Forward light scatter and visual acuity through scratched protective eyewear

Abstract

Background: Scratches and other surface abrasions on protective eyewear can shorten their life cycle by scattering light from the image, which will compromise visual performance. Although scratch and abrasion resistance testing is well developed, there is little information available regarding visual acuity through lenses scratched in an actual work environment. Methods: We collected protective eyewear from mine and smelting operations that had been returned because vision through the lenses was unacceptable due to scratches and abrasions. Forward scattered light was measured on both cleaned and partially cleaned lenses using both haze and light diffusion measurements. Visual acuity through a selection of lenses was determined for both high and low contrast letters under photopic light levels. Results: Lenses with scratches in the central region had haze values ranging from one to 30 per cent. The highest haze values were measured on lenses that were both scratched and covered with dirt, whereas haze values less than 10 per cent were measured on lenses that had the dirt removed and just had scratches in the central region. Light diffusion values were highly correlated with haze. Visual acuity for high and low contrast letters became worse as the percentage haze increased but the linear correlation was only moderate. Conclusion:The appearance of the scratches on the lens surfaces strongly suggests that they were a result of improper lens cleaning. The findings also suggest that haze values less than two per cent or a luminance reduction factor less than 3.0 cd/(m 2 lx) are unlikely to affect high and low contrast acuity.

The assessment of scattered light in ophthalmic materials

“…It may be seen that the abraded samples perform in a similar way to the samples of Bauer et al . (eye protector lenses) and the reference lenses of Hovis et al . and the line of best fit bisects those of the two previous studies.…”

“…9 shows the systematic differences between all the samples of all the studies. It may be seen that the abraded samples perform in a similar way to the samples of Bauer et al 16 (eye protector lenses) and the reference lenses of Hovis et al 20 and the line of best fit bisects those of the two previous studies. The performance of the lenses with inclusions is distinctly different, showing very much less light diffusion for the same amount of haze.…”

“…It is to be expected that the light diffusion measure will be much more affected by particle size than the haze method. This is a more extensive and specific illustration of the speculation of Hovis et al …”

“…A line of best fit has not been plotted. These data can also be compared with that of Bauer et al 16 and Hovis et al 20 Figure 9 again shows the lines of best fit for the current samples as well as the data of the two other articles. A linear plot shows the differences much more clearly.…”

“…The performance of the lenses with inclusions is distinctly different, showing very much less light diffusion for the same amount of haze. On the other hand, the scratched and dirty lenses of Hovis et al 20 show relatively more light diffusion for the same haze than the abraded samples and the samples of Bauer et al 16 show most of all. The most likely explanation would seem to lie in the scattering particle or defect size.…”