Considerations of a Consistent Model for the Reflection of Light from Human Hair

Transcription

1 HairS 13 Luebeck, 4 6 Sep 2013 Considerations of a Consistent Model for the Reflection of Light from Human Hair FJ Wortmann, G Wortmann School of Materials, University of Manchester, UK A Schulze zur Wiesche Fiantec GmbH, Hamburg, GER H-M Haake BASF Personal Care and Nutrition GmbH, Duesseldorf, GER 1 Hair Shine & Hair Cosmetics For the consumer the quality of hair shine is an important indicator of hair health. A popular hypothesis: Hair shine (lustre, gloss) is a very complex, largely subjective phenomenon, which is not realistically accessible to measurement. Pioneering work to measure hair shine has been undertaken by Reich & Robbins, using bent hair tresses in a shine box and an empirical correlation between perception and test results. C Reich & CR Robbins. JSCC 44, 221 (1993) Numerous attempts have been undertaken to refine the underlying principles of this approach. PD Kaplan et al. JCS 60, 111 (2009) 2 1

2 Hair Shine & Computer Graphics The successful implementation of shine for hair collectives on animals or humans in dynamic situations of body and light movements is an important condition for the realistic, natural appearance of a film character. One approach: Hair shine (lustre, gloss) is such a complex phenomenon, that the best option for its modelling is empirical, reverse engineering. wallpaper-sid-ice-age.html K Ward et al. IEEE Trans. Visualization and Comp.Graph.. 13, 213 (2007) 3 The Fundamental CG Approach: The Marschner Model A quite simple,two component model for light reflection specular & backside reflection already leads to a realistic appearance of hair, namely, when including hair cross sectionalgeometry. R Marschner et al. ACM Trans.Graphics 22, 780 (2003) Role of fibre geometry 4 2

3 Light reflection analysis by laser goniometry Progressive model developments (1) S? I DD B B S (1) On the basis of goniophotometric curves for hair of different colours, 3 components of light reflection were considered specular reflection - S backside reflection - B directional diffuse DD no uniform diffuse component FJ Wortmann, E Schulze zur Wiesche, A Bierbaum. J Cosm. Sci 54, 301 (2003) FJ Wortmann, E Schulze zur Wiesche, B Bourceau. J Cosmet Sci 55, 81 (2004) 5 Franz. J.Wortmann Light reflection analysis by laser goniometry Progressive model developments (2) Investigations have shown that light reflection in hair can show rather complex patterns, which requires to summarise distinct reflection modes into specific groups. B I S D (2) Current approach inclusion of 4 components specular reflection B-group of reflections I-group of reflections ideal diffuse reflection 6 Franz. J.Wortmann 3

4 Material for Experiments Asian, commercial hair 1-7x 30 standard, persulphate bleach Colour Changes of Asian Hair Tresses Lightness & red (+)/green ( ) L 40 a 35 b No of Bleaches Yellow (+)/blue ( ) CIE-Lab colour sphere Lightness (L+) increases with No of bleaches towards a limiting value of Redness (a+) increases & then decreases towards neutral. Yellowness (b+) increases & then decreases towards an equilibrium value of about 20. Changes are associated with the responses of the two types of melanin to the bleach. 8 4

5 Goniophotometric Curves for Asian Hair Virgin Asian Hair Simultaneous detection through 100 o Angle of incidence 40 o Hardware: Fiantec GmbH, Hamburg, GER Software: Obscure Systems UG, Aachen, GER Asian Hair, 3x bleach 9 Ternary Data Plot: Data Variability Variability of light reflection is very high between Samples Individual hairs Different points on single hairs Data groups drift towards higher I-, B- and D-values with bleaching, with increasing numbers of S=0 cases. D 10 5

6 Specular Reflection Despite low precision of the data, specular reflection shows a high degree of accuracy. With the number of bleaches a continuous drop is observed, which follows apparent 1 st order kinetics As is plausible, the fraction of specular reflection drops, when lightness increases with bleaching and interior and backside reflection become more pronounced. The logarithmic fit to the data roughly predicts a limiting value of about 60% for L=0. This is in good agreement with observations for gold-coated hair. Specular Refl., % Lightness 11 Internal & Backside Reflections Changes of internal reflection (I) and backside reflection (B) can be associated with the way reflection planes in hair interact with light. 12 6

7 Ideal Diffuse Reflection Ideal diffuse reflection goes through a maximum at around 4 bleaches. This is attributed to the increased visibility of the medulla and decreasing surface roughness through bleaching Strongly bleached hair 13 Overview of Light Reflections: Some Considerations For virgin, black (L=20) hair, there is a reasonable balance between surface and internal reflection. Effects of diffuse reflection, associated with dullness are small. For 1x bleach (L=30) the turn-over points for S as well as for (I+B) signify the change from primary, white, specular to secondary, coloured, silk gloss Beyond the turn-over point, the gloss -types approach their limiting values. For 3x bleach a maximum in D occurs, which signifies a maximum in hair dullness. 14 7

8 Conclusions The analysis of the goniophotometric reflection curve of small areas on single hairs, using a single wavelength, provides a large body of data, which gives insight into the complex mechanisms of light reflection, which can be related to hair gloss. Goniophotometric curves can be well analysed on the basis of 4 components of reflection. A distinct correlation is observed between decreasing specular reflection and increasing lightness of hair. The overview of the various types of light reflection identifies the range, where silk gloss takes precedence over specular reflection. A maximum for ideal diffuse reflection signifies a maximum of dullness, which is detrimental to hair shine. 15 Thank you for your attention! 16 8