j, Cosmet. Sd., 55, 539-551 (November/December 2004) The smoothness of shaven beards: The relation between stubble length distribution and skin parameters and tactile smoothness JOYCE WESTERINK, FRANK VAN DEN BERG, and JOHAN VAN EEMEREN, Philips Research Laboratories Eindhoven, Prof Holstlaan 4, 5656 AA Eindhoven, The Netherlands. Accepted for publication Aaagmt 26, 2004. Synopsis In this report we describe how the tactile smoothness of shaven beards depends on the distribution of the stubble lengths remaining after shaving and on various skin parameters like coarseness and scaliness. To this end, we set up two experiments on perceived smoothness with blinded human judges. In the first experi- ment we used artificial beards with various stubble length distributions and stubble densities as stimuli. In the second experiment we presented freshly shaven human beards, for which a wide range of objective skin and stubble parameters had been measured, as stimuli. The results of the two experiments are mostly in line with each other. We can conclude that there are individual differences between judges in the aspects they pay attention to when judging tactile smoothness. Nevertheless, a general line is discernable, and we can identify three parameters that influence the impres- sion of tactile smoothness: the orientation of the stubble, the density of the stubble, and the length of the hairs in the stubble length distribution. We have also established the relative importance of the latter two effects. INTRODUCTION In the judging of shaving systems for the removal of male beards, what ultimately counts for the consumer is the question: "Does it feel smooth?" To quantify this, measurement systems have been developed that assess the distribution of hair lengths remaining after shaving, and usually the average hair length is taken as a measure of a device's closeness (20). It is unclear, however, what its exact relation is with perceived smoothness as experienced by the users. Is it the only factor that contributes to the smoothness perception? In principle, there is the possibility that there are other factors of importance to perceived smoothness--for instance, certain skin characteristics. Few articles in the literature address the topic of the smoothness of shaven beards: Breuer eta/. (1) describe how they measured group-averaged judgments of shaving closeness for various types of systems. Their concept of closeness is directly related to what we call smoothness, but where "closeness" refers to a quality of a shaving system, "smoothness" 539

540 JOURNAL OF COSMETIC SCIENCE refers to the way in which skin and stubble are perceived. As expected, they find that the perceived closeness of a shaven beard gradually decreases over time. One of the ways in which they define the perceived closeness of a system is as the point in time after shaving at which 50% of the population judges the beard in question (still) to be smooth. They also indicate that they experienced great difficulties in obtaining reliable data from individual judges. Other relevant information in the literature focuses on tactile sense in general and the perception of roughness of various materials (metal grooves, sandpaper). Krantz (2) finds that smoothness is an important factor in object recognition, and Lederman and Klatzky (3) find a 1-to-1 relation between sliding movements of the fingers and texture percep- tion. Stevens and Harris (4) confirm with perceptual experiments that smoothness is interpreted as the opposite of roughness, and this roughness in turn is found to depend on various factors: fingertip force (5), skin temperature (6), skin hydration (7), and on more than one type of receptor (8). Green (9) finds that both shear and frictional forces contribute to the roughness perception and that their relative impact depends on the situation. Louw eta/. (10) find that the thresholds of perception of single Gaussian bulges depend on the width of the bulge, with a minimum height threshold of about 1 pm. However, for general textures (such as a shaven beard), adequate threshold- predicting models are not yet available. Another area of smoothness-related literature comes from the study of friction in cos- metics. Specifically, many standardized testing procedures for "skin feel" are described [see, e.g., Wiechers and Wortel (11), who present relations between some 35 attributes related to appearance, feel, and sensation]. Smith (12) established that "slipperiness" (skin friction) can contribute to perceived smoothness, whereas Koudine eta/. (13) hypothesize that it is the variatio, in frictional forces that determines smoothness per- ception. Summarizing the literature, we can learn that perceptual smoothness is well established in the fields of tactile research and cosmetics, but less so (as far as we know) in relation to shaving. It portrays tactile (skin) smoothness as an important perception in everyday life, in relation to which very small details can be detected and frictional forces play an important role. Based on the literature, we identified all potential factors of influence on the tactile smoothness of shaven beards and expect that stubble length and stubble density are of importance, as well as skin coarseness and friction. Our intention was to investigate which of these parameters are important for the tactile perception of smoothness. This has led to two experiments, one on artificial beards, investigating how perceived smoothness depends on stubble length and stubble den- sity, and one on human beards, investigating how perceived smoothness depends on stubble density, stubble length, skin coarseness, and friction. They will be described sequentially. EXPERIMENT 1: THE TACTILE SMOOTHNESS OF ARTIFICIAL BEARDS EXPERIMENT SETUP As a first step, we used artificial beards to investigate the relation between stubble length, stubble density, and perceived smoothness, as it is perceived by normal, un- trained people. Our aim was to answer two questions in particular: