plus full reflectance curve data, and do so at a high level of accuracy? How tight is the color tolerance that the customer uses? Wide-open tolerances may call for a simple RGB instrument, while 28 Quality Digest/September 2007 tight tolerances may require a 31-point spectrophotometer. How smooth is the surface you re measuring? Does it approach the brilliance of a first-surface mirror, or is it as rough as a roofing tile? Some textured surfaces, such as cloth, are angularly sensitive, meaning that color measurement is affected strongly by the orientation of the piece. Do you need to measure samples containing optical brighteners, where brightness is also affected by ultraviolet light, a part of the electromagnetic spectrum that is invisible to the human eye? Some products, such as optically brightened papers or textiles, contain optical brightening agents that essentially convert invisible ultraviolet rays into visible light. Assess the environment After a manufacturer understands a project s color-measurement specifications, the next practical and equally important consideration is assessing the suitability of the plant s instruments by taking a candid look at the environment in which they will be used. Often foremost among these considerations is whether a sample can be measured without destroying a salable part whether noncontact measurement can be used instead of destructive testing. A company that can utilize noncontact measurement techniques gains a tremendous competitive advantage in the cost of quality control and additional benefits related to advanced manufacturing techniques. Benefits from noncontact measurement may include an immediate reduction of scrap or spoilage, more uptime for production equipment, better utilization of quality control personnel (who no longer have to walk to and from a lab for tests), and more frequent quality control checks to catch problems sooner (see An In-Line Spectrophotometer Case Study on page 30.) Light, innovation, technology Manufacturers of color-measurement instruments have borrowed technology from a diverse crowd of industries cell phone companies, ink-jet manufacturers, aerospace firms to come up with innovative shop-floor solutions. By using components that are now mass-produced, colormeasurement manufacturers have also driven down the price of their products. There are three core reasons why spectrophotometers now offer enhanced quality and features at approximately one-fifth of the price of earlier in-line models: Smaller and highly reliable illumination sources due to remarkable improvements in light-emitting diodes (LEDs) Cost-effective innovations in the manufacture of color analyzers, i.e., stable interference-filter arrays driven by stepper motors 0907QDpg26-31(ColorAnalysis).ind /20/07 5:18:40 PM

4 Similar improvements in the optics and detectors that view samples, and in the electronics that analyze data The compact, intense sources of white light made possible by LEDs have been a boon to manufacturers of colormeasuring instruments. Developed just a few years ago for the flash option built into high-end cell phone cameras, new LEDs offer a nearly ideal optical component for spectrophotometer use a point source of bright light. The uniform LED burst of light allows instruments to read color samples from relatively far away. This new, more intense light source has allowed a new generation of spectrophotometers to utilize searchlight - type optics that partially offset the problem of variable readings at varied distances due to changes in light intensity. Coupled with imaging optics that can be likened to using a pinhole camera to keep image sizes proportional, spectrophotometers have greatly extended their range of vision. LEDs have other significant advantages. White light is a combination of the constituent of the visible spectrum. (Remember Roy G. Biv, the ploy your science teacher used to teach the colors of a rainbow red, orange, yellow, green, blue, indigo, and violet?) The light from LEDs has a flat degradation curve over time, meaning that the amount of each constituent color doesn t change much in the course of millions of flashes. The same can t be said for light from xenon flashes or incandescent bulbs. Over time, the constituent colors of their light shifts markedly, and light intensity diminishes. In addition, LEDs can withstand highvoltage electrical currents in a very short period of time to produce bright flashes. Because they rely on solid-state physics rather than electrical discharge through a gas or heated filaments in a vacuum, LEDs are very resistant to shock and vibration and more apt to remain at a constant temperature during measurements, reducing error. This allows spectrophotometers to operate accurately between 32 F and 120 F. New in-line spectrophotometers offer another critical advantage when measuring colors under varying lighting conditions. These instruments detect light only from the LEDs, and ignore all other sources of ambient light. Other sources of light incandescent, fluorescent, sodium vapor greatly affect the way colors are perceived. Previous spectrophotometers skirted the problem of error caused by ambient light by either relying on blinding flashes of light from a xenon flash to overwhelm any ambient light, or by the expensive process of shrouding samples to avoid ambient light. LED-illuminated spectrophotometers are much more refined. These newer instruments modulate LED pulses and tune the sensors so that they recognize only the LED light source, similar to the way a person tunes in one radio station from the many that are available in a given city. By utilizing techniques and processes mastered by the aerospace industry, spectrophotometers are now hardened to withstand the rigors of the shop floor. Quality Digest/September QDpg26-31(ColorAnalysis).ind /20/07 5:18:42 PM

