DCC Sparkle and Graininess Tolerancing Study. Current work on Color Difference Equations. DCC Color Education Course

Transcription

1 DCC Sparkle and Graininess Tolerancing Study presented at BYK-Gardner User Meeting, October 26, 2016 Larry Steenhoek, Jeff Alspach, Allan Rodrigues Current work on Color Difference Equations DCC Color Education Course

2 DCC Sparkle & Graininess Study Presentation Outline DCC Committee Purpose and Project Background Experimental and Data Analysis Procedures Sparkle Study and Results Graininess Study and Results Findings and Future Work Acknowledgements 2

3 Committee Purpose and Membership Purpose: Upgrade SAEJ1545 with the goal of standardizing automotive industry practices for establishing color and flake appearance measurement and methodology for setting meaningful tolerances. Membership: Chairman: Jeff Alspach, Axalta, Mt. Clemens, MI Car manufacturers: Audi, Ford, GM(need others) Coatings Suppliers: AkzoNobel, Axalta, BASF, PPG Instrument Manufacturers: BYK-Gardner, X-Rite, GTI Includes members from ASTM, CIE, DIN March 2013 Sub-committee Meeting Attendees 3

4 Project Background The BYK-mac multi-angle spectrophotometer is applied in measuring the color and flake appearance differences of automotive paints. Overall purpose: Determine tolerance threshold for sparkle difference Determine tolerance threshold for graininess difference Determine inter-company differences: Do observers from a variety of companies agree with initial observations taken at Axalta? Through industry participation, educate and raise awareness of spatial texture. Process: Estimate sparkle and graininess difference tolerances for several colors by visual assessment, analyzing with Binary Logistic Regression. 4

5 BYK-mac Geometry Courtesy of BYK-Gardner, GmbH 5

6 Basic Equations and Terms Used DE94 color differences with the gonioapparent S L Lightness correction: S L = 0.034L*; If L* 29.4., S L = 1.0 Sparkle difference ds and graininess difference dg as provided by the BYK-mac. Definitions (ASTM E284): Sparkle, n- The aspect of the appearance of a material that seems to emit or reveal tiny bright points of light that are strikingly brighter than their immediate surround and are made more apparent when a minimum of one of the contributors (observer, specimen, light source) is moved. DISCUSSION Sparkle is perceived under intense directional illumination only. Graininess, n- for gonioapparent coatings, the perceived contrast of the light/dark irregular pattern exhibited when viewed under diffuse illumination, scale typically >100 micrometers. DISCUSSION Also referred to as diffuse coarseness. 6

7 Experimental Procedures The panels are observed in the BYKO-Spectra Effect lightbooth. The viewing window on the lightbooth is shortened from 20 in. width to 5 in. width (helps focus on the center of the panels) 7

8 Experimental Procedures At least 30 observers were asked to make visual assessments of sparkle or graininess difference acceptability. All of the observers had experience in judging automotive color and appearance acceptability. (Note: 30 observers were from Axalta, additional observers from several companies, hosted by Thierry). Panel pairs were randomly presented. Panels supporting board set at 45 degree, allowing viewing perpendicular to the panels with the sparkle light at 15, 45 or 75 degrees. The light intensity is adjusted to 7 (1 to 10 scale). 8

9 Experimental Procedures Following is an example of the process used in selecting sample pairs for sparkle visual assessments: Used pairs of panels with known de94 and ds measured by BYK-mac at the appropriate angle Remaining panels Remaining panels de94 < 1 Remaining pairs used ds < 1.3 (assumes 1.3 close to 100% rejection) Choose pairs evenly spaced for: 0.30< ds <1.30, de94 < 0.85 (Added ds pairs close to expected high and low rejection) 9

10 Experimental Procedures Observers were asked: Ignoring any color differences, would you accept this sparkle (or graininess) difference between two adjacent car parts on your car? Accept, YES Reject, NO 10

11 Observer Data Smoothing This process compensates for subjective variation in each observer s assessment ds Original Response Add 0 0 Transform Sum of 3 Integer Divide by 2 Smoothed Response 1.48 Yes Yes 1.57 Yes Yes 1.60 No Yes 1.75 Yes No 1.84 No No Add 1 1 Berns et al, Visual Determination of Suprathreshold Color-Difference Tolerances Using Probit Analysis. Color Res Appl 1991; 16:

12 Binary Logistic Regression - Basics Goodness of Fit Tests Test DF Chi-Square P-Value Deviance Pearson Hosmer-Lemeshow Threshold Value at 50% Acceptance T 50 =177 Odds Ratios for Continuous Predictors Unit of Change Odds Ratio Sg

13 Typical Visual Assessment Results Std ID Bch ID de94 15 OK Rej Rej Freq 15 ds T 50 =1.15 Test DF Chi-Square P-Value Deviance Pearson Hosmer-Lemeshow Unit of Change Odds Ratio Sg A 50% frequency of rejection is the recommended tolerance 13

