Paper360 - July/August 2014 - (Page 32)
techlink | COLOR MEASUREMENT
Measuring Color online
Achieving the best color measurement and
control raises the question as to whether
scanning or single-point measurement is best
Color, shade and appearance
(meaning the properties of brightness, whiteness, opacity and gloss), are extremely important properties for paper and board products
because they carry a message to the end user:
they help identify the item, whether it be a
letter, a billing statement, a book, a magazine, a sale circular, a food package label, a
printed logo or even the contents of a box. If
the color of the paper on which the message
is printed is incorrect or inconsistent, it can
reflect badly on the product itself.
When dealing with near-white papers, we
generally use the term "shade" to describe
their color and the term "brightness" or
"whiteness" to relate other preferred reflectance attributes. For grades with lower reflectance of light, we can use the term "color"
(hue) to describe their reflectance (e.g., red,
orange, yellow, green, blue, indigo, violet).
For the sake of simplicity, "color" will be used
in this article as a generic term for either the
shade of near-white papers or the hue of more
Providers of modern online color measurement systems for paper include ABB,
Honeywell, HunterLab, Metso and X-Rite,
and nearly all paper machines that make
colored paper are equipped with color measurement and closed-loop control. Those
not equipped are at a severe disadvantage
to competitors as relates to manufacturing
costs, the processing of off-color broke and
Online color instruments can process and
display new data at typical rates of once every
15 seconds. The refresh rate of most of these
instruments is limited only by the recharge
time of the flash capacitor for the light source
(most often Xenon). This means that the operator has nearly continuous feedback on the
color of the paper as a grade/color change is
made. Closed-loop color control with 15-second measurement updates can follow most
changes in virgin furnish and adjustments in
broke usage while maintaining the finished
product well within color specifications.
The majority of modern fine white paper
machines are equipped with online color
measurement as well, regardless of whether
they produce uncoated freesheet (UFS),
coated freesheet (CFS), uncoated mechanical, coated mechanical or specialty grades of
paper. Even makers of containerboard grades
of paper often utilize online color measurement today as it is great for diagnostics as
well as for maintaining color consistency.
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Much effort has gone into making spectrophotometers excellent instruments for use in
the paper mill laboratory as well as online.
In the 1980s, that measurement was added
to the cadre of online measurements on the
Illustration of single point versus scanning color measurement.
paper machine in a number of paper mills.
The early instruments from Macbeth did a
great job and propagated. These instruments
measured in a single-point position on the
sheet; they did not scan the web.
AccuRay went the same route when it
began supplying color measurement and
control systems to the paper industry. They
provided the Macbeth ColorEye II sensor and
at first required customers to build a frame
to place that instrument on-sheet. Later,
AccuRay/ABB bowed to customer pressure
and began to build frames to support the
color sensor, providing a turnkey system for
color measurement. ABB eventually eliminated the independent color frame and incorporated the color sensor into the scanner.
When Measurex first offered color measurement, it incorporated the sensor into
the measurement package on the scanner
from the beginning; this was prior to ABB
eliminating its separate color measurement
frame. The Measurex approach eliminated
the need for a second frame to support a
color sensor, saving the space that would
have been required.
Table of Contents for the Digital Edition of Paper360 - July/August 2014
Over the Wire
Pulp and Paper Innovations
Strategic Importance of Water
Making Workers Heroes
Industry Emissions Improve
TAPPI Journal Summaries
Measuring Color Online
What’s New on paper360.org
Paper360 - July/August 2014