Corneal group refractive index measurement using low-coherence interferometry

Abstract

Purpose. The goal of the study is to measure the group refractive index of the human cornea in vitro to improve the accuracy of conical thickness measurements. Methods. Corneal buttons were trephined from 23 human cadaver eyes and the group refractive index ofthe cornea was measured at =840 urn using a low-coherence Michelson interferometer and the technique proposed by Sorin and Gray (Phot. Tech. Letz 4: [105][106][107] 1992). The effect of dehydration on the measurement was Studied by measuring the conical optical thickness as a function of time. Results. Preliminaiy measurements of the group refractive index at 840 nm gave flg 1.450 +/ 0.024 for the human cornea, which is much higher than a calculated group refractive index of flg 1.387. BCcLUS of dehydraüon, the optical thickness of the cornea decreased at a rate of 5.5 pim/min which led to an artificially high value for the group refractive index. Conclusion. The calculated group refractive mdcx of rig 1.387 ppC&S to be an accurate value for the purpose of cornea! thickness measurements using low-coherence interferometiy, and cornea! group refractive index measurements can be performed in vitro ifthe measurements are performed rapidly to avoid the effect of dehydration.

Simultaneous real-time imaging of the ocular anterior segment including the ciliary muscle during accommodation

“…The correction approach has been well-documented in the literature [26–30], including in our previous work [8,31]. The refractive index of each medium (the refractive index of 1.387 for the cornea [32], 1.342 for the aqueous humor [33], and 1.408 for the crystalline lens [34]) at a wavelength of 840 nm was applied in the algorithm of correction. The central corneal thickness (CCT), the pupil diameter (PD), the anterior chamber depth (ACD), the curvature radii of the anterior (CAL) and posterior (CPL) lens surface, and the central lens thickness (CLT) were computed.…”

In vivo measurement of the human crystalline lens equivalent refractive index using extended-depth OCT

“…Equivalent refractive index values were tested in steps of 0.001 over the range from 1.350 to 1.500, which was selected to exceed the range spanned by values reported in literature [3,4,9,[11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26]. The group refractive index of the lens was calculated from the equivalent index using the same approach as in Uhlhorn et al [30], since the group index was required to correct for OCT image distortions. We used lens dispersion data from Atchison and Smith [31] and scaled for wavelength by assuming a constant ratio between the group refractive and equivalent indices.…”

“…Intraocular distances between boundaries were corrected for optical path length by dividing the optical distances measured along the A-line passing through the apex of the cornea with the group refractive indices of the corresponding ocular media at 840 nm (n CORNEA = 1.387 [30], n AQUEOUS = 1.341 [31], n LENS = calculated value from first step of the algorithm, and n VITREOUS = 1.341 [31]). Radii of curvature for the anterior and posterior lens and corneal surfaces were corrected for distortion due to refraction of the OCT beam at the ocular surfaces and through the ocular media.…”

Comparison of two partial coherence interferometers for corneal pachymetry in high myopia and after LASIK

“…Nonetheless, it should be noted that corneal group refractive indices have been assessed in relatively few studies (Drexler et al. 1998; Uhlhorn et al. 1998; Lin et al.…”