1. COLOR or Shade: Keys to Acceptance CRABLE ENGINEERING LLC
Presented by:
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“Marketing psychologists state that a lasting impression is made within ninety seconds and that color accounts for 60% of the acceptance or rejection of an object, person, place, or circumstance. Because color impressions are both quick and long lasting, decisions about color are critical factors in the success of any visual experience.” - About Color
The fields of shade (or color) and appearance are critical to the acceptance of paper and board products, yet these product attributes are often overlooked. Or, systems to support them are often an afterthought in the design and operation of a paper machine.
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Now – What is Color (or Shade)?
(Shade is a term used for color generally when discussing a white or near-white object.)
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Color is a Perception
The eyes of humans have two sets of light receptors in their retinas (the neuronal layers at the back of the eyes). These are called rods and cones.
Color processing is done in the brain and is therefore subject to the interpretation of the viewer.
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Communicating Color...
You know, I told those guys in Color & Appearance to give the sheet some "snap." Does that look "snappy" to you?
?...
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Three Red Samples:
How would you describe their color differences?
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7. The Three Attributes of Color
Hue: the attribute of color described as blue, green, yellow, orange, red, etc.
Saturation (also called chroma): the intensity or "vividness" of a color.
Lightness: the degree of black, gray, or white in a color.
pastel
deep
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What Makes Color?
Observer (Eye-Brain)
Object
Light Source
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Color=(Light Source) x (Object) x (Observer)
Common
Light
Sources
for
Color
Viewing
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10. COMMON COLOR LIGHT SOURCES
Incandescent (most home lighting) Will they go away?
Cool White Fluorescent (most office lighting)
Daylight (outside lighting)
LEDs
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11. Describing the Color of a Light Source
The color of a light source influences the appearance of the objects it illuminates.
Lighting manufacturers often use terms like "warm," "cool" or "neutral" to indicate the color of a lamp.
The "Correlated Color Temperature" is a more specific term used to describe the color of a light source.
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12. Correlated Color Temperature (or CCT)
Assigning a correlated color temperature is an old practice that allows the color of a light source to be specified with a single number.
When a piece of metal is heated it changes color from red to yellow to white, to blue white.
The color at any point can be described in terms of the absolute temperature of the metal measured in degrees Kelvin (K):
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13. CCTs of Common Light Sources
A tungsten filament bulb (incandescent light) has a CCT of 3100K and is yellow in color.
Cool white fluorescent light has a CCT of 4150K and is greenish in color.
Daylight is blue-white; with the more common phases ranging between 5000K (D50) and 7500K (D75).
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Daylight
Ever-changing (must be specified).
The most common CCTs are 5000K (D50), 6500K (D65).
Rich in blue and UV energy.
Excellent color rendering properties.
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Fluorescent Lamps
Efficient (high number of lumens per watt).
Most common form of office lighting (usually with some daylight present).
Color is generally described as "warm, neutral, or cool white."
Color rendering capabilities vary with manufacturer and bulb type.
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16. Incandescent (tungsten filament bulbs)
Primarily used in the home.
Very yellow or "warm" in color.
Poor color rendering properties.
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17. The Influence of The Light Source on Color
To fully understand how a light source contributes to the color of an object, we
must know more than its Correlated Color Temperature.
The output of a light source is fully described by its Spectral Power Distribution.
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Definitions...
The spectral power distribution of a light source describes how much energy is present at each wavelength across the visible spectrum.
The visible spectrum is a small range of wavelengths, within the larger electromagnetic spectrum, that the human eye can see.
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The Electromagnetic Spectrum
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20. Relative Spectral Power Distribution of a Tungsten Filament Bulb
(Illuminant A; CCT = 3100K)
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21. Relative Spectral Power Distributions of
Two Cool White Fluorescent Sources
(CCT = 4150K) UV IR
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Relative Spectral Power Distributions for Two Phases of Daylight (D50 with a CCT of 5000K and D65 with a CCT of 6500K) UV IR
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23. Selecting the "Right" Light Source for Color Viewing and Measurement
Should approximate the color viewing condition(s) our customers use, if we know what those are.
