Jukka Räty

Complex refractive index of turbid liquids

We present a multifunction spectrophotometer for the measurement of light reflection from thick and transmission from very thin turbid liquid samples. Moreover, we present a method to get the complex refractive index of such turbid liquids. As an example of the high performance of the device and the method, we present data on inks that are used in offset printing.

Optical characterization of bisphenol-A-glycidyldimethacrylate–triethyleneglycoldimethacrylate (BisGMA/TEGDMA) monomers and copolymer

OBJECTIVE The fundamental optical characteristics of dental polymers as a function of curing time are exiguously reported in the literature. In this work the refractive index [1] and the extinction coefficient [2] of bisphenol-A-glycidyldimethacrylate-triethyleneglycoldimethacrylate (BisGMA/TEGDMA)-based polymers of different volume fractions have been studied as a function of the curing time. A comparison between the measured index of refraction and theoretical calculation based on molar refraction is made. Also the temporal behavior of both the refractive index and extinction coefficient is compared. METHODS Index of refraction: BisGMA/TEGDMA monomer samples were cured with a commercial light-curing unit by using camphorquinone and DMAEMA as the photoinitiator system. Curing times of 0, 40, 60, 300, 900 and 1800 s were used. The refractive indices were determined with a refractometer at 470 nm wavelength, where the peak absorption of the camphorquinone photoinitiator is located. Extinction coefficient: BisGMA/TEGDMA samples having different lengths were cured and the relative attenuation of light at 470 nm was measured as a function of the curing time (0, 40, 60, 300, 900, 1800 s). RESULTS The results show that in general the index of refraction increases during the photopolymerization process in a time frame from 0 to 40s and then approaches a steady-state value, which depends on the original composition of the admixture. The detailed refractive index change depends on the volume fractions of the constituent substances. The extinction coefficient decreases in a time frame from 60 to 900 s and reaches a steady-state value thereafter. SIGNIFICANCE Knowledge of the fundamental optical characteristics of commonly used dental polymers is essential in the development of advanced light-initiated curing methods.

The complex refractive index measurement of liquids by a novel reflectometer apparatus for the UV - visible spectral range

A novel reflectometer has been developed for the determination of the complex refractive index of liquids. In this study light reflected from the prism - sample interface has been analysed using the Fresnel equations to obtain values of the refractive index and extinction coefficient for several non-turbid liquids. The instrument allows one to control the angle of incidence and the wavelength so that other methods such as Kramers - Kronig analysis or the attenuated total reflectance technique can also be used. The wavelength can be varied in the range 250 - 650 nm. Tests with a diversity of liquids (water, ethanol, lignin solution, and so on) proved the validity of the device. The reflectometer is expected to have applications in industrial analysis of liquids.

Reflectance Study of Milk in the UV-Visible Range

Optical properties of milk were studied with the use of a recently developed reflectometer. The purpose of the study was, first, to test the applicability of the reflectometer in the estimation of real refractive index and scattering in turbid liquids and, second, to investigate the spectral properties of one particular turbid liquid, namely, milk. An expression for the reflectance of milk was constructed which takes into account the light scattering occurring in milk. With the application of a minimizing procedure to the reflectance data, it was possible to simultaneously obtain both the real refractive index and a scattering parameter for milk. The scattering parameter was observed to be linearly dependent on the milk fat concentration.

A multifunction spectrophotometer for measurement of optical properties of transparent and turbid liquids

A multifunction spectrophotometer was developed for the determination of optical properties of transparent and turbid liquids in the UV–visible spectral range. The geometry used in the device makes it possible to guide the light beam into the sample from desired directions, and enables controlled observation of transmission, reflection and light scattering from liquid samples. From the measured data, it is possible to calculate different variables of the samples, such as absorbance, complex refractive index, turbidity and rotation of linearly polarized light due to optical activity of the liquid. The operation of the spectrophotometer was tested using different solutions such as sugar diluted in water and lignin in sodium hydroxide and turbidity standards. This device is suggested for laboratory and industrial inspection of a variety of liquids.

Measurement of refractive index of liquids using s- and p-polarized light

A recently developed reflectometer was used for the determination of the wavelength-dependent refractive index of liquids. Reflectance from the prism-liquid interface was analysed using the formulae derived from the relation between Fresnels amplitude reflection coefficients r s and r p . Two sets of test liquids were prepared and the real refractive indices of the solutions were determined at 590 nm. The consistency of the results was experimentally tested using a commercial Abbe refractometer

On measurement of complex refractive index of liquids by diffractive element-based sensor

A sensor for measurement of the complex refractive index of absorbing liquids was developed. The sensor exploits diffractive element and machine vision.

Resolving Optical Constants from Reflectance of Liquids in the UV-Visible Range

Reflection spectra of liquids were measured in the UV-visible spectral range with the use of a recently developed reflectometer. The complex refractive index of two solutions, lignin and red food color, both diluted in water, were investigated. For the first time a maximum entropy model was applied in recovering complex reflectivities from non-normal incidence reflectance spectra of liquids. The optical constants in the UV-visible range were then resolved by Fresnels equations.

Measuring solvent barrier properties of paper

New methods for measuring barrier properties against solvents, acids and bases on dispersion coated paper were developed and investigated. Usability, reliability and repeatability were compared both between the new methods and with the standardized method for measuring barrier properties against water vapor. Barrier properties could be measured with all methods and the results obtained by the different methods were in correlation with each other. A qualitative method based on a trace color provided an indicative result, whereas further developed methods also took into account the durability. The effective barrier lifetime could be measured by measuring the conductivity through the substrate as a function of time, or by utilizing a glass prism where the change in refractive index caused by penetrated liquid was monitored, also as a function of time. Barrier properties against water and humidity were also measured and were found not to be predictors for barrier properties against either solvents, or acids or bases, which supports the need to develop new methods.

Measurement of refractive index of isotropic particles by incorporating a multifunction spectrophotometer and immersion liquid method

We propose a sequence of transmission and reflection measurements that makes it possible to get a relatively accurate estimate for refractive index of optically isotropic powder materials. This method makes use of immersion liquids and a multifunction spectrophotometer, which we have constructed recently. As an example we present data for CaF(2).