QUANTIFYING HAIR MOTION 379 straightforward. As per previous arguments, the increased maximum amplitude of motion at low humidity might be suggested as an indication of enhanced motion. Or, conversely, the ability to generate a given amplitude of motion using lesser frequency (energy) at high humidity might similarly be noted. REPRODUCIBILITY Any testing of hair fi bers and/or hair tresses is subject to issues relating to the high vari- ability of the substrate. It is therefore necessary to run a suffi cient number of replicate samples to afford an ability to perform suitable statistical analyses. Figure 8 shows a graph containing amplitude versus frequency curves for fi ve experiments on the exact same hair tress. The hair was initially subjected to motion testing at progressively higher frequencies. The tress was then carefully removed from the instrument, remounted and run three times more in the exact same manner. The dotted line shows a best-fi tting curve through these very tight data points. However, before one fi nal replicate experiment, the hair tress was given a somewhat vigorous shake. Figure 8 also shows how a very different shaped curve arises after this shaking. Here again a parallel is drawn to the related attributes of hair body and volume where sizable changes in this property can be induced by manipulation after the treatment process. For example, static air drying of hair will produce very different volume and body char- acteristic in comparison with the same hair dried with some degree of agitation. In short, this manipulation can have an overriding impact on the desired property which exceeds Figure 7. Amplitude versus frequency curves for common hair at low and high humidity.

JOURNAL OF COSMETIC SCIENCE 380 that of any product/treatment. For this reason a meticulous tress treatment and drying process is necessary in this work. TRESS SHAPE DURING MOTION The various aforementioned treatments and conditions can also affect the shape of the hair during motion. For example, in the previously described experiment involving the infl u- ence of relative humidity (see Figure 7), hair equilibrated at high humidity appeared decidedly more “puffed up” in comparison with the low humidity state. As a further ex- ample, heat straightening greatly decreases the technical volume of the hair because of the extremely straight and highly aligned nature of the fi bers. Such measurements represent the fundamental objective of the original BOLERO instrument, although here analyses can be further performed frame by frame on videos of moving hair (see Figure 9). As described earlier, the software is able to calculate an overall tress volume— however, via the backlighting approach, this total volume can be deconvoluted into a bulk volume and a volume of wispy fl yaway fi bers. Thus, all of these parameters can be tracked during the oscillating motion. Again, there is complexity to this process. The moving hair experiences differing degrees of momentum as it traverses the oscillating pathway. For example, the fi rst image in Figure 8 (frame #0) shows hair concave to the left-hand apex of motion as it moves to the right. The reverse scenario would occur soon after frame #20 as hair returns from the Figure 8. Reproducibility in the shape of amplitude versus frequency plots.