The "right" light source should have sufficient energy across the entire spectrum for optimal color discrimination.
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The Object
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25. The Interaction of Light with Paper
Incident white Light
Surface Reflected and Scattered Light
Red Light Reflected by Dyed Fiber & Fillers
Transmitted Light
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26. The Scattering Properties of Glossy and Matt Samples
Specular Reflection
Diffuse Reflection
GLOSSY SURFACE
MATT SURFACE
The surface properties of a sample influence the
quality and quantity of light that reaches our eye;
influencing the way an object appears.
In fact, we sometimes calender samples to get “the right look.”
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The Observer
Different people see color differently due to:
Age
Macular Pigment
Number and ratio of rods and cones
Some average or "standard" observer of color must therefore be established for consistent color measurements to be determined for any object.
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28. The Standard Observer Development
Source: Principles of Color Technology, 2nd Edition Fred Billmeyer, Jr. and Max Saltzman
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29. At 18 inches ~ dime & baseball.
The Standard Observer
2° (1931) and 10° (1964)
At 18 inches ~ dime & baseball. UV IR
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Spectral Curves
Yellow Object Curve
Blue Object Curve
% Reflectance
% Reflectance
UV IR
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Color Mixing
Yellow Object Curve
Blue Object Curve
Green Object Curve
% Reflectance + =
% Reflectance
% Reflectance
% Reflectance
Colorants like paint, dye, and ink reflect only certain wavelengths of light and absorb all others. Mixing two different colors will produce an entirely new color by combining their light absorbance.
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Light Source Output X Sample Reflectance = UV IR UV IR UV IR
The Light That Enters The Eye
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Light that Reaches
Our Eye...
"Red" Sensitivity
"Green" Sensitivity
"Blue" Sensitivity
"Red" Response
"Green" Response
"Blue" Response Y Z X
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Tristimulus Values
Three Numbers Required to Describe Color:
X = k * x(l) * R(l)
Y = k * y(l) * R(l)
Z = k * z(l) * R(l)
A spectrophotometer measures only R(l) or
% Reflectance across the spectrum. All else is math, done in a computer.
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35. Color is Three Dimensional
Tristimulus values are not perceptually uniform; (equal distances in tristimulus space will not appear visually equal).
Tristimulus values describe color but are
not intuitive.
Tristimulus values are therefore transformed into L*, a*, b* space.
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36. Equations for Transforming Tristimulus Values to CIE L*, a*, b* (1976)
Equations for Transforming Tristimulus Values to Hunter L, a, b (1942) [This original unit system, Hunter admitted, had flaws.]
L = 100 x (Y/Yn)1/2
a = 175 x {0.0102*Xn/(Y/Yn)}1/2 x {(X/Xn) - (Y/Yn)}
b = 70 x { *Zn/(Y/Yn)}1/2 x {(X/Xn) - (Y/Yn)}
Equations for Transforming Tristimulus Values to CIE L*, a*, b* (1976)
[An improved version – more linear.]
L* = 116 x (Y/Yn)1/3 - 16
a* = 500 x {(X/Xn)1/3 - (Y/Yn)1/3}
b* = 200 x {(Y/Yn)1/3 - (Z/Zn)1/3}
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38. MacAdam Ellipses
Note that there are larger tolerances for green and deep shades than there are for blue and white shades – that means that our eyes are more sensitive to small differences in white, blue, gray and tan colors.

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Color Differences
The color difference between any two samples is expressed in terms of "deltas": L* a* b*
0.5
1.0
dL*=1.0, da*=0.5, db*=0.5
delta L* or dL* = L*SAMPLE - L*STANDARD
delta a* or da* = a*SAMPLE - a*STANDARD
delta b* or db* = b*SAMPLE - b*STANDARD
delta E* or dE* = [(dL*)2 + (da*)2+ (db*)2]1/2
dE* is a measure of overall color difference. Establishing a reject limit
for dE* constrains the three dimensions of color so that they can't simultaneously be at their outer limits. (Our eyes perceive acceptable
color differences as ellipsoids, not rectangles.)
Saturated and deep colors can have broader tolerances, while whites
and neutral shades (grays) may need tighter tolerances.
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Control Strategies
If dL* is positive (+): sample is too light. Add dye.
If dL* is negative (-): sample is too dark. Cut dye.
If da* is positive (+): sample is too red (or not green
enough). NOW CHECK dL*! [Is the sheet light or dark?]
- Add yellow and blue for +dL*; cut red for -dL*.
If da* is negative (-): Sample is too green (or not red
enough).
- Add red for +dL*; cut yellow and blue for -dL*.
If db* is positive (+): sample is too yellow (or not
blue enough). NOW CHECK dL*! [Is the sheet light or dark?]
- Add blue for +dL*; cut yellow for -dL*.
If db* is negative (-): Sample is too blue (or not yellow enough).
- Add yellow for +dL*; cut blue for -dL*.
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41. Color Control Exercises: Case 1 (Using Red, Blue, and Yellow Dyes.)
Standard Measures: L* = 74.5, a* = 40.4, b* = What color is this?
Sample Measures: L* = 75.1, a* = 41.0, b* = What are the deltas? What adjustment should we make?
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42. Color Control Exercises: Case 2 (Using Red, Blue, and Yellow Dyes.)
Standard Measures: L* = 81.9, a* = -22.7, b* = What color is this?
Sample Measures: L* = 81.2, a* = -21.9, b* = What are the deltas? What adjustment should we make?
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43. Color Control Exercises: Case 3 (Using Red, Blue, and Yellow Dyes.)
Standard Measures: L* = 78.9, a* = -13.0, b* = What color is this?
Sample Measures: L* = 79.5, a* = -12.2, b* = What are the deltas? What adjustment should we make?
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44. Color Control Exercises: Case 4 (Using Red, Blue, and Yellow Dyes.)
Standard Measures: L* = 80.7, a* = 0.1, b* = What color is this?
Sample Measures: L* = 81.4, a* = 0.0, b* = 2.0 What are the deltas? What adjustment should we make?
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Metamerism
When two samples appear to be the same color but have different spectral reflectance curves, they may match under one light source but not another.
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46. Common Sources of Metamerism
Different dyes
Different levels of fluorescence
Different pulps
Different fillers The potential for metamerism between a color standard and production is almost always present.
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47. Important things to know about metamerism...
Metamerism is the 2nd major cause of color complaints.
Colors can match in one set of lighting conditions and still be rejected by the customer if viewed under a different light source.
The better we control the variables that contribute to metamerism the more consistent our products will be.
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SUMMARY 1
Color=(Light Source) x (Object) x (Observer)
Observer (Eye-Brain)
Light Source
Object
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SUMMARY 2
Color = (Light Source), an Array of Known
(shade) Values for Each Defined Light Source X
(Observer), a 3-Column Table of Known x, y, z Values for The Two Defined Observer Functions:
(2 degree and 10 degree)
(Object), a Measured Array of % Reflectance Numbers
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SUMMARY 3
Color = X, Y, Z tristimulus units for the Red, Green, and Blue cones (receptors) in our eyes.
X, Y, Z tristimulus units are converted to L*, a*, b* units for ease of use and discussion where L* = 0 to 100 for dark to light; a* = -100 green to +100 red; b* = -100 blue to +100 yellow (in this ‘opponent color system)
dL*, da*, db* are color differences: Sample – Standard values for each
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SUMMARY 4
dE* = SQRT(dL* **2 + da* **2 + db* **2)
dE* is the total (summed) color error, and putting a limit on it prevents all 3 of dL*, da*, and db* from being at specifications limits at the same time. (See Slide 39.)
It takes all four of dL*, da*, db*, and dE* being within specification for good color reproduction.
Beware of metamerism!
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