fundamentals of colors (2 of 2)

Fundamentals of Color and Appearance

VisualQuality Control

© GretagMacbeth

Section 3:


ASTM D1729-89

•Defined method to control variables which can occur during visual evaluation

•Extremely important when attempting to communicate color results within companies or to suppliers & customers


Key Elements of a Visual Color Evaluation Program

• Light Source Selection• Viewing Conditions• Color Standards & Sample Preparation• Color Vision Testing• Metamerism Evaluation• Communicating Color Differences


Light Source Selection

• Make certain that everyone use the same light source

• If there is an established procedure, choose the specified light source(s)

• Choose a light source that fits your needs


Light Sources


Viewing Geometry


Correct Viewing Geometry


Metamerism


Reflectance Curves of a Metameric Pair


Geometric Metamerism

• Samples that match at one angle of illumination, but do not match when the angle of illumination or viewing angle is changed

• Often occurs with materials such as velvets, suedes, broadlooms, and plastics


Observer Metamerism

• Samples that appear to match to a group of observers, but do not match to an individual observer

• This individual could have slightly variant color vision

• Color vision and discrimination tests provide some insight into the differences between observers


Color Standards

• To make visual color evaluations, a trial must compared to a standard

• A physical standard is a piece of material that represents the ideal color


Color Standard Characteristics

• Stability

• Reproducibility

• Availability

• Consistency

• Uniformity

• Representation


Sample Preparation

• Fluorescence• Surface (texture, curvature)• Surround• Pressure or tension• Size & overall shape

• Directionality• Opacity, translucency

or transparency• Photochromism• Thermochromism


Visual Tolerancing

• Perceptibility - our ability to see color difference• Acceptability - the color difference we are willing

to accept• Acceptability is not absolute and is entwined with

the psychology of color perceptions• Specialized color charts help companies establish

visual tolerances for their colors


Munsell® Color Tolerance Set

• Target color plus six other representing the limits for each of the three dimensions of color

• Limits can be established in any color space

• Color and limits are specified by customer


Color Vision and Color Discrimination Tests

• Dvorine Book of Pseudoisochromatic Plates• Farnsworth-Munsell 100 Hue Test• Inter-Society Color Council Color Matching Aptitude Test• Japanese Color Aptitude Test• Farnsworth Lantern• HVC Color Vision Skill Test


Communicating Color Information

• Understanding the three attributes - hue, value & chroma is critical

• Standardizing the language minimizes the error caused by a miscommunication


Communicated in terms of redder, yellower, greener or bluer

Communicating Changes in Hue


Value is almost always communicated using the terms lighter and darker

Communicating Changes in Value


Is often described as more saturated or less saturated

Communicating Changes in Chroma


Visual Color Evaluation Documentation

• Ensures that everyone is following your standard

• This written method should be communicated to:– customers– suppliers– anyone involved in your color decisions


Visual Color Evaluation Method – Sample Outline

1. Viewing ConditionsLighting Product: GretagMacbeth SpectraLight® III with UVPrimary Light Source: D65 Filtered TungstenSecondary Light Sources: Cool White Fluorescent and Illuminant AViewing Geometry: 0/45Surround: Munsell N 7/

2. Color Standards & Sample PreparationSample Size (standard and trial): 3” x 5” (desired)Sample Orientation: Side-by-side, always touchingColor Standard Storage: Filed in nonacidic envelopeColor Tolerance: Munsell® Color Tolerance Set

3. Color AnalysisColor Vision Testing: Farnsworth-Munsell 100 Hue TestColor Communication: Based on Munsell Hue Terms


Visual Color Evaluation Limitations

• Visual color evaluation is subjective• Color differences are difficult to

quantify & communicate• Illumination needs to be controlled

(intensity, angle of view, and so forth)• The surround needs to be controlled


Instrumentation

© GretagMacbeth

Section 4:


Colorimeter:• Filter Based (3 or 4)

• Fixed Illuminant (C/D65)

• Fixed Observer (2˚ or 10˚)

• Tungsten Halogen Light Source

• No Metamerism Testing

• Colorimetric Data (XYZ, L*a*b*)

• Quality Control

Spectrophotometer:• Fixed Grating & Array

• Multiple Illuminant

• Choice of Observer

• Pulsed Xenon Light Source

• Metamerism Detection

• Spectral & Color Data

• Q.C., R&D, Formulation, etc.

Instrumentation


Spectrophotometers

• Analyzes spectral distribution of reflected or transmitted light wavelength by wavelength, across the visible spectrum

• Measures the ratio of reflection or transmission by a specimen relative to a reference standard


Pulsed Xenon vs Tungsten Halogen


0/45 and 45/0 Geometries


Integrating Sphere Optics


SCI vs SCE Geometry


SCI vs SCE UsesSCI:

• Includes all angles of illumination

• Measurement independent of sample surface (gloss or texture)

• Measurement of true color

• Sample must touch the sphere

• Widely used for color matching

SCE:• Good correlation with visual

assessment

• Characterizes effects of sample surface

• Appearance only arrangement

• Non-contact possible, for on-line applications

• Values similar to 0/45, depending on gloss level


Use and Care of Instrument Standards

• Permanent reference base against whichinstrumental measurements are made

• Accuracy and precision of measurementsdepend upon “initial calibration”

