Using a transmission-spectrum-based method, the refractive index of a 50 μm thick sample of poly(methyl methacrylate) (PMMA) was measured as a function of wavelength. To mitigate the effects of nonplane-parallel surfaces, the sample was measured at 16 different locations. The technique resulted in the measurement of index at several thousand independent wavelengths from 0.42 to 1.62 μm, with a relative RMS accuracy <0.5×10(-4) and absolute accuracy <2×10(-4).

Refractive index measurements of poly(methyl methacrylate) (PMMA) from 0.4-1.6 μm.

Optical measurement techniques such as particle image velocimetry (PIV) and laser Doppler velocimetry (LDV) are now routinely used in experimental fluid mechanics to investigate pure fluids or dilute suspensions. For highly concentrated particle suspensions, material turbidity has long been a substantial impediment to these techniques, which explains why they have been scarcely used so far. A renewed interest has emerged with the development of specific methods combining the use of iso-index suspensions and imaging techniques. This review paper gives a broad overview of recent advances in visualization techniques suited to concentrated particle suspensions. In particular, we show how classic methods such as PIV, LDV, particle tracking velocimetry, and laser induced fluorescence can be adapted to deal with concentrated particle suspensions.

/pdf/refractive-index-and-density-matching-in-concentrated-1fni51l18a.pdf

Refractive-index and density matching in concentrated particle suspensions: a review

The refractive index of liquid solutions at the He–Ne laser wavelength, 0.6328 μm, is presented. The measurements were carried out using the conventional minimum deviation method of an equilateral hollow glass prism. The refractive indices of sucrose, sodium chloride, glucose, and caster sugar solutions for a range density varying from distilled water to a saturated condition were measured. The result shows that at higher of concentrations a slight curvature can be seen from the plot of refractive index vs concentration of solution. However, the refractive index of sucrose shows a linear relationship with concentration. The accuracy of the measurements is estimated to be better than 0.3%.

/pdf/refractive-index-of-solutions-at-high-concentrations-1in4w98sr7.pdf

Refractive index of solutions at high concentrations

The refractive index of a material is one of the most important optical parameters. In this review article we have discussed different methods and techniques for the measurement of refractive indices of various materials. We have considered the literature of the past two decades from 1980 to 2001 and have shown how the techniques have been developed and improved for the measurement of refractive indices. Some applications of refractive index have also been discussed.

https://iopscience.iop.org/article/10.1238/Physica.Regular.065a00167/pdf

Refractive Index Measurement and its Applications

Terahertz spectroscopy on polymers: A review of morphological studies

J. M. Cariou, J. Dugas, L. Martin, and P. Michel Laboratoire de Physique des Solides, Associé au CNRS, Université Paul Sabatier, 31062 Toulouse CEDEX, France. Received 30 July 1985. 0003-6935/86/030334-03$02.00/0. © 1986 Optical Society of America. For polymers, the thermal volume expansion coefficient is always much higher than the coefficient for inorganic materials. We have recently shown that the variation of the refractive index of polymers vs temperature is essentially due to the variation of density. From the Lorentz-Lorenz relation, the function (n + 2)/(n 1) appears perfectly linear in the various temperature ranges where no phase transition occurs. The slope of the representative curve exactly follows the thermal expansion coefficient α which keeps a constant value in each temperature range. Thus, α may be easily deduced from the refractive-index measure­ ment at various temperatures. At the same time, the tem­ peratures where discontinuities of (n + 2)/(n 1 ) or its derivative appear conveniently fit with the transition tem­ peratures which may be so determined. A correct observation of such behavior is that the tempera­ ture variations are so slow that the material is continuously kept in thermal equilibrium. Indeed, it is well known that the various molecular processes which induce the transitions are characterized by very large relaxation times. If the material undergoes temperature changes too fast, it remains in a metastable state which is not a true equilibrium state. Waxier et al. have interferometrically measured some optical properties of Plexiglas and Lexan and determined the evolution of dn/dt, with the temperature between -160°C and +60°C. They obtained irregular but continu­ ous curves which do not evidence, even in a crude way, any systematic change in their behavior which may be related to a phase transition. Nevertheless, in the temperature range studied, at about -30°C PMMA (Plexiglas) undergoes the so-called β transition which is attributed to the beginning of the rotation of lateral chains of methacrylate radicals. A relevant discontinuity of the coefficient of thermal expan­ sion has been observed for PMMAs various tacticities. In the same way, for polycarbonate, the glass transition at 125°C is out of the temperature range studied by these authors. However a so-called α peak was observed at ~60°C and was widely discussed by Neki and Geil. No noticeable anomaly appears in the Waxier results. We have measured the refractive index of commercial samples of PMMA (Altuglas) and polycarbonate (Lexan) between about -100°C and +150°C. To avoid any stress which could take place by contact with another material, the measurement method chosen was the minimum deviation of a prism. The PMMA prism was drawn from bulk material

Refractive-index variations with temperature of PMMA and polycarbonate.

Transport of solar energy with optical fibres

The refractive indexes of PMMA, polystyrene, and poly-(methyl pentene), three transparent polymers having various morphologies, have been measured between about -100°C and +150°C The prism method was used in order to make the sample quite free of any contact and mechanical interaction Thermally stimulated current experiments were concurrently performed to observe and to confirm the existence of the transitions occurring in the various samples It is verified that the refractive index variations, on the one hand, perfectly follow the Lorentz-Lorenz law and, on the other hand, systematically exhibit anomalies every time a morphological change temperature is overstepped by the material So, it is shown that measurement of the thermal variations of the refractive index of polymers is another means to follow the behavior of their specific volume while avoiding the application of any stress from the experimental device

Thermal variations of refractive index of PMMA, polystyrene, and poly (4-methyl-1 -pentene)

Theoretical limits of optical fibre solar furnaces

We describe an original technique that permits a specially accurate measurement of the variations of the transmittance of optical fibers vs the incidence of launched light. For large-diameter large-aperture fibers, which prefer light power transmission, the transmittance may be modeled using geometric optics. Taking into account the reflection losses at both ends, the core attenuation and a very weak lack of reflectivity at the core–cladding interface, good agreement is obtained between calculated variations and experimental results for the three kinds of fiber tested. The remaining difference has to be attributed to the mode coupling process which may be thus evaluated. The too high attenuation coefficient, which is attributed to the cladding to account for the losses at the core–cladding interface, characterizes in fact a thin perturbed layer where the materials contact. Thus, this method is a good means to evaluate separately the various processes contributing to the attenuation of open beams along the fibers and to determine which process should be worked on to improve the transmittance.

Louis Martin

Papers

Refractive-index variations with temperature of PMMA and polycarbonate.

Transport of solar energy with optical fibres

Thermal variations of refractive index of PMMA, polystyrene, and poly (4-methyl-1 -pentene)

Theoretical limits of optical fibre solar furnaces

Accurate characterization of the transmittivity of large-diameter multimode optical fibers