Quantification of forward scattering based on the analysis of double‐pass images in the frequency domain

Purpose To assess forward intraocular scattering by means of a new parameter (Frequency Scatter Index, FSI 3 ) based on the analysing double‐pass retinal images in the frequency domain, which minimizes the impact of aberrations on the results. Methods A prospective observational study was carried out in the Department of Ophthalmology, Hospital Universitari Mútua de Terrassa (Spain) on a group of 19 patients diagnosed with nuclear cataracts of various LOCSIII grades and a control group (CG) with nine healthy eyes. We recorded double‐pass retinal images with a custom set‐up based on a high‐sensitivity digital camera. The FSI 3 was then computed using spatial frequencies below three cycles per degree. A preliminary validation of the FSI 3 was performed on an artificial eye and two eyes of volunteers with and without commercial diffusers, and under defocus. Results The FSI 3 was hardly affected by defocus values up to 2.50 D. The mean (and standard deviation) FSI 3 values were as follows: for the CG, 1.19 (0.21); and for LOCSIII grades nuclear opacity 1, 2 and 3, 1.30 (0.12), 1.62 (0.21) and 1.85 (0.21), respectively. Eyes with cataracts showed FSI 3 values significantly different than healthy eyes (p = 0.001). A good correlation ( ρ  = 0.861, p < 0.001) was found between the FSI 3 and objective scatter index provided by a commercial instrument. Conclusion Since aberrations have little impact on the FSI 3 , the light scatter assessment becomes less dependent on the patient's refractive error compensation and the presence of higher‐order aberrations. The FSI 3 can further the clinical and scientific understanding of forward intraocular scattering.