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by Michael H. Brill, Ph.D.

Ten Tips for Maximum Color Measurement Performance

Although color spectrophotometers are designed to measure specimens both accurately and repeatably, they accomplish these measurements only within tolerances. Spectrophotometers aren’t perfect measuring devices, and how well they measure often depends upon factors under the control of the system operator.

After you’ve followed buying basics—i.e., purchased from a reliable supplier with a history of producing high-quality instruments and satisfied customers, and selected the model spectrophotometer that has the features and capabilities necessary to meet the needs of your specific application—the following 10 tips can help maximize the performance of your spectrophotometer:

1. Maintain the spectrophotometer according to manufacturer recommendations, including periodic testing and preventive maintenance by qualified service personnel.

2. Operate the spectrophotometer in a temperature-controlled, clean environment and, if possible, leave it under power at all times.

3. Maintain the white and black calibration standards so they’re clean and safe from potential damage.

4. Recalibrate more often than the manufacturer recommends, perhaps every two to four hours and immediately before important jobs, even if the manufacturer suggests that recalibration is normally necessary only every eight (or more) hours.

5. Consider that almost all colorants—whether dyes or pigments—change color as temperature changes. If possible, measure specimens at the same temperature each time.

6. For specimens that aren’t opaque, prepare them for reflectance measurement by increasing specimen thickness so they’re opaque (or nearly so), unless you’re intentionally measuring specimens over white and/or black backgrounds.

7. For specimens with directional surface orientation, always measure at the same single orientation, or measure them at the same four orientations (i.e., 90 degrees apart) and average the data.

8. For specimens with inconsistent color and/or surface effects, measure multiple times (moving the specimen between reads to increase the effective area measured) and average the data.

9. If practical, use the largest specimen-measuring area available (e.g., large area view, small area view and ultra small area view, in decreasing order of preferability) to achieve averaging over a larger specimen area.

10. If possible, always measure standards and batches (of colors to be compared) using the same specimen measuring area as well as the same spectrophotometer, under the same conditions.

 

LEDs: An Alternative Light Source

Another technology reaching the color-analysis market is based on light-emitting diodes (LEDs). Different LEDs can emit different light spectra, but the intensity and spectrum of the light in a given LED does not drift; in fact, it becomes more stable if a constant current is maintained through the LED.

Flashed-xenon light sources are currently the rule in spectrophotometers. A xenon source generates a lot of light (enough to overpower any ambient light), and the spectrum of that light resembles daylight. However, xenon is a high-maintenance light source, requiring especially careful recalibration. Furthermore, fine-tuning a xenon-source spectrum to a particular daylight requires the use of filters (which eat energy). Thus LEDs are becoming an attractive alternative technology. Differentially driving a set of different colored LEDs can adjust a spectrum without filters.

For many years, LEDs have been used in low-cost color analyzers that measure the visible spectrum only. Such systems pulse a series of differently colored LEDs whose combined output spans the entire visible spectrum. These instruments flash the LEDs in microseconds and measure the reflected signals using a silicon full-spectral responder. This data is then used to estimate the spectral reflectance of the specimen. Datacolor's PrintFIX product uses such an alternative to correct the printing of digital photographs. LED-based products are also manufactured by Color-Tec, BYK-Gardner, GretagMacbeth, X-Rite and others.

LEDs fit the portability needs of the quality control world because of their durability and low power requirements. Color-measurement instruments based on LEDs are beginning to find their way to the factory floor, where their use complements xenon-based spectrophotometers.

For several decades, industries have been integrating instrumental color management systems into their processes, thereby increasing the objectivity and the repeatability of the evaluation procedure. In today's global market, where leaders look for every competitive advantage, innovative color-measuring tools have been developed with bottom-line efficiencies in mind. This new generation of systems can deliver streamlined color management across every aspect of the coloring process, whether that means receiving raw materials or approving the finished product. Here's a closer look at the advances of benchtops and portables as well as guidance for selecting the best spectrophotometer to suit your needs.

What's right for you?
In the past, choosing a spectrophotometer was relatively simple. Any sampling done in the color laboratory dictated a benchtop, while a portable was the device of choice in production environments. Now, however, with the converging needs of color quality control throughout a global supply chain, the division of tasks between laboratory and production isn't so clearly defined. Instrument manufacturers have responded with a wide range of choices, encompassing benchtops, compact benchtops, portables and hand-helds.

The system that's best for your organization depends on a number of factors, including the requirements of your specific application, where your operations fit in the overall supply chain, and what you most need the instrument to measure--the color itself or the total appearance of the product, including texture and gloss. To understand what instrument is best suited to measure certain color characteristics, it helps to review spectrophotometer basics.

