Light scattering by pigment epithelium granules in the human retina

Determination of the intraocular irradiance and potential retinal hazards at various positions in the eye during transscleral equatorial illumination for different applied pressures

PURPOSE

With diaphanoscopic illumination of the eye, the intensity of light entering its interior depends on the transmission properties of the eyewall. Light that passes through the eyewall can cause damage to the retina. Therefore, in this study, the intraocular irradiances are determined at different positions on the retina, directly behind the illuminated eyewall, the opposite eyewall and near the macula of ex-vivo porcine eyes. These irradiances are examined for their dependence on the pressure applied on the eyewall with the illuminating fiber and for the influence of the pigmentation of the eye.

METHODS

In total 221 ex-vivo porcine eyes were investigated. For transscleral illumination an illumination fiber with a diffusing adapter cap is pressed against the equatorial eyewall. The illumination fiber is pressed onto the eye and the pressure is measured in the anterior chamber. Three different pressures are applied, 23, 78 and 132 mmHg. A detection fiber with diffusing fiber tip is inserted into the eye at the desired position. The eyes were divided in groups with high and less pigmentation to investigate the influence of the pigmentation on the intraocular irradiance.

RESULTS

The intraocular irradiances Eintra increases for various increasing applied pressures with the illumination fiber on the eyewall and for various positions inside the eye. With this the irradiances weighted with the photochemical and thermal hazard weighting function, EA-R and EVIR-R, also increases. Differences in Eintra, EA-R and EVIR-R could be found for different pigmented eyes as these values are higher for less pigmented eyes than for strong pigmented ones.

CONCLUSION

The hazard to the retina during diaphanoscopic illumination of the eye depends on how strong the surgeon presses the illumination fiber on the eyewall. Depending on the applied pressure and the measuring position in the eye, the specified limit for the photochemical hazard to the retina is partly exceeded. The pigmentation of the eye also plays a role. The irradiance in less pigmented eyes appears to be higher than in strongly pigmented eyes. Because of this, the surgeon should be able to adjust the intensity of the light source to the color of the patient's eye.

Monte Carlo modelling of the spectral reflectance of the human eye

The interpretation of in vivo spectral reflectance measurements of the ocular fundus requires an accurate model of radiation transport within the eye. As well as considering the scattering and absorption processes, it is also necessary to account for appropriate histological variation. This variation results in experimentally measured spectra which vary, both with position in the eye, and between individuals. In this paper the results of a Monte Carlo simulation are presented. Three histological variables are considered: the RPE melanin concentration, the choriodal haemoglobin concentration and the choroidal melanin concentration. By considering these three variables, it is possible to generate model spectra which agree well with in vivo experimental measurements of the nasal fundus. The model has implications for the problem of extracting histological parameters from spectral reflectance measurements. These implications are discussed and a novel approach to interpretation of images of the ocular fundus suggested.

Fundus reflectance and the measurement of crystalline lens density

We have developed an optical technique for measuring the in vivo absorption of the human crystalline lens based on using the retina as a reflector for a double-pass measurement of lens density. We compare results obtained by the reflectometric technique with results obtained in the same subjects by a psychophysical method. We find that the estimates obtained with the two techniques are highly correlated, and both measure similar effects of aging on the optical density of the lens. In addition, we show that the significant variations in retinal reflectance attributable to differences in ocular pigmentation and drusen have only a minor effect on the estimate of lens optical density. However, correcting retinal reflectances for the effect of lens absorption shows that the effect of lens absorption may be overestimated with both methods. The overestimation may arise from inaccuracies in the shape of the assumed lens spectrum.

Direct measurement of human-cone-photoreceptor alignment

We have developed an imaging reflectometer to measure cone-photoreceptor alignment. One makes measurements by bleaching the cone photopigment and imaging the distribution of light returning from the retina, which is illuminated from a small source imaged in the plane of the eye's pupil. If the source is near the optimal entry pupil position as determined psychophysically, the distribution of light returning from the retina is peaked, and the magnitude of the peak depends on the location of the source in the pupil of the eye. If the source is far from the optimal entry pupil position, then there is no measurable peak. The location of the peak varies across individuals and coincides with the reported location of best visibility of the measuring light and with previous psychophysical and reflectometric measurements of the Stiles-Crawford peak. The source of this directionality must arise either from the photoreceptors or from behind the photoreceptors because the peak is not present if measurements are made when the cone photopigments have high optical density.

Optical properties of ocular fundus tissues--an in vitro study using the double-integrating-sphere technique and inverse Monte Carlo simulation

Various models have been published calculating the light transport at the ocular fundus either for interpretation of in vivo reflectance measurements or for the prediction of photocoagulation effects. All these models took the absorption spectra of the pigments located at the ocular fundus, melanin, haemoglobin, xanthophyll, and the photoreceptor pigments, into account. However, light scattering inside the single fundus layers has not been investigated in detail and was, therefore, neglected in the calculations or only considered by very rough approximations. This paper presents measurements on specimens of retina, retinal pigment epithelium, choroid, and sclera using the double-integrating-sphere technique. Absorption coefficients, scattering coefficients, and anisotropy of scattering were calculated by an inverse Monte Carlo simulation from the measured collimated and diffuse transmittance and diffuse reflectance. Conclusions are drawn for the interpretation of fundus reflectance measurements, which are a useful tool in diagnostics and photocoagulation dosimetry.

The resonator theory of colour vision

The resonator theory claims that light scattering by the melanin granules of the retinal pigment epithelium causes light to be reflected back through the outer segment of the cones, in a direction opposite that of the incident light. These two opposite directed wave motions may produce standing waves inside the cone outer segment, which then acts as a resonating cavity for light waves. The wavelength specificity of the single cone is determined by the length and the diameter of the cone outer segment. The variation of the length and the diameter of the cone outer segments throughout the retina is then the basis of colour discrimination.