Harvesting the weak angular reflections from the fundus of the human eye : on measuring and analyzing the light wasted by the retina

Summary of the thesis “Harvesting the weak angular reflections from the fundus of the human eye” by Jan van de Kraats University Medical Centre Utrecht. Defended October 16, 2007. This thesis is on the modeling of the optical reflection of the human fovea, and on the three instruments build for retrieving the data. Instrument one, the densitometer, measured reflected light from a small spot on the retina at 14 wavelengths. Not only in a state with the visual pigments bleached, but even in the dark adapted state. An optical model was derived to explain the data in terms of absorbers (lens, macular pigment, blood and melanin) and reflectors in the eye (mainly cones and choroid). Important messages were that the origin of the directional reflection was at the cone discs, and that the layers posterior to the receptors show inherent non-directionality. A second instrument was developed, the Profile Spectrometer. This for speeding up the measurement time, important in working with patients. A spot of white light illuminated the retina. Reflected light fills the whole pupil of the eye, but only the light passing through a (virtual) slit from an imaging spectrometer was used. In one second a two dimensional image from a CCD detector could be gathered, incorporating both the detailed spectral and directional reflection information from the retina. A new model explaining simultaneously the directional and the non-directional part of the foveal reflection was developed. In this model the reflection from the cones shows a one over wavelength squared behavior. A new insight was that light losses by scattering were not lost for the reflection from the layers posterior to the receptors. Macular pigment is present in the central region of the retina. It benefits visual performance, and keeps the retina healthy by scavenging free radicals. To measure its optical density, a third instrument, the Macular Pigment Spectrometer was built. In order to keep it low cost, an external fiber optic spectrometer was applied. Thanks to the small illumination and detection pupil of the instrument, it made measurements in eyes with undilated pupil possible. In order to gain knowledge of the optical transmission spectra of the eye media as a function of age, an extensive literature investigation was performed. Five spectral templates were derived that proved sufficient and necessary to describe all literature data in great detail. One formula was derived to calculate the transmission of the mean human media for every wavelength, at any age. With these new data the optimal spectral shape of the filter in intraocular lenses (implanted after cataract surgery) was derived. Such spectral filtering prevents the harmful short wavelengths from reaching the retina. At the same time, the perception of this violet and blue light asks for minimal attenuation. We measured several types of existing intraocular lenses and calculated the protection and perception values. It appeared that most older types of lenses, and even one claimed as a modern type, transmitted more dangerous UV radiation than a young human lens. With the new concept of 'virtual age' for the filter characteristics, selection of lenses should become much easier