Increased straylight within the eye may severely affect the quality of vision. Despite the clinical interest, there is a lack of reliable optical instruments suitable for this type of measurement. In this context, we have evaluated a new instrument in a clinical setting.
The instrument uses a specially designed extended source that is projected into the retina. It is composed of a central disk (visual angle 3 degrees radius) and a concentric annulus (3-9 degrees) that are illuminated by high-brightness green LEDs (530±10 nm). The LEDs illuminating the annulus and the disk are modulated at different temporal frequencies (464 and 862 Hz respectively). The contribution of each part of the source to the signal recorded after double-pass through the eye from the central fundus is analyzed in the Fourier domain. A straylight parameter is computed from these signals. Measurements were performed in 28 eyes of 16 cataract patients (4 IOL). For these patients straylight was additionally assessed psychophysically (C-Quant, Oculus, Germany). A subjective grading of cataract was given by a specialized physician (MT). As reference, measurements of straylight were also performed in 16 normal eyes with no known pathology.
The logarithm of the straylight parameter ranged from 0.64 to 2.36 (Mean: 1.33) in cataract patients and 0.51 to 0.79 (Mean: 0.63) in healthy eyes. The difference was statistically significant (p=0). Figure shows the correlation between the optical and the psychophysical measurements (Pearson’s r = 0.859). Furthermore, Bland-Altman calculations showed that %95 of the measurement differences were within ±0.41 band. The variability of the optical measurement was low having a coefficient of variance equal to 0.015. While the optical measurements were completed in all patients, 8 patients could not perform the psychophysical measurements (%50).
A new optical instrument allows measuring a straylight parameter both in healthy and cataract eyes with high repeatability rapidly and easily for both the patient and the clinician. Measurements are in line with psychophysical values and clinical observations.