• Should be handled with care & stored properly


Instrument Precision

• Repeatability - the deviation from mean experienced when measuring a single specimen on a single instrument

• Reproducibility (interinstrument agreement) - the deviation from mean experience between instruments of a “given type”

• Instrument Accuracy - the deviation from an accepted reference level


GretagMacbeth Spectrophotometers

Instrument Geometry Repeatability Interinstrument

Color-Eye® XTH

Color-Eye® 7000A D/8 0.01 0.08

Color-Eye 3100 D/8 0.02 0.12

Color-Eye 2180 D/8 0.04 0.12

ColorChecker® 545 45/0 0.04 0.12

Color-Eye 2145 45/0 0.04 0.12


Sources of Measurement Difference• Instrument Geometry (sphere or 45/0)• Choice of Color Calculations (CIELab, CMC, LCh)• Instrument Setup (SCI or SCE, UV Included or Excluded)• Area of View (SAV or LAV)• Sample Preparation• Sample Measurement• Orientation• Backing or Thickness• Pressure• Tension• Single or Multiple Measurement


Multiple Light Source Multiangle Measuring System


Single Light Source MultiangleViewing System


Multiple vs Single Light Source Systems

Multiple Light Source System• GretagMacbeth/Xenon• Short Measurement Time• Good Wavelength Resolution• Good Interintrustment Agreement*

Single Light Source System• Other/Tungsten• Long Measurement Time• Poor Wavelength Resolution• Poor Interintrustment Agreement


Glossmeter Design


InstrumentalQuality Control

© GretagMacbeth

Section 5:


Selection of Standard

• Size

• Shape

• Thickness

• Opacity

• Quantity

• Stability


Consistency in Color Quality Control

• Sample Preparation

• Instrument Selection & Configuration

• Mathematical Configuration


Color Standard Selection

• Physical Standard - must meet recommended guidelines

• Numerical Standard - defined by spectral data


Physical Standards

Advantages• Allows for visual comparison

• Decreases dependency on absolute agreement between color measuring systems

Disadvantages• May change over time

(deterioration, handling)

• May be difficult to produce or obtain several pieces that match


Numerical Standards

Advantages• Electronically stored

numerical standards are easily accessible

Disadvantages• May not be valid on a

system different from the one on which it was first measured


Sample Preparation

• Directionality

• Opacity

• Photochromism

• Thermochromism

• Fluorescence


Typical Illuminants

• Daylight Illuminants• Illuminant A• Fluorescent Illuminants


Color Difference

• Numerical comparison of trials to the standard

• The difference in absolute color coordinates between a trial and a standard

• These differences are called Deltas

• Deltas may be positive or negative


Rectangular Coordinates L*a*b*


Polar Coordinates L*C*h


Creating a Tolerance

• Consistency is critical

• Specify exact calculation

• Avoid “fudge” factors between different color equations

• Numbers should be confirmed by visual acceptability


Problems in Setting Color Tolerances

• It is better to start with a wide tolerance and tighten it to acceptable limits

• Avoid setting a tolerance at the minimum perceptible difference

• Tolerances should be agreed upon by customer and supplier


Rectangular Tolerances


Polar Tolerances


Elliptical Tolerances


Limitations of Color Measurement Systems

• Correlation with visual response

• Interinstrument agreement


Phases of a Color Quality Program

• Design & Color Specification

• Color Matching & Formulation

• Visual & Instrumental (Hardware/Software) Analysis

• Quality Control of the Color Manufacturing Process

• Applications Procedures & Methods

• Training & Education

• Retail & Consumer Preferences


INSTRUMENTAL ANALYSIS1. Color Measurement

Spectrophotometer: GretagMacbeth Color-Eye® 7000A Sphere-Based (D8)CIE Observer: 10 degreeCIE Illuminant: D65Secondary Illuminants: Illuminant A and Cool White Fluorescent Color Scale: L*a*b* (CIELAB)Measurement Mode: Reflectance, SCE, UV included, LAV Number of Measurements: AVERAGE = 3Color Difference: CIELAB Delta E* 1.0 with no single component greater

than 60% of total

2. Gloss MeasurementGlossmeter: 60 degreeTarget Gloss and Tolerance: 45 ± 2 gloss units

Instrumental Color Evaluation Method - Sample Outline


VISUAL ANALYSIS

1. Viewing ConditionsLighting Product: GretagMacbeth SpectraLight® II with UVPrimary Light Source: D65 Filtered TungstenSecondary Light Source: Cool White Fluorescent and Illuminant AViewing Geometry: 0/45Surround: Munsell N 7/

2. Color Standards & Sample PreparationSample Size (standard and trial): 3" x 5" (desired)Sample Orientation: Side-by-side, always touchingColor Standard Storage: Filed in nonacidic envelopeColor Tolerance: Munsell® Color Tolerance Set

3. Color AnalysisColor Vision Testing: Farnsworth-Munsell 100 Hue TestColor Communication: Based on Munsell Hue Terms

fundamentals of colors (2 of 2)

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