Photophobia and light-induced interference with visual comfort and performance are the main complaints of RP patients [14, 15]. Discomfort is usually reported as glare, reflections, flicker and non-uniformity of illumination, all interfering with task performance . Probable causes of these symptoms in RP patients are: (1) general photophobia caused by light scatter, since the retinal pigment epithelium can no longer absorb light normally. This causes poor adaptation to different illumination levels because of the lack of photoreceptor function [17, 18]. (2) increased levels of intraocular light scatter caused by posterior subcapsular cataracts that decrease retinal image quality [19, 20].
What is now termed the video terminal syndrome is a multifactorial condition with several potential contributory causes, such as uncorrected refractive error, especially astigmatism, presbyopia, and tear film abnormalities . This experiment also takes into account that the refractive error will affect patient discomfort. Before the experiment, the patients were corrected for their refractive errors, and plus lens additions were added for the appropriate viewing distance for patients who needed. We then measured reading speed and comfort of RP patients in the most comfortable way, instead of testing the patients with their distance refractive correction. And in order to exclude the video terminal syndrome that includes prolonged tear film ruptures caused by long-term reading, the reading time of each reading session was interrupted for 5 minutes to reduce eye strain associated with computer use based on physiologic correlates of eye fatigue.
Carracedo et al. showed that only 11% of RP patients wearing a CPF-527 filter reported improvement in visual comfort for indoor activities of daily living . In our study, 95% of subjects (21/22) wearing CLF reported improved comfort during computer use. Possible reasons for the difference between the two studies were the filter type and the visual task. Our CLF filtered 100% of wavelengths <400 nm and 71% of wavelengths between 400-500 nm, with a total luminance transmittance of 74.5%. The CPF-527 filter Carracedo et al. used removed 90% of the wavelengths <550 nm with a luminance transmittance of 21%. Total transmittance may be one contributing factor to visual comfort and an explanation for the difference. Second, we investigated visual comfort when RP patients read text on a computer screen that transmitted more blue light that might cause eye strain. Carracedo et al. studied the visual comfort during a broader range of their subjects’ general daily activities. These are important differences between visual task conditions.
Declines in CS with progression of RP lead to difficulty in daily tasks [23, 24]. Van den Berg and Carracedo et al. found that RP patients wearing filters had improved CS [22, 25], while Cedron-Sanchez et al. showed that filters improved visual discrimination for their RP patients . In our study, CLF wear did not enhance CS. Differences in spectral and luminance transmission of the various filters used in these studies may be a reason for these disparities in CS. Van den Berg and Carracedo et al, both used CPF-527 filters that filter out 98% the wavelengths below 527nm with an overall 32% transmittance.
Another reason for the discrepancies between studies may be the differences in contrast sensitivity tables. Gonzalo Carracedo et al,point out that contact filters improve contrast sensitivity at medium and high frequencies, while glass filters only improve contrast sensitivity at high frequencies. In our study, the Mars contrast sensitivity table was used. The visual Angle of each letter at 0.5m was 2°, corresponding to logMAR VA of 1.380. which is near the normal peak frequency of the CSF. Our RP patients had such good vision that they may have easily met the vision standard of 1.38. The finding of Colombo et al is also different from ours . Although they also used Mars charts and selective blue-violet light filtering spectacle lenses, they included patients affected by retinal diseases other than RP.
Virgili et al. pointed out that reading difficulty for RP patients is closely related to progressive reduction of visual field, gradual loss of vision and significant reduction of high frequency CS . Szlyk et al found statistically signiﬁcant correlations between the clinical measures of vision such as CS and the functional performance of daily tasks, where better CS was associated with better reading performance . We used random text sequences to eliminate any learning effect for accuracy of reading speed. The fact that CLF wear neither enhanced CS nor expanded the visual field of RP subjects  is one explanation for no change in reading speed with filter wear. Another reason may be that lens wear and transmittance of filters affect the visual acuity of RP patients differently. Visual acuity improved with refractive correction lens wear, while filter absorbance reduces luminance. The absorbance of the CLF alone is 74.5%. This can reduce visual brightness, but the contrast sensitivity is not expected to change with such a small luminance change. Thus, the eyeglass wear and CLF filter used in this study is consistent with the lack of effect of the CLF on vision and and reading speed.
Although CS and reading speed did not improve with filter wear, our subjects reported that their visual comfort improved. This finding suggests that patients with RP who experience photophobia when reading on a computer screen can be prescribed a CLF to improve their comfort and quality of life.
Conclusion： CLF wear did not improve RP subjects’ CS or reading speed for screen text, but did reduce the appearance of screen brightness and improve subjects’ reported visual comfort. Improvement in comfort alone may be sufficient justification for filter use as a vision aid for RP patients during vision rehabilitation.