14 Binary Logistic Regression Analysis T 50 =1.43 Test DF Chi-Square P-Value Deviance Pearson Hosmer-Lemeshow Unit of Change Odds Ratio Sg

15 Summary of Sparkle Results Color Center Appearance Parameter T 50 L* C* Black ds 15 (Axalta) ds 15 (Thierry) ds 15 (Combined) ds 45 (Combined) ds 75 (Axalta) ds 75 (Thierry) ds 75 (Combined) Bright Yellow ds 15 (Axalta) ds 15 (Thierry) ds 15 (Combined) Cyber Gray ds 45 (Axalta) ds 75 (Axalta) Blue #9 ds 15 (Axalta) Threshold T 50 3D Scatterplot of T 50 vs L* vs C* 3D Scatterplot of Threshold vs L* vs C* L* C*

16 Graininess Study Graininess: Visual texture observed under cloudy day with diffused illumination High Graininess Low Graininess Images Courtesy of BYK-Gardner, GmbH 16

17 Graininess Study BYKO-Spectra Effect lightbooth: Color evaluation source at 45. Light diffusion was enhanced with frosted plastic over light source T 50 = 1.31 T 50 =

18 Summary of Studies To Date Participants Color Center ds15 ds45 ds75 dg NO = Color difference between available panels too large to accurately judge ds or dg? = May be possible, no observations to-date Number of Observers Black Cyber Gray NO NO Brt Yellow* NO NO NO Blue #9??? 28 Blue #14??? 30 Company * Bright Yellow has too much scattering pigment to observe graininess or sparkle at other angles Number of Observers Axalta 30+3 Byk 3 Konica Minolta 1 Toyal 1 GM 2 BASF 1 Celanese 1 Keystone Aniline 1 Abedmego Environmental 2 Bosch 1 Independent Consultant 1 Total 47 18

19 Findings Acceptability tolerance for sparkle differences were determined for 4 colors at 15, 45, 75 degree viewing. Observations by different groups of observers (different companies, OEMs, suppliers, etc.) were all within the predicted confidence intervals. Some colors did not exhibit sparkle at all angles, e.g., light scatter within Bright Yellow suppresses sparkle at viewing angles greater than 15 degrees. Some sparkle was visible when assessing graininess with the current BYK viewer, resulting in observer confusion. An additional diffuser for suppressing the sparkle worked well for graininess difference observations in this study. Sparkle and graininess differences are difficult to assess when obvious color differences are also observed. We limited color differences between panel pairs to DE94 less than 1.0. It is very difficult to create panel sets meeting these constraints. 19

20 Findings (Cont.) Due to the fact visual color harmony audits are done under a variety of lighting conditions, the differences observed in our study (under controlled conditions in a light booth) might be masked or magnified in an audit. Achieving a good correlation of visual and instrumental differences for color and spatial appearance differences can help deliver reasonable tolerances. Over controlling instrumental assessment and under controlling visual assessments or vice versa will hinder implementation of instrumental control. Sparkle and graininess acceptability varied by color and angle of view/measurement. Therefore tolerances need to be set accordingly. 20

21 Future Work Additional visual assessments for additional sparkle angles and other metallic colors to estimate the sparkle and graininess difference tolerances. Explore generalized tolerances, e.g., possible relationship to color, lightness, hue, chroma, contrast, absorption vs. scattering pigmentation, etc. Observers often found it difficult to ignore color difference when judging sparkle or graininess differences, suggesting that assessments of color/sparkle/graininess differences are not independent of these individual components. Need industry agreement to standardize assessment when these differences are observed in combination. 21

22 Overall Progress and needs on Specification of Gonioapparent Color & Appearance Spatial Appearance Definitions in ASTM E284 (Standard Terminology of Appearance)? ASTM standard on visual assessment of spatial appearance? ASTM standard on measurement of spatial appearance? Revision of SAE J1545 (Standard for instrumental color difference measurement for exterior finishes, textiles, and colored trim) and J361 (Visual evaluation of interior and exterior automotive trim) to address sparkle and graininess Note: Standards organizations are reluctant to standardize around instruments supplied by only one manufacturer. Can spatial appearance assessment be addressed in a generic fashion? 22

23 Acknowledgements BYK-Gardner for their support and providing the BYKO-Spectra Effect lightbooth for the visual assessments Members of the DCC Committee DCC for their sponsorship Our colleagues in Axalta Coating Systems for their inputs and participation in visual assessments Thierry Corporation for hosting observers in collecting additional data. Participants from automotive companies and suppliers in on-going experiments 23

24 Any Questions?

25 Current Work on Color Difference Equations

26 Uniform Color Scales CIELAB or CIE76 C * a * 2 b 2 * h tan 1 ( b*/ a*) DH 2 2 * DE * DL* DC * 2 h = hue angle DH = metric hue difference This system is more consistent with how we perceive color DE DL 2 2 * Da * Db * 2 DE DL 2 2 * DC * DH * 2 26