Geometry guidelines
The term "geometry" refers to the relative placement of the material specimen to be measured, the light source and the spectrophotometer's measuring sensor. The most common geometries found in modern instruments are d/8 (i.e., illuminate the specimen diffusely and measure at 8 degrees from the specimen's perpendicular) and 45/0 (i.e., illuminate the specimen at 45 degrees and measure it perpendicularly). Each instrument has a particular domain where it's most useful.

In a d/8 spectrophotometer, the specimen is illuminated by use of an integrating sphere. Light that enters the sphere from the source reflects off the coated interior surface of the sphere and strikes the specimen from all angles. The reflected light is then measured at an angle of 8 degrees relative to a perpendicular line drawn from the specimen. An integrating-sphere spectrophotometer tends to minimize the influence of surface irregularities on the light reflected from the specimen, allowing measurement of reflectance due to color rather than surface variations. This is especially helpful in laboratory shade matching and in color production, where it's preferable to measure the color difference between two specimens with little consideration for differences due to construction or surface irregularities.

A 45/0 spectrophotometer makes use of one or more light sources to illuminate the specimen at an angle of 45 degrees, with a lens placed at 0 degrees through which a sensor measures the amount of light reflected from the specimen surface. This type of instrument is more sensitive to surface irregularities than the d/8 instrument and measures "appearance" as well as color. For this reason, 45/0 instruments are often beneficial in quality-control applications where differences in surface texture and finish are important--in other words, where there's a need to evaluate more closely what the color looks like in an actual finished product.

Traditionally, a d/8 spectrophotometer, developed for detecting and evaluating subtleties in color appearance in the controlled environment of the lab, was considered superior in repeatability and inter-instrument agreement.

The latest advances in 45/0-based spectrophotometers take these laboratory-grade specifications into production environments. Users can now measure color on surfaces regardless of shape or texture, with the measurement result correlating directly to the total appearance of the surface.

To understand the significance of total appearance, consider an automotive supply chain. Suppliers as well as manufacturers now can be assured that the colors in all the many varying parts of a vehicle match or coordinate with one another when assembled.

The right light
All spectrophotometers have traditionally used one of two light sources--either a tungsten filament bulb similar to a common projector bulb, or a pulsed xenon flash lamp similar to that used in a camera's flash bulb. Pulsed xenon is considered superior and has become the standard source for use in laboratory instruments because its high intensity allows for good measurement repeatability on dark and high-chroma specimens. The short pulse length also minimizes the possibility of specimen heating, which can change the color of the specimen. Because xenon bulbs are rich in ultraviolet (UV) energy, they can excite any fluorescing chemicals or colorants (such as optical brighteners) present in a specimen and lead to inaccurate match predictions.

Be sure that the benchtop instrument you select offers filters to control the UV, not only for shade matching but also to calibrate the amount of UV energy for accurate whiteness calculations. Some instruments even provide automatic positioning of a special filter to ensure accurate UV control. If the instrument doesn't provide a method for calibrating ultraviolet energy, the consequences can be quite time-consuming--i.e., standards must be remeasured each time a batch is evaluated for whiteness. Also, keep in mind that stored standards can be used for evaluating whiteness only if they're measured on an instrument equipped with a UV filter.

Until now, adjustable UV filter options were available only on laboratory instruments. Some of the more advanced hand-held color measurement instruments can now deal with the UV power in the source as well. The PRO version of Datacolor CHECK, for example, features an unfiltered xenon bulb that contains a UV component. The instrument can be fitted with a cutoff filter, which keeps all the energy in the UV region from reaching the specimen, or it can be equipped with an adjustable filter that allows the user to adjust the UV content of the light source, which will vary as the instrument ages. Another light source increasingly popular in hand-helds is LED (see the sidebar on page 26).

Viewing variables
Generally, spectrophotometers must be equipped with a range of aperture sizes to allow measurement of both small and large specimens.

Lab and production standards are often prepared with the intent of using the largest area view available on the spectrophotometer. This is to minimize the influence of any variants that occur in the coloration process. Smaller ports are necessary for measuring the smallest specimens, especially the challenging circumstances when only threads or small clippings are provided as standards for shade matching.

The best benchtop devices will provide the widest range of aperture sizes to deliver optimal flexibility, from 3 mm to 30 mm. Once the purview of benchtops, such flexibility is now offered by hand-helds as well. Be sure to ask what, if any, aperture options are available in the portable you're considering.

Handling gloss
Many materials--shiny paint chips, plastic panels and magazine pages, to name a few--that serve as standards for shade matching appear glossy when viewed from a particular angle. When such materials are measured for new shade formulation or when specimens are measured behind a glass plate, it's important to exclude the glossy reflection from the specimen measurement.

