Abstract
Purpose: :
Different optical methods have been developed to get free from reflections anterior to the macular pigment (MP) when measuring MP density. Zagers and van Norren have analysed the directional component radiated from foveal photoreceptors, thereby eliminating virtually stray-light. Since the choice of the templates for spectral lens absorption is delicate, we propose to measure the specular reflection from the inner limiting membrane (ILM) in the perifovea as a reference to determine the MP density Ddir.
Methods: :
In the fovea the radiant flux of the pattern radiated from photoreceptors was measured at 470 nm and 532 nm (sample field: 2 deg). In the perifovea (6 deg from the fovea) the radiant flux of the beam reflected specularly by the ILM was measured for the same wavelengths. The distribution of light in the pupil was determined with a CCD camera cooled by liquid nitrogen (Princeton Instruments). The study population consisted of 27 male subjects free of ocular pathology (ages: 45-55). The background reflectance was also measured for the same wavelengths and eccentricities to calculate the MP density Dnd.
Results: :
The mean densities Ddir(460) and Dnd(460) were equal to 0.437±0.089 and 0.197±0.042 D.U., respectively. They were highly correlated (r=0.738, p=0.0003). The mean directionality factors ρ(470) and ρ(532) were 0.235±0.027 and 0.203±0.020 mm-2, respectively. They were also correlated (r=0.779, p=0.0001), with a slope of 1.034.
Conclusions: :
The relationship between the directionality factors that we measured at 470 nm and 532 nm for the different subjects is compatible with the model of Marcos. The mean value of Ddir is similar to MP density estimates provided by autofluorescence or spectral reflectance methods. Comparison of Ddir(460) and Dnd(460) implies that 60±11% of the light reflected from the fovea comes from layers anterior to the macular pigment at 470 nm.
Keywords: macular pigment • photoreceptors • optical properties