Analytical technique description:
A Colorimeter makes possible the conversion of a color in a numerical code, according to international standards, making it universally understandable. The subjective perception of a color can change depending from the context of the object and the light source that illuminates it. A colorimeter instead has a sensibility equivalent to the one of human eye, but gives accurate and reproducible measurements, when the same light source and the same illuminant system is used.
A Spectrophotometer can moreover display the graphic of spectral reflectance, in addition to give color numerical code.
In Cultural Heritage field, an objective, unique and reproducible definition of the color of a given area is very important for making measurements having different aims: as an example, monitoring possible variations of the color of a given area during time (changes can be due to treatments, or deterioration, or following interventions), or making comparison between different areas, in order to identify a pigment, or a compound, etc.
A reflection Colorimeter is able to convert the colors perceived from human eye into a numerical code (3 numbers, called tristimulus values). As an example, if CIE-L*a*b* Cartesian system is used (that is a color solid where each point is univocally defined by three spatial coordinates which correspond to a visible color), we will have a number for L* parameter, a number for a* parameter and a number for b*. L* is lightness (0 = absolute black, 100 = absolute white), a* is red-green axis value (-60 for green, +60 for red) and b* is yellow-blue axis value (-60 for blue, +60 for yellow).
A Spectrophotometer measures the light reflected by an object for each wavelength and in each wavelength wave band, displaying the data on a graphic and giving more specific information about color.
Instrumental details:
Colorimetro MINOLTA, modello CR-200 (Florence Unit, Milan Unit).
Spettrofotometro MINOLTA, modello ChromaMeter CM-700d
(Florence Unit, Milan Unit).
Via Madonna del Piano, 10 - 50019 Sesto Fiorentino (FI)
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Institute for the Conservation and Valorization of Cultural Heritage