C O L O R or Shade : Keys to Acceptance Presented by: CRABLE ENGINEERING LLC

COLOR or Shade: Keys to Acceptance

Presented by:

CRABLE ENGINEERING LLC

1CRABLE ENGINEERING LLC

“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.


Now – What is Color (or Shade)? (Shade is a term used for color generally when discussing a white or near-white object.)


Color processing is done in the brain and is therefore subject to the interpretation of the viewer.

Color is a Perception


Communicating Color...

You know, I told those guys in Color & Appearance to give the sheet some "snap."

Does that look "snappy" to you?

?...


Three Red Samples: How would you describe their color differences?


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.

The Three Attributes of Color

pastel deep


What Makes Color?

Light Source

Object

Observer (Eye-Brain)


Common Light

Sources for

Color Viewing

Color=(Light Source) x (Object) x (Observer)


COMMON COLOR LIGHT SOURCES

Incandescent (most home lighting) Will they go away?

Cool White Fluorescent (most office lighting)

Daylight (outside lighting)

LEDs10CRABLE ENGINEERING LLC

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.


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):


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).


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.


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.


Primarily used in the home.

Very yellow or "warm" in color.

Poor color rendering properties.

Incandescent (tungsten filament bulbs)


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.

The Influence of The Light Source on Color


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.

Definitions...


The Electromagnetic Spectrum

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Relative Spectral Power Distributions of Two Cool White Fluorescent Sources

(CCT = 4150K)

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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.

Selecting the "Right" Light Source for Color Viewing and Measurement


The Object


Transmitted Light

Surface Reflected and Scattered Light

Red Light Reflectedby Dyed Fiber &

Fillers

The Interaction of Light with Paper

Incident white Light


The Scattering Properties of Glossy and Matt Samples

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.”

Specular ReflectionDiffuse Reflection


Different people see color differently due to:Age Macular PigmentNumber 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.

The Observer


The Standard Observer Development

28Source: Principles of Color Technology, 2nd Edition Fred Billmeyer, Jr. and Max Saltzman


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x: 2 degree

y: 2 degree

z: 2 degree

x: 10 degree

y: 10 degree

z: 10 degree

The Standard Observer 2° (1931) and 10° (1964)

At 18 inches ~ dime & baseball.

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Spectral Curves

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+ =

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.

Color Mixing

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Light Source Output X Sample Reflectance =

The Light That Enters The Eye

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"Red" Sensitivity

"Blue" Sensitivity

"Green" Sensitivity

"Red" Response

"Green" Response

"Blue" Response


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.

Three Numbers Required to Describe Color:

Tristimulus Values


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.


Equations for Transforming Tristimulus Values to CIE L*, a*, b* (1976)

[An improved version – more linear.]

L* = 116 x (Y/Yn)1/3 - 16a* = 500 x {(X/Xn)1/3 - (Y/Yn)1/3}b* = 200 x {(Y/Yn)1/3 - (Z/Zn)1/3}

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 {0.00847*Zn/(Y/Yn)}1/2 x {(X/Xn) - (Y/Yn)}

Equations for Transforming Tristimulus Values to Hunter L, a, b (1942)

[This original unit system, Hunter admitted, had flaws.]

36


•

*

*


MacAdam EllipsesNote 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.

http://upload.wikimedia.org/wikipedia/commons/f/f4/CIExy1931_MacAdam.png

The color difference between any two samples is expressed in terms of "deltas":

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

Color Differences

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.

L*

a*

b*0.5

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1.0

dL*=1.0, da*=0.5, db*=0.5


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*.

Control Strategies


Color Control Exercises: Case 1(Using Red, Blue, and Yellow Dyes.)

Standard Measures:L* = 74.5, a* = 40.4, b* = 27.8

What color is this?

Sample Measures:L* = 75.1, a* = 41.0, b* = 28.7

What are the deltas?What adjustment should we make?



Standard Measures:L* = 81.9, a* = -22.7, b* = 13.4

What color is this?

Sample Measures:L* = 81.2, a* = -21.9, b* = 13.1




Standard Measures:L* = 78.9, a* = -13.0, b* = -10.5

What color is this?

Sample Measures:L* = 79.5, a* = -12.2, b* = -10.4




Standard Measures:L* = 80.7, a* = 0.1, b* = 2.2

What color is this?

Sample Measures:L* = 81.4, a* = 0.0, b* = 2.0



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.


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.


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.


SUMMARY 1Color=(Light Source) x (Object) x

(Observer)

Light Source

Object

Observer (Eye-Brain)


SUMMARY 2Color = (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)

X (Object), a Measured Array of %

Reflectance Numbers


SUMMARY 3Color = 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


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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C O L O R or Shade : Keys to Acceptance Presented by: CRABLE ENGINEERING LLC