ABB’s High Performance Color Measurement sensor improves the quality of paper and board production by using light-emitting diode (LED) light sources and a high-speed spectrometer to make faster and more reliable measurements of colorimetric properties such as color, brightness, opacity and whiteness.

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_{Michael O’Hora ABB Pulp and Paper Dundalk, Ireland michael.ohora@ ie.abb.com

Håkan Hjalmers ABB Pulp and Paper Dundalk, Ireland hakan.hjalmers@ ie.abb.com}

Color – the term is used here to also cover brightness, opacity and whiteness – is a quality of paper that few people consciously consider, yet it plays a crucial role in paper manufacturing and use. For example, print quality, readability and visual aesthetic appeal are all significantly affected by paper color. A company’s brand recognition may rely on the color of the paper they use and color is important, too, when specific shades are required for security documents such as passports or identification cards →01.

Online monitoring of paper color during production is, therefore, critical for ensuring product consistency and, thereby, sellability. The online instrumentation involved must have the performance characteristics needed to maintain a stable process with minimal off-specification product – especially during shade changes or when rapidly moving the process to a different grade of paper (heavier, lighter, etc.) However, papermaking processes present a unique challenge for color measurement due to the widespread use of optical brightening agents (OBAs) that fluoresce in the blue region of the visible spectrum to increase apparent whiteness and brightness.

A colorful history

Having installed its first online color sensors on paper machines in the early 1980s, ABB has a long history of innovation in this field. To date, the laboratory and online color instrumentation approaches pursued by ABB and others have been dominated by designs based on gas-discharge or tungsten-halogen lamps. While these technologies are well-proven and well-standardized, they have inherent drawbacks regarding lamp lifetime, spectral degradation over time and pulse generator charge-up time (which limits measurement rate). Another significant disadvantage is the need to filter lamp output to control ultraviolet (UV) content so that OBA effects can be taken into account. Here, a UV filter has to be physically moved in front of the discharge lamp, severely limiting the rate at which measurements can be made.

From lamps to LEDs

In recent years, LED technology has advanced dramatically, especially regarding stability, repeatability, spectral range and optical power. Furthermore, as LEDs are discrete emitters with a variety of well-defined spectral ranges, it is possible to select individual emitters that span separately the UV and photoptic regions of the spectrum. In this way, LED emitter pairs employed in a color sensor can be sequenced to produce alternate UV-included and UV-excluded illumination modes many times every second to give an almost continuous stream of color data on a moving web of paper. Thus, the necessity to physically move a UV filter, as described above, is eliminated.

ABB’s High Performance Color Measurement sensor embodies this approach, using modern LED technology and a high-speed spectrometer that can measure the colorimetric properties of moving paper while scanning across the sheet during production →02. The sensor has been explicitly designed to overcome the many challenges encountered when measuring fluorescent and non-fluorescent paper in an industrial environment.

Sensor design

The ABB High Performance Color Measurement sensor module is designed to be mounted on either side of the paper or both sides of the paper, typically on an ABB Network Platform scanner that traverses it across the paper sheet, measuring colorimetric properties, in some tens of seconds. A backing tile module (BTM) is mounted on the opposite side of the process. The BTM allows colored tiles to be mounted in the optical path of the sensor for measuring supplementary colorimetric and optical properties, such as opacity (see below).

In the sensor, a white LED and blue-UV LED are sequenced – both on, then only white on – to produce UV-included and UV-excluded illumination modes. With both LEDs off, a third mode can be invoked optionally for baseline measurements to account for the intense extraneous light sources often found around paper machines →03.

A toroidal mirror produces an undirected annular illumination pattern on the paper at 45 °, focused to a 10 mm spot. Light scattered and reflected normal to the paper is gathered and directed via a mirror to the spectrometer (thus following the favored 45 °/0 ° design).

With up to 50 full readings of both UV-included and UV-excluded per second, the sensor gives unprecedented color measurement resolution in the machine- and cross-direction of the paper machine, with greater accuracy and lower short-term variability than traditional approaches. The sampling rate is 10 to 100 times faster than gas-discharge lamps and data is averaged continuously to deliver better measurement information for control applications. This high sampling rate means each measurement sent to the host is an average of hundreds of readings across the sheet, which, especially in a scanning application, drastically reduces color process noise from moisture and temperature streaks.

