Imaging the human iris: a hyperspectral approach

Purpose: Hyperspectral (HS) imaging is a promising optical technique that allows the detection of both spatial and spectral information in a single data acquisition. Here we investigate the capability of a prototype HS instrument to measure the iris spectral reflectance in vivo. Moreover, we evaluate the intra- and inter-session repeatability of quantitative HS measurements of the human iris. Methods: Eight healthy volunteers (all males, average age 27 ± 2 years) participated in the study. Irises colors were classified using the grading system of Franssen et al [Grading of Iris Color with an Extended Photographic Reference Set, J Optom 2008; 1:36-40]. Each HS acquisition (duration: 4125 ms) consists of 22 images (HS cube) captured between 480 – 900 nm, in 20nm intervals. Each acquisition was repeated 6 times. The instrument was calibrated using a NIST white reflectance standard, to ensure quantitative and repeatable measurements. Reflectance was analyzed across the 22 spectral bands in an annular region of interest concentric to the pupil. Repeatability was assessed from images captured in 3 sessions, at the same exposure time, with realignment of the iris and refocusing between each session. Results: Inter-subject variability was highest at short wavelengths (Fig. 1) and reflectance exhibited opposite trends in the visible and near infrared. This might be the result of melanin scattering. The influence of water absorption is detected above 850 nm. Repeatability was highest for lighter irises and for wavelengths from 520 to 900 nm. In that range, the coefficient of repeatability (95% of confidence interval) for measurements between sessions was ±10% and ±5% for dark and light irises, respectively. The within-session repeatability was ±7.5% and ±4%, respectively. Conclusions: Automated in vivo HS imaging of the human iris appears to be reliable and reproducible. This technique could potentially be exploited for monitoring iris pigmentation changes with time induced by prostaglandin analogues. In addition, the iris spectral reflectance could be correlated with the development of certain ocular pathologies.