# A 'Look' at Curl in Eyelashes

Eyelashes protect the eye from debris and are found in almost all primates. They also serve as sentinels of sensation, warning our eyes that a foreign object is nearby. Eyelashes also play an important role in beauty and fashion. They are often treated with mascara formulations to enhance certain properties of the lash, making it more striking to the observer. Typically, the objective of an eyelash treatment is to increase volume (thicken), enhance curvature, and darken the lashes. There have been several research accounts on the subject of eyelashes or mascara treatments, primarily focusing on the physical attributes imparted on the eyelash by the mascara. Several parameters were calculated to characterize an eyelash fiber assembly and included eyelash curvature, length, and volume or thickness.

In order to perform measurements on eyelashes, we glued commercial eyelashes on a Teflon cylinder similar to the manner employed when they are placed on a person’s eyelid.

We first take a digital photograph with a perspective similar to that of an observer when positioned in the same plane—90 degrees clockwise to the focus. As seen in Figure 1, untreated and mascara treated eyelashes result in a different geometric arrangement of the fibers. The ends of the lashes are in a higher position in the treated sample as compared to the untreated control. Likewise, mascara treatment results in an increase in the radius of curvature of the eyelashes. The %Curl of an eyelash can be measured from a digital image if we know the arc length (L) and radius (r) of the curl, as seen in Eq. 1, where r is the radius and is defined by Eq. 2.

Using Equations 1 and 2, the %Curl was calculated for two eyelash samples, similar to the ones shown in Figure 1, resulting in values of 23.09±2.22 and 32.09±1.42 for untreated and mascara treated samples, respectively. This correlates well with visual observations of the image in Figure 1. Please keep in mind, however, that treatment of the eyelash will depend on the person administering the treatment, mascara formulation, and accompanying mascara brush. In this study, treatment was administered with 10 brush strokes of the mascara, starting from the base of the lash and dragging the brush to the lash tips.

In addition to the measurements already discussed, we can also measure the alignment, gluing together, and darkening of the fibers. The alignment of eyelash fibers was measured using Fourier transform, similar to the method described earlier for African hair treated with a relaxer. Figure 2 shows two images of eyelashes, untreated and mascara treated, with a view looking down the axis of the cylinder. A section of the image was segmented (Figures 2c and 2d) and Fourier transform was performed on the new image. From the Fourier transform plots (Figures 2e and 2f) we can discern a preferential geometric arrangement for mascara treated eyelashes, which is manifested as two distinct distributions. The untreated sample, on the other hand, provides a weak and scattered distribution in frequency space.

The gluing together and darkening of fibers is also measured by taking a photograph along the axis of the mounting cylinder. A plot of luminosity versus distance is obtained by scanning the image circumferentially along the middle of the eyelash assembly (Figure 3). From the plot of luminosity versus distance we found that the number of peaks decreased as a result of mascara treatment. Specifically, the number of peaks per unit area was 10.94 peaks/mm and 5.94 peaks/mm for untreated and mascara treated eyelashes, respectively. Equally interesting, the fibers became darker as a result of the treatment. When comparing Figures 3b and 3c, the peak height is much greater in the latter case. In fact, measurements revealed an average peak height of 110.29±20.47 and 165.22±33.38 for untreated and mascara treated eyelashes, respectively. The peak height was calculated by subtracting the minimum value of a negative peak from 255, which corresponds to the highest possible luminosity value. This certainly demonstrates the ability of mascaras to emphasize eyelashes by making them darker due to the pigment in the mascara and more distinguishable by binding multiple fibers together.