Simulation of color perception of layered dental composites using optical properties to evaluate the benefit of esthetic layer preparation technique.

OBJECTIVES: Esthetic restorations require that dental restorative materials have similar optical properties to teeth. To improve the color perception, the inhomogeneous morphology of the native tooth can be imitated by layering two optically different restorative materials. However until now the benefit of this method has not been satisfactorily demonstrated. METHODS: The optical parameters, absorption coefficient µ(a), scattering coefficient µ(s), anisotropy factor g and effective scattering coefficient µ'(s), were determined for the enamel and dentin material of the restorative material systems Artemis(®) and Herculite XRV(®). This was carried out for each material system in the wavelength range between 400 and 700nm using integrating sphere measurements followed by inverse Monte Carlo simulations. RESULTS: Using the optical parameters and a forward Monte Carlo simulation, the color perception of layered samples could be predicted with a sufficient degree of accuracy. The total color impression was shown to be dependent on the sample thickness and the transparency/translucency of the single layers of enamel and dentin materials. CONCLUSION: The study demonstrated that the use of two materials is well-suited for the restoration of front teeth with their relatively high proportion of enamel. This study will continue further with the compilation of a data pool of optical parameters which will enable the application of calculation models to optimize the optical approximation of the natural tooth.

Influence of osmolarity on the optical properties of human erythrocytes.

Plasma osmolarity influences the volume and shape of red blood cells (RBCs). The volume change is inversely related to the hemoglobin concentration and as a consequence to the complex refractive index within the cell. These morphological changes can be linked to changes in the optical behavior of the cells. The optical parameters, absorption coefficient µa, scattering coefficient µs, and effective scattering phase function of red blood cells are investigated in dependence on osmolarity in the spectral range from 250 to 1100 nm. Integrating sphere measurements of light transmittance and reflectance in combination with inverse Monte-Carlo simulations are carried out for osmolarities from 225 to 400 mosmol/L. Osmolarity changes have a significant influence on the optical parameters, which can in part be explained by changes in the complex refractive index, cell shape, and cell volume. Spherical forms of RBCs induced by low osmolarity show reduced scattering effects compared to the normal RBC biconcave disk shape. Spinocytes, which are crenated erythrocytes induced by high osmolarity, show the highest scattering effects. Even only a 10% change in osmolarity has a drastic influence on the optical parameters, which appears to be of the same order as for 10% hematocrit and oxygen saturation changes.

Optical properties of dental restorative materials in the wavelength range 400 to 700 nm for the simulation of color perception.

Aesthetic restorations require dental restorative materials to have optical properties very similar to those of the teeth. A method is developed to this end to determine the optical parameters absorption coefficient mu(a), scattering coefficient mu(s), anisotropy factor g, and effective scattering coefficient mu(s) (') of dental restorative materials. The method includes sample preparation and measurements of transmittance and reflectance in an integrating sphere spectrometer followed by inverse Monte Carlo simulations. Using this method the intrinsic optical parameters are determined for shade B2 of the light-activated composites TPH((R)) Spectrum, Esthet-X, and the Ormocer Definite in the wavelength range 400 to 700 nm. By using the determined parameters mu(a), mu(s), and g together with an appropriate phase function, the reflectance of samples with 1-mm layer thickness and shade B2 could be predicted with a very high degree of accuracy using a forward Monte Carlo simulation. The color perception was calculated from the simulated reflectance according to the CIELAB system. We initiate the compilation of a data pool of optical parameters that in the future will enable calculation models to be used as a basis for optimization of the optical approximation of the natural tooth, and the composition of new materials and their production process.

Influence of oxygen saturation on the optical scattering properties of human red blood cells in the spectral range 250 to 2,000 nm.

The intrinsic optical parameters absorption coefficient mu(a), scattering coefficient micros, anisotropy factor g, and effective scattering coefficient micros were determined for human red blood cell (RBC) suspensions of hematocrit 33.2% dependent on the oxygen saturation (SAT O(2)) in the wavelength range 250 to 2,000 nm, including the range above 1,100 nm, about which there are no data available in the literature. Integrating sphere measurements of light transmittance and reflectance in combination with inverse Monte Carlo simulation were carried out for SAT O(2) levels of 100 and 0%. In the wavelength range up to 1,200 nm, the absorption behavior is determined by the hemoglobin absorption. The spectral range above the cells' absorption shows no dependence on SAT O(2) and approximates the absorption of water with values 20 to 30% below the respective values for water. Parameters micros and g are significantly influenced by the SAT O(2)-induced absorption changes. Above 600 nm, micros decreases continuously from values of 85 mm(-1) to values of 30 mm(-1) at 2,000 nm. The anisotropy factor shows a slight decrease with wavelengths above 600 nm. In the spectral regions of 1,450 and 1,900 nm where water has local absorption maxima, g shows a significant decrease down to 0.85, whereas micros increases.

Influence of shear rate on the optical properties of human blood in the spectral range 250 to 1100 nm.

