Barbara K. Pierscionek

Refractive index gradient of human lenses

We report nondestructive measurements and the modeling of the refractive index profiles in human lenses. Results in the equatorial and sagittal planes are compared with destructive measurements of refractive index, modeled data as well as with microdensitometric measurements of protein concentration. These comparisons highlight the differences between current models and measured data on human lenses.

Age-related response of human lenses to stretching forces.

Five human lenses of varying ages were subjected to radial stretching forces which mimic the action of the ciliary muscle in vivo. Although the number of lenses was small, it was found that the human lens becomes more resistant, with age, to radial stretching forces and that lenses over the age of 50 years showed very little response to stretch. In prepresbyopic lenses, stretching has a greater effect on the shape of the anterior surface than it does on the shape of the posterior surface. The five lenses studied showed individual variations in lens size and shape and a trend to increased curvature with age, particularly marked for the anterior surface.

A dual optical probe for volume fraction, drop velocity and drop size measurements in liquid-liquid two-phase flow

This paper describes the development and application of a dual optical probe for local volume fraction, drop velocity and drop size measurements in a kerosene-water liquid-liquid two-phase flow. A sampling tube procedure and interface tests were used to calibrate the leading optical sensor and both optical sensors for measurements of the local volume fraction and drop velocity as described in this paper. Measurements were carried out in a large-scale vertical two-phase facility mainly at the pipe centre-line to demonstrate the advantages of using optical fibres with normal cut ends in a kerosene-water two-phase flow. Finally the applicability of this probe for measurements in gas-liquid two-phase flows and gas-kerosene-water three-phase flows has also been demonstrated.

Refractive Index of the Human Lens Surface Measured with an Optic Fibre Sensor

The refractive index of the human eye lens has been measured directly with a reflectometric, optic fibre sensor. Thirteen human lenses ranging in age from 27 to 84 years were examined. The results show that the refractive index is higher on the anterior than on the posterior surface with most of the values ranging from 1.364 to 1.381 and 1.338 to 1.357, respectively. There is no topographical variation along either surface and no evidence of age-related differences. Results using the sensor are compared with those from the Abbe refractometer on homogeneous materials. The refractive index of distilled water was found to be 1.332 with the fibre sensor and 1.3326 with the Abbe refractometer; of 0.5% agarose gel, it was 1.334 and 1.3355, respectively.

Variations in refractive index and absorbance of 670-nm light with age and cataract formation in human lenses

A newly-developed fibre-optic sensor (wavelength of 670 nm) was used to measure the index gradient in 16 human lenses ranging in age from 38 to 78 years. Two of these lenses had nuclear cataract. The refractive index, calculated from the proportion of reflected light at the sensor/sample interface, appeared to increase with age and further with cataract formation. This was not consistent with previous studies. The increased losses of reflected light, which gave apparent increased values of refractive index, were reconsidered to be resulting from light absorption and an estimate of relative absorption was made by deducting excess light loss from a base level expected to yield the maximum index value. The characteristics of the relative absorption are similar to those of a red fluorophor reported in other studies.

Surface refractive index of the eye lens determined with an optic fiber sensor

The use of a fiber-optic sensor for measurement of the refractive index on the surface of eye lenses is described. The technique makes use of the fact that the amount of light reflected at the interface of two media (Fresnel reflectance) depends on the refractive-index difference between them. The sample is probed with a single-mode fiber, and the refractive index is calculated from the proportion of light reflected at the probe-sample interface.

Refractive index distribution in the porcine eye lens for 532 nm and 633 nm light.

AimsTo measure the refractive index distribution in porcine eye lenses for two wavelengths from the visible spectrum: 532 and 633 nm, in order to determine whether there are any discernible wavelength dependent differences in the shape of the profile and in the magnitude of refractive index.MethodsRays were traced through 17 porcine lenses of the same age group and of similar size. Ray trace parameters were used to calculate the refractive index distributions for 633 nm light in all 17 lenses and for 532 nm light in 10 lenses. The effect of the refractive index at the edge of the lens, on the rest of the profile, was considered because the mismatch between refractive index at the lens edge and the refractive index of the surrounding gel necessitated a further step in calculations.ResultsThe shape of the refractive index distributions is parabolic. There is a small wavelength dependent difference in the magnitude of the refractive index across the profile and this increases very slightly into the centre of the lens. The value of the refractive index at the edge of the lens does not appreciably affect the index profile.ConclusionsThe wavelength dependent differences in refractive index between light of 633 and 532 nm are small but discernible.

Explanation of isogyre formation by the eye lens

The eye lens is considered to be birefringent and this properly is thought to be responsible for the image of a cross which is formed within the lens when it is placed between crossed polarizers. This paper presents an alternative explanation for the pattern, as a phenomenon of refraction.

Aging changes in the optical elements of the eye

The cornea and the lens are the optical elements of the eye, responsible for correctly focusing light rays on the retina. As the eye ages, both the cornea and the lens undergo structural changes, some of which affect function. This review deals with such age-related changes and any functional manifestations arising from them.

Mathematical description of isogyre formation in refracting structures

The cross pattern (isogyres) formed by certain crystals when they are placed between crossed polarizers and illuminated with convergent light results from birefringent properties of the medium. A similar image is seen in refracting structures which exhibit little or no birefringence, such as the eye lens. This paper provides a mathematical explanation of the formation of isogyres by refraction.