27 CIE94 and CMC Equations CIEDE2000 General Form of the Color Difference Equation DE DL * K L S L 2 DC * ab K C S C 2 * DH ab K H S H CIE94 Equation CR&A, 18, (1993) For solid colors: S L = 1.0 For metallics: (not in CIE94 recommendation) S L = 0.034L*; If L* <= 29.4, SL = 1.0 S C = C* ab where C* ab is that of the standard The CMC Equations For L>16, S L L* L* For L<=16, S L = C * S ab C C * ab S H = (FT + 1 F) S C DE DL* K LSL 2 b* DC * KCS ab C DH * K H S ab H 2 R T DC * KCS ab C DH * K H S ab H S H = C* ab The parametric factors KL:KC:KH = 1:1:1 are generally satisfactory in CIE94 CMC typically uses 2:1:1 where (C * ) F ab * 4 (C ) 1900 ab T = abs[0.4 cos(35 + h ab )] unless h is between 164 and 345 then T = abs[0.2 cos(168 + h ab )] SL a* L* 50 L*

28 Lightness Weighting Factor S L CIE 2000 CMC(1:1) CMC(2:1) E S L L* 28

29 AUDI 2000 Where f i are defined separately for L, C, H for solid or gonioapparent colors Suggested Next Step de AUDI 2000 CIE de 2000 dde combined Lex, K. & Kiggle-Bockler, Combined Color difference Method fo Multi-Angle Application BYK-Gardner User Meeting,

30 DCC Education Activity

31 DCC Course: Matching & Control of Metallic and Pearl Colors Presentations by experts from different coatings companies and instrument manufacturers Hand-on exercises reinforcing concepts, and use of spectrophotometers Visual Match Evaluation and Colorimetry (Allan Rodrigues) - Components of seeing color that lead to colorimetric equations - Consistent color match terminology - Multi-angle viewing of gonioapparent materials - Visual exercises to reinforce understanding these concepts. Effect Pigments for Automotive Topcoats (Claudia Royal, Axalta Coating Systems) - Characteristics of effect pigments leading to their use - Types of effect pigments: aluminum, mica, synthetics, multi-layer, glass, non-flake (micronized TiO 2, Graphite) - Guidelines for durable and reproducible color formulation - Visual examples of these flakes in finishes 31

32 DCC Course (contd.) Impact of the Application Process on Color in Metallic/Pearl Coatings (Chris Fowler, BASF) - Appearance properties we are looking for (gloss, DOI, no mottle, sag, craters, etc) - Factors affecting appearance (surface roughness, coatings formulation, rheology, application) - Application sequences (spray, flash, bake, etc), booth environment, equipment types - Spray processes (conventional, HVLP, rotational, variables, limitations of each process) Colorimetry Advances and Measurement of Gonioapparent Colors (Allan Rodrigues) - Improvements on CIELAB - Multiple angle viewing and colorimetry - Flake characterization (sparkle, graininess) and measurement - Setting tolerances General Discussion (Student inputs, standards committees, DCC activities) 32

33 Other Color Courses Eastern Michigan University Short Course (Kendall Scott) Instrument manufacturers provide courses Universities provide degrees in color science as well as short 1-week courses (e.g., RIT) Inputs on courses or 1-day workshops DCC should provide - Paint vs. plastics color - Exclusively on rheology and application - Textiles leather, thread, carpet, fabric - How to match Bumper/Body/Parts. - Application based class, WBBC vs. SBBC - Matching European colors to North American colors - Your inputs??? 33

34 Thank you Any Questions?

35 CIE DE2000 DE 00 2 DL' k L S L DC' k C S C 2 2 DH' k H S H DC' R T DH' k C S C k H S H 1 2 L = L* a = a*(1+g) b = b* G * C 7 ab * C 7 ab 25 7 S L 1 2 S 2 C C ' L ' L ' 50 S H C ' T T cosh ' cos 2h ' 0.32cos 3h ' cos 4h ' R T sin 2DR C D 30exp h ' 275 R C 2 C ' 7 C '

36 AUDI

37 Audi2000 for C* < = 10 37

38 Combined Color and Appearance Equations CIECAM02 was developed for color imaging systems. Some use it as a color difference formula It is the color appearance model by CIE TC8-01 (Color Appearance Modelling for Color Management Systems) The model consists of: - A chromatic adaptation transform, CIECAT02 and - Equations for technically defined dimensions of color appearance: brightness (luminance), lightness, colorfulness, chroma, saturation, and hue. CIECAM02 inputs are the tristimulus values of the stimulus, the tristimulus values of an adapting white point, adapting background, and surround luminance information, and whether or not observers are discounting the illuminant. Reference: Fairchild MD, Color Appearance Models: CIECAM02 and Beyond, IS&T/SID 12 th Color Imaging Conference, Tutorial T1A, 11/9/04. 38