Concentration Additivity of Coefficients for Elastomer Pigmented to Skin Colors

Reflectance measurements using a non-contact system of 45° illumination and 0° (normal) observation provide more accurate data for translucent natural or restorative materials, yet the concentration additivity of optical coefficients has not been evaluated for Kubelka-Munk (KM) absorption (K) and scattering (S) coefficients derived using this system.  Objective: The specific objective of this study is to determine the accuracy of concentration additivity to estimate K and S of those pigments which produce natural skin colors in maxillofacial elastomer.  Method: Samples at varying thicknesses of each of 19 shades of skin-colored maxillofacial elastomer were measured using a non-contact 45°/0° spectral reflectance system on each of black, gray and white backings.  Reflectance values at each wavelength from 380nm to 780nm in steps of 2nm for each shade were fit by non-linear regression to KM theory using an internal reflection correction appropriate for the 45°/0° reflectance measurement geometry using the value of the internal reflection coefficient for completely scattering translucent materials.  Then K and S values separately at each wavelength were fit by linear regression to the concentrations of the tan, black, red, and yellow pigments used to make these shades.  The color difference between the shade color and that predicted using concentration additivity was calculated.  Results:  Each regression was statistically significant (P<0.0001 for K at every wavelength, P<0.0001 for S similarly).  The adjusted R-squared values were r²³0.968 for K over all wavelengths, and r²³0.969 for S.  The mean color difference was 3.3.  Conclusion: Although concentration additivity may provide an adequate model to relate the concentrations of pigments required to duplicate the color of a patient's skin color, further research into the optical effects of multiple pigments in maxillofacial elastomer is warranted.

This study was supported by Student Research Program of The Ohio State University College of Dentistry.