5 An In-Line Spectrophotometer Case Study As one of the world s largest manufacturers of color cards and color systems, Chicago-based Color Communications Inc. specializes in exact duplication of the colors consumers will see when they apply paints and stains to the interiors and exteriors of their homes. Over the past 35 years, CCI has produced billions of color-merchandising products for companies around the globe, such as the paint chips offered by the paint manufacturers at home improvement retailers like Home Depot and Lowe s. The company s work is so precise that some color tools produced by CCI are used by its customers as standards for their internal color-quality control. Customers come to us because they have a need for accurate color, says Jerald Dimas, vice president, technical, at CCI. There is a premium cost to our product because of that accuracy. You can t print colors using four-color process printing for an accurate representation. We use the actual paint in our process. That s one of our competitive advantages: High-speed production with a high degree of accuracy. To ensure color quality during a production run, CCI technicians take test samples by stopping the equipment, cutting the desired sample from the production roll, and conducting spectrophotometer tests. A high-profile project for one client required more than 20,000 such measurements in the last eight months alone. The costs of quality add up quickly: Part of the production run is spoiled by taking the sample and the equipment remains idle during the tests. In addition, quality control personnel spend time conducting the tests and transporting samples to and from the factory floor. By multiplying the resources expended to test one sample by the tens of thousands of tests it performs annually, CCI determined that it had a great opportunity for cost savings and enhanced quality control practices. To pursue this opportunity, CCI agreed to act as one of a number of beta test sites for a new-generation spectrophotometer developed by X-Rite Inc. of Kentwood, Michigan. X-Rite s new tool can measure samples without destructive testing, match the accuracy of lab-confined spectrophotometers on a relative basis, and withstand the harsh environments of a factory floor. There was one final key to make the system work for CCI: cost. The instrument is priced at less than $20,000 a unit, nearly one-fifth the cost of other lab-bound or noncontact spectrophotometers. The company estimates that the instrument will pay for itself in less than a year through savings in scrap, equipment downtime, and labor. CCI uses the system to measure color at a distance of up to four inches from the sample, which is more than adequate for CCI s needs and allows for versatility in mounting the instrument. Because the instrument does not employ destructive testing, CCI has been able to compile data as often as it wishes while the process is running. Under the destructive testing system, CCI tended to conduct tests at the beginning, middle, and end of the product run simply because of the resources required for each test. We have a very active Six Sigma program here, and one of its tenets is to define and measure for constant improvement, Dimas said. We want to catch a problem at the source, and in order to catch it, we have to measure it. To engineer quality into their products, CCI has begun to correlate the process data that it collects from the new instruments with other quality control systems that monitor dwell and cure times, film thickness, and substrate variations. Initial results indicate that the new system will allow CCI to reduce the number of off-line samples from tens of thousands of trials to hundreds over the course of a year. These newer instruments handle mechanical shock and prolonged vibration due in part to toughened enclosures and sealing systems, with LEDs providing a shock-resistant, solid-state method of illumination. Reliable interference-filter arrays driven by rugged stepper motors are at the heart of color analyzers that essentially break the flash of light bouncing off a sample into 31 spectral components. Battle-hardened spectrophotometers housed in NEMA enclosures are so tightly sealed and packaged that they can withstand the pressurized hoses of a car wash without leakage. The newer spectrophotometers also work properly under dusty or oily shop conditions when protected by enclosures that direct a constant flow of filtered, compressed air past lenses and other optics. When bringing lab-quality spectrophotometers to the shop floor, quality 30 Quality Digest/September 2007 control personnel should take into consideration the environments in which the instruments will operate. Specific issues to consider include: The distance from which the sample will be read The allotted measurement time The area and shape of the color sample The substrate or texture of the sample Variation and extremes in shop-floor temperature Whether the instrument will operate under dusty or oily conditions Whether the instrument will perform under varying ambient light The access that technicians and operators will require to properly calibrate instruments The frequency of calibration The need to integrate with existing software and hardware Companies are discovering that the advantages of placing this new generation of spectrophotometers into factory environments greatly outweigh the need for additional planning and upfront research. About the author Kenneth Phillips is a product manager for the industrial color and appearance division at X-Rite Inc. in Kentwood, Michigan, the world s largest manufacturer of color-measurement solutions. In his 18 years of experience, Phillips has worked in a wide range of industries to develop strategies and solutions for product portfolios, including the automotive aftermarket collision repair industry, and the automotive paints and finishes industry. QD Comments Send feedback to comments@qualitydigest.com. 0907QDpg26-31(ColorAnalysis).ind /20/07 5:18:45 PM

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