Ilpo Niskanen

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.

Novel nanostructured PCC fillers

New filler and pigment technologies are needed to improve the optical properties of paper. Filler contents in different paper grades are approaching the maximum levels achievable with current papermaking practices. Much work has been done to maximize the light scattering potential of fillers and pigments by modifying their particle size distribution or specific surface area. The refractive index (RI) is an optical constant of pigment, and less attention has been paid to the possibility of increasing this parameter. In the present study, a novel nanostructured filler-grade precipitated calcium carbonate (PCC) pigment was synthesized. Zinc-based nanostructures, physically contacted with the host PCC material, increase the differences in RI between filler-fiber and filler-air interfaces, yielding increased light scattering. The effective RI of the novel filler was measured using a method which combines a multi-function spectrometer with the immersion liquid method. This method enables effective RI measurement from pigment suspensions, irrespective of the shape, size, and nanostructures occurring on the host pigments. When compared to conventional PCC, the results gained with the nanostructured PCCs suggest an increase in the effective RI. When used as filler in paper, nanostructured PCC yields improved light scattering, i.e., better opacity.

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 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).

Estimation of Effective Refractive Index of Birefringent Particles Using a Combination of the Immersion Liquid Method and Light Scattering

A method to detect the effective refractive index and concentration of birefringent pigments is suggested. The method is based on the utilization of the immersion liquid method and a multifunction spectrophotometer for the measurement of back scattered light. The method has applications in the measurement of the effective refractive index of pigments that are used, e.g., in the paper industry to improve the opacity of paper products.

Determination of the refractive index of microparticles by utilizing light dispersion properties of the particle and an immersion liquid

The knowledge of the refractive index of a particle is important in sensing and imaging applications, e.g., in biology, medicine and process industry. The refractive index of tiny solid particles such as microsize particles can be determined by the so-called liquid immersion technique. This study deals with three different types of interrogation methods to get the refractive index of a particle in a liquid matrix. These methods utilize thermo-optical properties and wavelength-dependent refractive index of the particle and the immersion liquids, as well as, the classical method using a set of in advance prepared set of immersion liquids with different refractive indices. The emphasis is on a method to get especially the wavelength-dependent refractive index of microparticles and exploiting different wavelength-dependences of immersion liquid and a solid particle because identification of a particle is more reliable if the refractive index of the particle is known at several wavelengths. In this study glycerol-water mixtures served as immersion liquids to obtain the refractive index of CaF2 at several discrete wavelengths in the spectral range 200-500 nm. The idea is to find the maximum value of light transmission of suspension by scanning the wavelength of a commercial spectrophotometer. The light dispersion-based method is suggested as a relatively easy, economic and fast method to determine the refractive index of a particle by a spectrophotometer at several wavelengths of light. The accuracy of the detection of the refractive index is suggested to be better than ± 0.005 refractive index units.

Fresnel reflectance in refractive index estimation of light scattering solid particles in immersion liquid

The refractive index of homogenous particle population can be determined by the so-called immersion liquid method. The idea is to find a known liquid whose refractive index matches the index of the particles. We report on a method that simultaneously obtains the refractive index of particles and that of the immersion liquid. It is based on a system using internal light reflection and Fresnel’s theory. The method includes a series of straightforward reflection measurements and a fitting procedure. The validity of the method was tested with CaF2 particles. The method has applications within scientific studies of microparticles and nanoparticles or micro-organism in suspensions. It can be also be utilized in industry for the detection of the refractive index of products involving particles for the purpose of improvement of product quality.

Optical sensing of concentration and refractive index of pigments in a suspension

We describe an immersion liquid and data analysis method for the simultaneous determination of the refractive index and concentration of pigments by measurement of light transmission of suspensions. A new innovation is that, in the event that two different pigments are simultaneously present in a suspension, it is possible to detect the refractive index of an unknown pigment with the aid of half-width of transmittance and, furthermore, to obtain the concentration of the unknown pigment.

A method for the detection of the refractive index of irregular shape solid pigments in light absorbing liquid matrix

The immersion liquid method is powerful for the measurement of the refractive index of solid particles in a liquid matrix. However, this method applies best for cases when the liquid matrix is transparent. A problem is usually how to assess the refractive index of a pigment when it is in a colored host liquid. In this article we introduce a method, and show that by combining so-called multifunction spectrophotometer, immersion liquid method and detection of light transmission and reflection we can assess the refractive index of a pigment in a colored liquid, and also the extinction or absorption coefficient of the host liquid.

On estimation of complex refractive index and colour of dry black and cyan offset inks by a multi-function spectrophotometer

The optical properties of inks have much importance in quality inspection of prints. For instance, gloss and gloss mottling of a print depend not only on the surface roughness of the print but also on the spectral properties such as the wavelength-dependent complex refractive index of the ink. We have developed a device, the so-called multi-function spectrophotometer (MFS), that allows the measurement of reflectance and thereby assessment of the complex refractive index of the dry ink. Here, we report on the complex refractive index of dry black and cyan offset inks. Such information has importance in the study of gloss of prints. Furthermore, we show that the colour of these dry inks can be obtained with our apparatus by using light scattering data from the dry ink.