2008 TRI/PRINCETON CONFERENCE 165 Figure 13. On very dark hair that is not perfectly combed, the visual width can be signifi cantly narrower than the width measured on the profi les. This also partly explains the lack of sensitivity observed for very dark hair. because of the chroma band (red and blond hair), measuring the width on the image does not signifi cantly change the results. EXPERIMENTAL RESULTS The optical set-up has been tested on different type of hair in order to validate the new luster formula. 8 panelists have been asked to judge the increase of luster observed be- tween an untreated and treated hair tresses put side by side. They have been asked to decide which sample had more luster. If they can decide, they check same luster. If they see a difference, they have to decide between 3 choices: O slightly more luster (less than 30% increase) O more luster (about 50% increase) O much more luster (about or more than twice as much luster) The hair tresses are 8 inch long and 3.5 g. The hair sample was treated with a silicon shine spray. MEASUREMENT ON BLACK HAIR Most of the panelist estimated the treated sample to have more or much more luster which corresponds to an increase clearly higher than 50% (Table I). The effect of the treatment can be clearly observed on the images, the treated sample be- ing darker outside the peak of the distribution (Figure 14). However this darkening is mostly in the specular light and not in the diffused light. Reich-Robbins and TRI formu- lae give respectively a 16% and 27% increase of luster which is clearly much less than the visual sensation. Bossa Nova Technologies formula gives a 116% increase of luster, which Table I Results of the Panelist Rating of Treated Versus Untreated Samples Same luster (less than 10% increase) Slightly more luster (less than 30% increase) More luster (about 50% increase) Much more luster (about or more than twice as much luster) Percentage of panelists 0% 12% 38% 50%

JOURNAL OF COSMETIC SCIENCE 166 is much more correlated to the visual change than the increase observed with Reich Rob- bins and TRI formulae. On black hair, the diffused light is negligible in front of the wings of the specular light. So in the Bossa Nova Technologies formula, D + Sout can be approximated to Sout (Equa- tion 3). In this case, the Bossa Nova Technologies luster formula becomes a spatial con- trast luster formula with the luster being the ratio of the light in the peak of the specular light over the light in the wings of the specular light. D S L S D S S S Wvisual out BNT in out in out = + ( )*Wvisual ~ * (3) Equation 3. Experimental results on dark hair. D is negligible in front of Sout. The Bossa Nova Technologies formula can be simplifi ed to a spatial contrast formula. MEASUREMENT ON RED HAIR Most of the panelist estimated the treated sample to have more luster which corresponds to an increase higher than 50% (Table II). The effect of the treatment can be clearly observed on the images (Figure 15). Shine and chroma bands are superimposed after the treatment which leads to smaller width of the distribution and higher contrast between the peak of the distribution and wings of the distribution. Reich-Robbins and TRI formulae give respectively a 63% and 34% increase of luster, which is less than the visual sensation for the TRI formula but consistent with the visual sensation for the Reich Robbins formula. Bossa Nova Technologies formula gives an 85% increase of luster which is also coherent with the visual change observed. On red hair, the diffused light is about the same level as the wings of the specular light. The Bossa Nova Technologies formula cannot be simplifi ed (Equation 4). D S LBNT S D S out in out ~ ( ) *Wvisual = + (4) Equation 4. On red hair, Sout and D are of the same order of magnitude. Figure 14. Images and profi les of untreated and treated black hair. The treatment increases the peak of the distribution and decreases the light in the wings of the distribution. The contrast of the treated sample is much higher as seen on the images.