With more frequent and continuous measurement, paper manufacturers benefit from increased precision, leading to more consistent color and improved final product quality. The sensor enables manufacturers to precisely control color while minimizing off-specification product and improving shade consistency, as well as reducing the use of expensive fluorescent whitening agents, dyes and pigments.

Backing tile module design

As described above, the BTM is located on the opposite side of the process to the color measurement module. The BTM features a patented tile enclosure mechanism that helps automate sensor calibration, standardization and check sample routines to minimize operator intervention and ensure long-term sensor capability. The BTM features a six-tile carousel: The white backing tile facilitates continuous color measurement, while a black backing tile enables the sensor to make “infinite thickness” color calculations (a method patented by ABB) →04. To ensure long-term sensor stability, black and white standardization tiles are included to help the color sensor correct for variables that may affect accuracy (such as dust or dirt build-up). In the absence of the web, tiles are used to standardize the reflectance measurement and check the instrument’s performance over time against an internal standard. An internal check sample automatically verifies and certifies online sensor performance.

The BTM utilizes a high-velocity air stream – a so-called Bernoulli hold-down – to stabilize the paper web passline. This hold-down eliminates sheet flutter and enables non-contact color measurement.

Color correlation

The sensor has been extensively tested to ensure correlation with both laboratory instrumentation and existing gas-discharge online instruments. In particular, the performance of UV-included and UV-excluded measurement algorithms were rigorously examined to ensure accurate performance on fluorescent materials.

→05 shows data for a set of fluorescent plastic samples measured by the sensor and a laboratory instrument (a Lorentzen & Wettre Elrepho, calibrated with traceable standards provided by the manufacturer). This plot compares the ISO brightness of the samples measured in UV-included and UV-excluded modes, based on a D65 standard illuminant. The LED sensor shows excellent correlation in both illumination modes, despite the difference in illumination sources (LED versus xenon).

When dealing with color, L*a*b* is a color space that is very often used. This color space was defined by the International Commission on Illumination (CIE) in 1976 and expresses color as three values: L* for perceptual lightness and a* and b* for the four unique colors of human vision: red, green, blue and yellow. The two measurement sets (laboratory device versus the color sensor) for each of the three color space values lie almost on top of each other.

It should be noted that LED spectral characteristics differ somewhat from the standard illuminants defined by the CIE, which means LED devices need more source correction than gas-discharge-based instruments.

The High Performance Color Measurement sensor has also been trialed on several paper machines, where it tracks other means of measurement very well.

Easy maintenance and upgrade

The sensor features a lightweight, compact, modular design with no moving parts in the measurement module, thereby enhancing reliability and lowering maintenance and lifecycle costs. The lifetimes of the LED sources are many times longer than those of lamp designs and major parts can be replaced without recalibration. Safety is also improved as high-voltage pulsing and powerful capacitors have been eliminated. Real-time health reporting to the host is provided.

The High Performance Color Measurement sensor can be applied to the top or bottom of the web or to both sides and is designed for a quick and easy upgrade path as the measurement module is plug-compatible with ABB’s Smart Color sensor and is also fully compatible with the previous generation Smart BTM. The sensor works on ABB’s Network Platform scanner in conjunction with ABB’s Multivariable Color Control to minimize off-specification product and reduce shade change and start-up times. The sensor’s new Color Profiles feature enables the optimization of white top, size press and coating operations.

An even brighter future

As LED brightness and spectral characteristics improve, the sensor can be further enhanced. Additionally, the sensor’s high measurement rate will allow the separation of machine- and cross-direction information in the signal, opening up more opportunities for improved color control that were not possible with slower gas-lamp devices.

Future work is likely to also include the provision of control algorithms that fully exploit the unprecedented resolution of the sensor and enable papermakers to automatically and precisely rapidly change to a target shade and deliver superior consistency of color, brightness, fluorescence, opacity and whiteness in their products.

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