Investigation of dispersion effects in ocular media by multiple wavelength partial coherence interferometry

We report on quantitative measurements of group refractive indices and group dispersion in water and in human ocular media such as the cornea, the aqueous humor, the lens, artificial intraocular lenses, as well as a total value averaged over the media along the axial eye length of normal subjects and pseudophakic patients in vivo using dual beam partial coherence interferometry. Different optical thickness values due to the dispersion of the cornea are demonstrated using two spectrally displaced light sources. The displacement can be used to indirectly calculate the group dispersion of the human cornea in the spectral region between 810 nm and 860 nm. If the object under investigation is dispersive, resolution is limited due to a broadening of the detected signals. This broadening increases with group dispersion, i.e., the extent to which the group refractive index of the medium varies with wavelength and thickness of the tissue under investigation as well as with the spectral bandwidth of the light source. Measurements of the group dispersion in the cornea, lens and vitreous of pseudophakic and normal human eyes, show that the cornea and the lens are more dispersive than water-by a factor of about 5 and 2, respectively-in the investigated spectral region. The cornea is approximately threefold more dispersive than the human crystalline lens, the aqueous humor is less dispersive than water and the group dispersion of all ocular components together, averaged over the axial length of normal and pseudophakic eyes, was only slightly higher compared to that of water. Since the highly dispersive cornea and lens together have only a thickness of about one sixth of that of the axial eye length, it seems that their contribution to the group dispersive effect along the whole axial eye length is only small.