Optical method for estimating the equivalent refractive index of the crystalline lens in vivo

Abstract

In numerous studies it has been necessary to represent refractive indices of the crystalline lens by an equivalent refractive index (here denoted nL). Its traditional value of 1.416, based on in vitro measurements, may be in error by more than 0.025 in some cases. In vivo estimates are subject to recognized deficiencies of ultrasonography. This study describes and evaluates a new and exclusively optical technique. Using exact ray tracing, nL is found by combining routine optical section data with optical depths that may be measured by a ranging system termed OCDR. Gullstand's no. 1 eye is used to calculate the effects of errors in eye data on the accuracy of the technique. From pooled errors estimated for 22 parameters, a standard deviation of 0.0045 is anticipated for the estimate of nL, although allowance for additional uncertainty associated with the refractive index profile of the lens must await further research.

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.…”

“…There are challenges in acquiring accurate measurements of the equivalent index. For instance, precise measurements of the lens curvature are needed in both in vitro [3,4,12,15,16] and in vivo [9,11,13,14,[17][18][19][20][21][22][23][24][25][26] experiments. Additionally, in in vitro experiments, extracted lenses can also undergo changes from their in vivo state, particularly swelling over time [27], which can affect the shape and thickness of the lens as well as its equivalent index.…”

“…For in vivo studies, biometry of ocular surfaces is input into a computational model eye from which the equivalent refractive index is solved, so the difficulty lies in obtaining accurate values of these biometric parameters. Studies up to date have either estimated ocular distances and curvatures [18,24] or have combined data from multiple instruments and imaging techniques, including Scheimpflug [11,14,17,19], phakometry [9,13,20,22,23], and ultrasound biomicroscopy [9,11,13,22,23]. The inclusion of estimated ocular distances and curvatures in the computational eye model reduces the accuracy of the predicted equivalent refractive index, while the use of multiple instruments increases measurement uncertainty given differences in instrument precision and potential differences in patient alignment and centration between instruments.…”

Refractive Index Measurement of the Crystalline Lens in Vivo

“…[1][2][3][4][5][6][7][8][9][10][11][12][13] Measurement of the lens refractive index is practically difficult because the refractive index is not homogeneous within the lens, changing from the lens center to the lens surface and forming a special gradient of the refractive index distribution. [14][15][16][17] Advanced imaging techniques, such as Scheimpflug imaging, 1-5 MRI, [6][7][8][9][10] optical interferometry, 11 and optical coherence tomography (OCT), 12,13 have been used to assess the lens refractive index in vivo. Scheimpflug imaging is a geometrical optical imaging modality in which an image of the lens is formed by a camera from a side view at an angle of 45°from the optical axis of the eye while a slit light beam directly illuminates the eye.…”

“…However, accessing the lens refractive index in the human eye in vivo has been a challenge for a long time, even though the task has been approached using a variety of techniques. [1][2][3][4][5][6][7][8][9][10][11][12][13] Measurement of the lens refractive index is practically difficult because the refractive index is not homogeneous within the lens, changing from the lens center to the lens surface and forming a special gradient of the refractive index distribution. [14][15][16][17] Advanced imaging techniques, such as Scheimpflug imaging, 1-5 MRI, [6][7][8][9][10] optical interferometry, 11 and optical coherence tomography (OCT), 12,13 have been used to assess the lens refractive index in vivo.…”