It should be noted that the area under the curve obtained by RS includes the red reflectance, thus, it is does not solely represent skin pigmentation. Skin pigmentation or melanin provides photoprotection and can be calculated as melanin index, MI. (see following section on Melanin Index) Even as early as 1961, the potential for confounding of the MI obtained by RS by dilatation of superficial blood vessels was recognized. The device, Mexameter MX 18, internally corrects for the red reflectance in calculating the melanin index. The melanin index is derived from the multilayered skin model and Lambert-Beer's law.