Attenuation is added postsimulation by applying the Lambert–Beer law, which takes into the account the absorption coefficients and total path-length traveled in the vessel and fundus. The advantage of applying the attenuation due to absorption at this stage, rather than during the propagation of each photon packet, is that the same Monte Carlo data can be used with a range of absorption values for each layer. As scattering varies with wavelength,19,20 separate simulations are performed for 25 equally spaced wavelengths from 580 to 630 nm. In the simulations, the scattering coefficients and values (i.e., the mean cosine of the scattering angle) are taken from published data.19,20 For purposes of validation, it was necessary to place a phantom blood vessel above a scattering medium to simulate the scattering from the anterior fundus tissues.5 In the experiments, Spectralon® (Labsphere, New Hampshire) (21) played the role of the scattering medium. At a wavelength of , the optical parameters for blood are: scattering and . For Spectralon, and at . The result of the simulation is a set of 2-D images of the vessel over the Spectralon at 25 separate wavelengths and a range of absorption coefficients. The path-length distribution in each layer (vessel and Spectralon) is recorded for each case.