Thickness of Compressed Hair Layer: A Pilot Study in a Manikin

Recent advancements in 3D anthropometry enable to link a subject’s 1D measurements to its 3D shape to achieve better fit, functionality, comfort and/or safety. Statistical shape models of the human head retrieved from medical images (CT or MRI) allow for such parameterized models, with a recently proven accuracy of 1.6 mm with only four scalp parameters. This induces opportunities for better sizing systems and mass customization of head mounted products. Due to the nature of medical images, hair is never present in these models. Head shapes constructed from conventional optical 3D scans however do capture the presence of hair. In current optical 3D scans, subjects wear a swimming cap, hairnet or wig cap to flatten and compress the hair layer and prevent artifacts, interferences and misinterpretations during scanning due to the presence of hair. Thus models based on medical images provide parameterized scalp models with proven accuracy. This might be required for the design of certain head mounted wearables were sensors/actuators should make proper contact with the scalp through the hair. However, many other products to be designed for closely fitting the human head will rest on a (flattened) layer of hair. Thus in these situations it might be required to take account of the flattened and compressed hair layer in the design process. Up till now, knowledge of the compressed hair layer geometry is rarely needed in the field of industrial design with only anecdotic numbers available. The uprising of accurate parameter driven 3D models of the human head and entailed applications will induce the need for further accurate quantification thereof. Firstly, we present two a method to assess the thickness of the flattened and compressed hair layer by capturing hair thickness at a given point with a linear dial gauge. Secondly, we present another method to quantify the hair layer from points measured on scalp and hair layer. From this geometric information one can also deduce whether and to what extend the parametric scalp model approximates the head model with hair layer with the same accuracy as it approximates the scalp. Effect of hair thickness was evaluated by measuring a manikin with and without wigs and caps that flattened and compressed the wigs. All experiments were thus conducted in vitro, but both methods can be transferred without burdens to in vivo experiments. A hair thickness between 1.3 ± 0.4 mm was observed with the first method and 1.5 ± 1.3 mm with the second method. A small number of measurements indicate that the head form with flattened and compressed hair is predicted by the parametric scalp model with the same accuracy as it predicts the scalp form. These pilot results indicated that for the design of head mounted products that rest on the flattened hair layer, a margin of at least 1.5 mm should be taken into account for eventual variations in hair thickness. For the design of (personalized) head mounted products, already established parameterized scalp models can be used without loss of accuracy towards the presence of a flattened hair layer. Further large scale and in vivo studies are required to confirm and fine-tune these results.