Spectra were evaluated with an updated version
20 of the original
26 Van de Kraats fundus reflectance model. The model describes the spectral aspects of light reflected from the fundus for all positions in the pupil profile (corresponding to angles at the retina), using a limited number of absorbing and reflecting layers. In short, the incoming light is thought to be reflected at the inner limiting membrane, at the discs in the outer segments of the cone photoreceptors (Rd), at the pigment epithelium, and at the choroid. Known spectral characteristics of the different absorbers in the eye (lens, MP, melanin, and blood) were used to optimize the density of the absorbers and the reflectance at the interfaces to fit the measured data. The recent version of the model simultaneously fits the optical SCE from the cone photoreceptors and the nondirectional reflection from more posterior layers in an extended wavelength range of 400 to 950 nm. In addition, the model incorporates the most recent spectral shapes of lens absorption templates.
27 MPOD is fitted by using the absorption curves for zeaxanthin and lutein published by Handelman et al.
28 The relative contributions of zeaxanthin and lutein to central MP are set at 70% and 30%, respectively. For the blood layer, a linear thickness gradient is used instead of a homogeneous layer of constant thickness, mimicking the range of pathways through the center or edge of small and large blood vessels. This thickness gradient varies from 0 to a certain maximum value, found with the model fit procedure. To use the model fit in advanced AMD (with noisy, or absent SCE), we made it more robust by fixing a number of non-age-dependent parameters. This involved ρ, a measure for the steepness of the optical SCE in the pupil plane, set at a mean value of 0.149 based on data of 102 healthy subjects.
20 In addition, two parameters of the eye media that showed minimal changes with age were set to the values corresponding to those of a subject of 65 years:
d LY, density of the young lens component was fixed at 1.58, and
d RL, a measure for the amount of Rayleigh scatter losses was set at 0.58.
20 The fundus reflectance model also provided information on the goodness of fit of each measurement by means of a χ
2 value. The mean χ
2 value ± SD of the healthy subjects was 13.2 ± 6.6. This was 24.9 ± 57.6 for the patients with AMD, indicating more noisy measurements in this group, mainly caused by spatially irregular lens densities together with AMD-related foveal disturbances. To limit the number of parameters generated by data that showed a very bad fit, measurements with a χ
2 exceeding 33.1 (mean of normal subjects + 3 SD) were discarded. This resulted in the loss of 1 of 230 healthy subject measurements (0.4%) and 55 of 388 AMD patient measurements (14.2%). In five eyes, all measurements were thus discarded. After this correction, the mean χ
2 ± SD was 13.0 ± 6.0 for the healthy control subjects and 12.7 ± 5.6 for the patients with AMD.