The natural hair color characterized by the type, quantity and organization of the melanin pigments in the hair fiber can be altered by two processes: a natural one; other, chemical. The first occurs mainly due to the photodegradation of these pigments by solar radiation . The second, via oxidative degradation of these pigments (discoloration) and the insertion of new pigments, colored or not, in the fiber (coloring).
However, the color of a given material is the result of the overlapping of wavelengths in the visible light range that are reflected by the material, between 400 and 700 nm. It is dependent not only on the physical aspects involved in this reflection but also on the psychological or sensory aspects of color stored in the brain. Therefore, the equipment used for color measurement should take into account both physical aspects of color measurement and psychological aspects of color perception.
Scales created for color measurement take into account that most colors are produced by the combination of red, blue and green colors within the visible light spectrum. Among these scales, two are the most commonly used: CIELAB system and Munsell system.
The CIELAB system was created in 1931 by the Commission Internationale de l'Éclaraige (CIE) and underwent several revisions over the years. Today this system takes into account three Cartesian coordinates of color measurement: L *, lightness, light - dark; a *, green - red; and b *, yellow-blue
In Lab space, the color of an object can be represented by a point ("E" ) in this space that the coordinates are L *, a * and b *. And any color difference ("∆E" ) in this space can be calculated simply by the distance between the points "E" _"0" (standard color of the object) and "E" _"f" (color after change). That is,
Another great advantage of this system lies in the fact that the color change can be viewed in all three dimensions. For example, a negative 〖"∆L" 〗^"*" indicated that the object became darker, a positive 〖"∆a" 〗^"*" , that the object became redder and a negative 〖"∆b" 〗^"*" , that the object became more yellow.
The Munsell system is based in a solid of colors, as shown in Figure 2. The vertical axis represents the site of neutral colors and it is called value, it varies from 0 to 10 and indicates how light (10) or dark (0) is a material. The distance of the central vertical axis, known as chroma, is the function of the intensity or color saturation of the sample and varies from 0 (neutral color) to 12 (total saturation) in this scale. The color or hue, as defined in this system, is represented by different plans or around the vertical axis of the Munsell solid. The plans are divided into five main hues (red, yellow, green, blue and purple) and five intermediate hues (yellow-red, green-yellow, blue-green, purple-blue and red-purple). Each hue segment is divided in a scale from 0 to 10, in which the main and the secondary hues occupy always position 5 of the scale.
It is interesting to notice that despite two systems presenting different nomenclatures and definitions to characterize a determined hue, both are based in a spherical space and may present mathematical relations that correlate them.
Another interesting fact is that the hues presented in these two spaces are so many and so diverse that a difference of hue observed by a device may not detected by the human eye, even if they are trained.
To control this problem, the formula of hue different of the CIE from 2000 was developed. Despite what it may seem, this formula is not an attempt of creating a color space in which the limits of perception of the human eye are uniform. It defines, actually, a calculation in which the color difference approximates itself with the limits of perception of the human eye on the CIELAB5 space color.
Thus, this is the color space chosen during our analyses that have an extensive list of the attributes evaluated: