Article • Comparison of Accommodative Responses to E-Ink Vs. LCD Vs. Standard Ink on Hard Copy
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Article • Comparison of Accommodative Responses to e-Ink vs. LCD vs. Standard Ink on Hard Copy Patricia M. Cisarik, OD, PhD • Southern College of Optometry • Memphis, Tennessee Jennifer Nguyen, OD • Southern College of Optometry • Memphis, Tennessee ABSTRACT Background: Adverse visual symptoms with electronic devices have been reported, but causes have yet to be clearly defined. Previous studies have neither equated stimulus luminance across devices nor studied subjects with verified normal accommodation prior to testing. Pupil size during device use also has not been explored. We compared pupil sizes and accommodative responses to targets of similar size displayed with ink on paper, an e-ink reader, and an LCD device in young, healthy adults with normal accommodation. Methods: Subjects were 42 healthy adults, aged 22 to 33 years, with near acuity 20/20 (6/6) or better in the tested eye. Normal accommodation was verified with accommodative amplitude and facility (+/-2.00 flippers) tests. The reading text was presented at 40 cm, and text size and device luminance were matched across devices. While the subject read the passage aloud, pupil size was measured. Accommodative responses were obtained twice with near retinoscopy and twice with an open-field autorefractor for each device. Results: One-way ANOVA for correlated samples and Tukey’s HSD test were used to compare mean pupil size and mean accommodative response across devices. The differences in mean pupil size for paper versus e-ink (5.1 ± 0.9 vs. 5.5 ± 0.8, p<0.01) and LCD versus e-ink (5.1 ± 0.9 vs 5.5 ± 0.8, p<0.01) were significant. By retinoscopy, the difference in mean accommodative response was significant only for e-ink and LCD (1.48 ± 0.76 D vs. 1.78 ± 1.26 D, p<0.01). By autorefraction, no significant differences in mean accommodative response were found across device type. Linear regression showed that accommodative response was not predicted by pupil size for any of the devices. Conclusions: In this group of young adults with normal accommodation, accommodative response was not significantly different for ink on paper, e-ink, or LCD devices of matched luminance. Pupil size did not predict accommodative response for any device. Keywords: accommodation, electronic device, e-reader, MEM retinoscopy open-field autorefraction Introduction available have simply increased the amount of Electronic devices used for reading and time spent on near tasks is uncertain. other near tasks have been associated with Several studies have attempted to investigate adverse visual symptoms, such as tired eyes, whether digital display use is correlated with eyestrain, and dry eyes.1-4 Whether these clinical evidence of reduced visual function.2-6 symptoms result from changes in visual For example, in their study of 76 video display responses instigated by properties of the terminal (VDT) users who averaged almost 7 devices themselves or whether work demands hours per day of computer use, Shrestha et and interest in the vast array of digital content al. found accommodative infacility to be the Optometry & Visual Performance 269 Volume 7 | Issue 5/6 | 2019, December most common functional problem identified greyish background) than used for most digital (+/-2.00 D binocular flippers at 40 cm, target displays in this century (LCD, e-ink, etc.), and size equivalent to N8, diagnostic criterion set the VDT users spent at least 50 percent of their at 10 cycles/minute).2 Given that the test was workday viewing the terminal. Not reported administered binocularly, whether vergence by the authors is whether the presence of dysfunction contributed to the reduced test accommodative or vergence dysfunction results is unknown. Their subjects, whose was an exclusion criterion for participation; mean age was 28.5 ± 5 years, were recruited additionally, no data were collected while the from a population of patients presenting to subjects viewed the VDTs. a clinic specifically seeking eye and vision While desktop terminals are still widely used care; therefore, the results of the study may today, hand-held digital devices of varying not be representative of the responses that sizes have become ubiquitous in daily work and would be obtained from a randomly sampled leisure life. In 2014, Hue et al. compared reading population. With respect to other factors that from electronic devices versus hard-copy text can contribute to computer vision syndrome, and found that reading rate was slower and Shrestha et al.2 acknowledged the roles of accommodative lag was larger for the iPod ergonomics: furniture used, presence of glare, electronic device than for the hard-copy text, temperature, humidity, location of VDT, task whereas visual symptoms were greater for the performed, and undiagnosed or untreated Kindle electronic device than for the hard-copy vision problems; however, neither these text.7 A direct comparison of the two electronic factors nor the parameters of the VDTs used devices on the same group of subjects was by their subjects were assessed in the study not made. Interestingly, the accommodative and are unlikely to have been uniform across assessment, done with an infrared optometer, their subject population. Finally, whether the showed no significant change in response accommodative infacility identified in their over the 12-minute reading time for any of the subjects would lead to similar symptoms with conditions tested. The report states that the the same duration of near work using hard targets were matched for contrast (80%) and copy is unknown. viewed under the same ambient illumination, In two well-controlled studies that examined but no information was given about whether the effects of VDT use on visual functions in the target luminance values were similar. As new users of VDTs in a single office, Yeow et al. with many previous studies on visual function found a slightly greater reduction in near point and digital device use, the subjects were not of accommodation over two years for the VDT screened for accommodative and vergence users compared to non-VDT users.5,6 Their study disorders or for dry eye prior to enrollment. involved repeated testing every three months, Benedetto et al. explored the effects of used a site where all VDT users worked with display technology on objective (number of the same type of terminal, and controlled for eye blinks per second) and subjective (Visual subject age. Additionally, their control group Fatigue Scale score) indicators of visual consisted of subjects who worked in the fatigue.1 Their twelve pre-presbyopic subjects same office environment and did similar work participated in three separate prolonged tasks as the experimental group but who did (70-minute) reading sessions, using a device not use a VDT during their workday. The VDT with a different display technology (LCD vs. terminals used by the subjects in the study e-ink vs. hard copy) at each session. Viewing were an older type of technology (phosphor distance was 60 cm; displays were equated P31, producing greenish characters on a for page size, font size, typeface, and number Optometry & Visual Performance 270 Volume 7 | Issue 5/6 | 2019, December of words per page; and all sessions took place Table 1. Subject Inclusion Criteria in a controlled and standardized room. Visual 20/20 or better visual acuity at near (40 cm) in the tested eye No strabismus, amblyopia, ocular disease affecting crystalline lens anatomy or Fatigue Scale score was evaluated before function, or other ocular disease in the tested eye and after the reading sessions, whereas the No current or recent (within 30 days of testing) use of medications known to affect accommodation or otherwise cause blurred vision blink rate was evaluated during the reading Not over-minused with habitual correction to avoid confounding of the sessions. Their results showed a significant accommodative response data (see text for details) Normal accommodation: verified with measurement of monocular increase in the mean of the before- versus accommodative facility (+2.00/-2.00D lens flipper test, at least 11 cpm in test eye) and amplitude of accommodation (push-up to blur value at least 15 – after-reading session Visual Fatigue Scale score 0.25(age in years) diopters) in the test eye for the LCD device only. With respect to blink rate, analysis showed a significantly lower disorders, nor was a cross-over design used mean blink rate while using the LCD device to determine whether similar results would compared to the e-ink and hard-copy devices, occur if the groups switched reading devices. but no significant difference in the mean Additionally, no information was given relative blink rate was found between the e-ink and to the equivalence of the luminance or the hard-copy devices. Due to the differences in contrast of the targets for the two devices. luminance across the devices, the researchers The present study was initiated specifically compared the tonic pupil size during reading to evaluate the accommodative response and with each of the devices. They found that the pupil size to targets created with different mean pupil size during reading with the LCD display technologies in young, healthy adults. device was significantly smaller than when To the extent possible, given the devices and reading with either of the other two devices. the technology available to us for the study, Thus, the effects of the smaller pupil (e.g., we equated the stimuli across the devices increased depth of focus) created by the for size and luminance and controlled for higher luminance on the LCD compared to accommodative disorders by screening the other two devices used in this study could subjects for accommodative dysfunction prior have contributed to the differences found in to enrollment. their subjects. Accommodative responses were not measured, closer reading distances were Methods not evaluated, and their subjects were not Subjects screened for accommodative and vergence After receiving study approval from the disorders or for dry eye.