The intrinsic optical parameters-absorption coefficient mua, scattering coefficient mus, anisotropy factor g, and effective scattering coefficient mus'--are determined for human red blood cells of hematocrit 42.1% dependent on the shear rate in the wavelength range 250 to 1100 nm. Integrating sphere measurements of light transmittance and reflectance in combination with inverse Monte-Carlo simulation are carried out for different wall shear rates between 0 and 1000 s(-1). Randomly oriented cells show maximal mua, mus, and mus' values. Cell alignment and elongation, as well as the Fahraeus effect at increasing shear rates, lead to an asymptotical decrease of these values. The anisotropy factor shows this behavior only below 600 nm, dependent on absorption; above 600 nm, g is almost independent of shear rate. The decrease of mus' is inversely correlated with the hemoglobin absorption. Compared to randomly oriented cells, aggregation reduces all parameters by a different degree, depending on the hemoglobin absorption. It is possible to evaluate the influence of collective scattering phenomena, the absorption within the cell, and the cell shape.

Optical properties of platelets and blood plasma and their influence on the optical behavior of whole blood in the visible to near infrared wavelength range.

The optical parameters absorption coefficient, scattering coefficient, and the anisotropy factor of platelets (PLTs) suspended in plasma and cell-free blood plasma are determined by measuring the diffuse reflectance, total and diffuse transmission, and subsequently by inverse Monte Carlo simulation. Furthermore, the optical behavior of PLTs and red blood cells suspended in plasma are compared with those suspended in saline solution. Cell-free plasma shows a higher scattering coefficient and anisotropy factor than expected for Rayleigh scattering by plasma proteins. The scattering coefficient of PLTs increases linearly with the PLT concentration. The existence of physiological concentrations of leukocytes has no measurable influence on the absorption and scattering properties of whole blood. In summary, red blood cells predominate over the other blood components by two to three orders of magnitude with regard to absorption and effective scattering. However, substituting saline solution for plasma leads to a significant increase in the effective scattering coefficient and therefore should be taken into consideration.

Empirical model functions to calculate hematocrit-dependent optical properties of human blood.

The absorption coefficient, scattering coefficient, and effective scattering phase function of human red blood cells (RBCs) in saline solution were determined for eight different hematocrits (Hcts) between 0.84% and 42.1% in the wavelength range of 250-1100 nm using integrating sphere measurements and inverse Monte Carlo simulation. To allow for biological variability, averaged optical parameters were determined under flow conditions for ten different human blood samples. Based on this standard blood, empirical model functions are presented for the calculation of Hct-dependent optical properties for the RBCs. Changes in the optical properties when saline solution is replaced by blood plasma as the suspension medium were also investigated.

Determination of oxygen saturation and hematocrit of flowing human blood using two different spectrally resolving sensors.

The blood parameters oxygen saturation and hematocrit were determined by two different spectral sensors using reflectance spectra from 550 to 900 nm and partial transmission spectra centered at 660 nm. The spectra were analyzed by the method of partial least squares. One sensor consists of a miniature integrating sphere, while the other was fiber-guided. The results show that the geometry of the sensors and different blood flows do not influence the spectral analysis significantly. Independent of the sensor geometry, both hematocrit and oxygen saturation could be determined with an absolute predicted root mean square error of less than 3%. Furthermore, the analysis showed that hematocrit prediction requires eight wavelength regions and oxygen saturation prediction requires four wavelength regions using reflectance spectroscopy. This implies that if the measurement is restricted to reflectance, a spectrometer is indispensable for determining both blood parameters. Hematocrit determination could be improved using reflectance measurements in combination with transmission.

Determination of optical properties of human blood in the spectral range 250 to 1100 nm using Monte Carlo simulations with hematocrit-dependent effective scattering phase functions.

The absorption coefficient mu(a), scattering coefficient mu(s), and anisotropy factor g of diluted and undiluted human blood (hematocrit 0.84 and 42.1%) are determined under flow conditions in the wavelength range 250 to 1100 nm, covering the absorption bands of hemoglobin. These values are obtained by high precision integrating sphere measurements in combination with an optimized inverse Monte Carlo simulation (IMCS). With a new algorithm, appropriate effective phase functions could be evaluated for both blood concentrations using the IMCS. The best results are obtained using the Reynolds-McCormick phase function with the variation factor alpha = 1.2 for hematocrit 0.84%, and alpha = 1.7 for hematocrit 42.1%. The obtained data are compared with the parameters given by the Mie theory. The use of IMCS in combination with selected appropriate effective phase functions make it possible to take into account the nonspherical shape of erythrocytes, the phenomenon of coupled absorption and scattering, and multiple scattering and interference phenomena. It is therefore possible for the first time to obtain reasonable results for the optical behavior of human blood, even at high hematocrit and in high hemoglobin absorption areas. Moreover, the limitations of the Mie theory describing the optical properties of blood can be shown.

Model function to calculate the refractive index of native hemoglobin in the wavelength range of 250-1100 nm dependent on concentration.

The real part of the complex refractive index of oxygenated native hemoglobin solutions dependent on concentration was determined in the wavelength range 250 to 1100 nm by Fresnel reflectance measurements. The hemoglobin solution was produced by physical hemolysis of human erythrocytes followed by ultracentrifugation and filtration. A model function is presented for calculating the refractive index of hemoglobin solutions depending on concentration in the wavelength range 250 to 1100 nm.

Determination of the complex refractive index of highly concentrated hemoglobin solutions using transmittance and reflectance measurements.

The complex refractive index of highly concentrated hemoglobin solutions as they appear in red blood cells are determined in the wavelength range of 250 to 1100 nm using transmittance and Fresnel reflectance measurements. The determined real parts of the refractive indices are on average 0.02 units higher than the values found in the literature. The wavelength dependence of the measured data in the UV region differs from the calculated data using the Kramers-Kronig relation.