Stray Light of Spectacle Lenses Compared to Stray Light in the Eye

Abstract

Abstract: : Purpose: When measuring ocular stray light for clinical evaluation, it is important that the stray light of spectacle or trial lenses does not contribute significantly to the measured amount of stray light. We therefore investigated the stray light characteristics (magnitude and angular dependence) of spectacle lenses and compared these to the ocular stray light characteristics. The results might also give more insight into the importance of spectacle lenses in glare situations and into the tolerance to exoptic stray light. Methods: Stray light can be defined as the height of the skirts of the Point Spread Function (PSF) of an optical system. The PSF of the eye is well known from literature, while the PSF of spectacle lenses was determined experimentally with stray light angles ranging from 4 to 30 degrees. Fourteen spectacle lenses were measured 'as worn' (uncleaned) and thirty spectacle lenses were measured after cleaning. Subsequently, the PSF was fitted to the function a₁₀×(θ/10)^b, in which a₁₀ is the PSF value at 10 degrees and b is the slope (shape) parameter of the PSF. Results: All measurement data fitted the stated equation well (R²>0.95, with 73% of the lenses R²>0.99). The slope parameter b for both 'as worn' and clean lenses had an average value of –2.0, with a standard deviation of 0.5. This is very similar to that of the eye. All of the 'as worn' lenses remained below the stray light values of the eye, although some of them did approach the values of the eye. The amount of stray light of cleaned spectacle lenses is in general at least an order of magnitude smaller than that of the eye. This result already holds when simply cleaning with breath and a clean cloth. Another result was how the stray light values of the cleaned lenses increased with the age of the lenses. In terms of stray light, plastic lenses degraded notably faster (by a factor of about 4) than glass lenses. Conclusions: Because the shape of the PSF of spectacle lenses is similar to that of the eye, the ratio between the two is more or less constant with angle. This might help in tolerating stray light from spectacle lenses. The amount of stray light found in the 'as worn' lenses might indicate that a subject tolerates an amount of stray light up to levels approaching those of the eye. To reach the amount of stray light of the eye, the spectacle lens needs to be contaminated by as much as 1 or 2 clear fingerprints. For ocular stray light measurements, a patient can use prescription lenses without significantly increasing the measured amount of stray light. However, the lenses will need to be cleaned and not obviously damaged.

Keywords: optical properties • physiological optics