The glossy reflection--also known as "specular gloss"--is automatically removed when using a 45/0 instrument. If you're evaluating a d/8 instrument, be sure it offers a specular gloss port to remove this obstacle to measurement accuracy. The specular port should be located at an 8-degree angle from the perpendicular to the measured specimen, opposite the sensor's exit port. With the right software program, it can open automatically when selected during the calibration routine. The specular gloss can then be safely included when comparing two identical materials, and excluded when comparing two materials with significantly different gloss levels, as long as the same measurement technique is used.

Application challenges
Certain applications require specific spectrophotometers to handle specific challenges. (Think of the translucent plastic used in traffic signals or etched glass: You can view an object through it, but it will appear hazy.) If you need to measure color in translucent specimens--which are neither completely transparent nor totally opaque--you'll find that most color-measuring instruments aren't designed to yield the best results. You need a benchtop instrument developed to accommodate the ways in which transmittance properties must be measured, with features such as a transmittance compartment large enough to insert and remove specimens easily, and a measurement geometry that complies with the industry standards covering this highly specialized area.

Metallic-flake and pearlescent finishes represent another example where an application-specific spectrophotometer is required. These special-effect pigments are helping manufacturers lead the way in buying trends, brand recognition and product differentiation due to their eye-catching appeal. Yet the very characteristics that make them so appealing--the manipulation of light to create mutable or dazzling special effects--also make them notoriously difficult to match and measure.

Because special-effect pigments look different when viewed from different perspectives, they must be measured at multiple angles to get an accurate read. This requires a benchtop unit equipped with a special multi-angle configuration that accommodates variation in both illuminating and viewing angles. Also, inquire about links to quality-control software and access to color-control solutions developed particularly for special effect pigments.

Accuracy is critical
Accurate color measurement lies at the heart of effective color management throughout the supply chain, and it's behind many derivative benefits--from higher speed to market to reduced cost of goods. To fully appreciate the importance of accurately capturing electronic color data, consider the potential devastation a color difference can make by the time it's repeated--and compounded--throughout a supply chain.

Yet measurement accuracy and agreement among instruments aren't guaranteed, regardless of the model you choose. Instrument performance varies over time due to age and environment, and no two vendors manufacture identical devices.

Historically, operators are responsible for ensuring the accuracy of a spectrophotometer. Prior to measuring specimens on the spectrophotometer, users must perform diagnostic tests to check the accuracy of the measurements. Such tests include a drift test to check for read-to-read repeatability, a diagnostic tile test to monitor long-term repeatability and a standardization--or calibration--with a white tile.

Users also can improve instrument performance by controlling environmental factors such as temperature and humidity. A constant-temperature environment will eliminate measurement variability caused by thermal changes within the instrument and specimen. Therefore, it's best to maintain power to the spectrophotometer to eliminate warm-up thermal drift and to keep the instrument room at a nearly constant temperature. But not every facility has the means to provide a stable color-measurement environment. This costly problem is often compounded when specimens are measured at different locations.

Fortunately, there are now specimen-conditioning cabinets that are inexpensive, compact and easy to use. The best of such cabinets not only condition specimens quickly and correctly at a fixed
temperature and relative humidity, but they also supply a constant and stable exposure to a specific defined illumination. This latter feature ensures stability of conditioning to light.

Embracing the entire cycle
Spectrophotometers designed specifically for measuring colored materials are at the center of any modern color formulation, color production or color quality-control system. The process of transferring color from a design concept to the consumer is complex. To help realize enterprisewide benefits, from portfolio and pipeline management to front-end practices, the technology underlying these important devices must embrace the entire color production cycle. The best color--- measuring instruments are designed to deliver this power.

Throughout virtually every industry in the world, color is a fundamental indicator of quality. Delivering material that's the wrong color can imperil future business, and failing to get the color right the first time can significantly increase labor and raw-materials costs. With increasing competition and pressure to bring products to market in record time, it's more important than ever to deliver an on-spec color faster and more efficiently. Integrating a color-measuring instrument into your workflow will significantly improve the efficiency of the coloring process and the color consistency of your finished product.

Editor's note: Discuss this article by clicking on the "Color Analysis" link at www.qualitydigest.com.

About the author
Michael H. Brill, Ph.D., is the principal color scientist at Datacolor in Lawrenceville, New Jersey, where he initiates new product ideas and manages the corporate intellectual property. Brill is a co-inventor of the Emmy Award-winning Sarnoff vision model. He's a member of the Editorial Board of Color Research and Application.