Effects of Flicker on Reading Comfort - Essay Example

?Effects of Flicker on Reading Comfort Objective: The purpose of this study is to investigate the effects of moderate rate frequency of flicker from LEDs light on reading comfort. The study hypothesises that when a reading material is illuminated by LEDs lamp powered on AC, produced flicker may influence reading comfort. Methods: University students with good binocular vision and no history of migraine or photosensitive epilepsy were invited to participate in a crossover randomised double masked study. Participation involved attending five reading sessions for 30 minutes under five different types of LEDs light using DC and AC. Two types of questionnaire, an initial questionnaire to identify the previous experience of discomfort and a symptom questionnaire were used. Results: The results indicated insignificant difference between discomforts caused by sustained reading and using computer. The condition of 50 Hz induced the greatest impact on reading comfort compared to DC. The most affected symptoms of discomfort were vision discomfort followed by eyestrain, blurred vision and headache. The mean difference of 75 Hz, 100 Hz and 140 Hz compared to DC lighting situation was not significant. Conclusion: The study shows that reading under 100 Hz LEDs light has no different effects on comfort than reading under LEDs light powered on DC. In addition, the study shows insignificant difference between discomfort induced by reading and by computer use in terms of incidence and nature. Further studies are required with larger and balanced sample size, different direction of illumination and longer period of reading time to address the possible limitations of this experiment. Methods Participants Students of the University of New South Wales (UNSW) were invited by posters and emails to participate in the experiment. Participants who completed the experiment received a nominal reward. The inclusion criteria were good binocular vision, lack of ocular and systemic abnormalities that possibly influence reading ability, and age had to be between 19 to 30 years old since light sensitivity often higher at this age group. The exclusion criteria were any known case of migraine or photosensitive epilepsy. A total number of 18 students were recruited (2 male and 16 female), all signed an informed consent form prior commencing the study. Ethics and the study protocol were reviewed and approved by the UNSW Human Research Ethics Advisory (UNSW HREA) Panel. Design and Procedure In a double masked randomized crossover study, discomfort symptoms were self-reported by using two types of questionnaires; an initial questionnaire and symptoms questionnaire. The participants had to attend five reading sessions under LED light, but with different rate of flicker in each session. They were randomly assigned to the sessions by Latin squares, and the reading sessions had to be separated by at least one day. At the beginning of the first session, the initial questionnaire had to be completed, whereas symptom questionnaire had to be filled immediately at the beginning and at the end of every session to evaluate reading discomfort. During the reading sessions, the ambient light was turned off, and the room was completely dark, so the only light source was that installed in lighting booth. The booth was positioned on a table, and a “twilight” novel was placed inside the booth directly under the light. Each subject had to read a part from the book on every session for 30 minutes with the habitual glasses. Comfortable sitting was enhanced by providing an adjustable chair placed in front of the booth to read with habitual reading posture. Questionnaires Two types of questionnaires were used in the study: the initial questionnaire and the symptom questionnaire. The initial questionnaire was designed to examine the previous experience of discomfort related to reading and computer use and to detect unknown cases of photosensitive epilepsy or migraine. The first part of the questionnaire was divided into three sections. Each section listed possible visual and non-visual symptoms of discomfort that include eyestrain, blurred distance vision, blurred near vision, tendency to skip words or letters, glare, headache, top neck ache, bottom neck ache, backache and dizziness. The first section asked the subjects to rate the frequency of experienced symptoms during reading and computer use conditions by using Likert scale describing the frequencies as always, occasionally, rarely and never. Then, the second section question the average time in minutes before noticing each symptom following continuous reading or using computer. The last section determined the required time for each symptom to disappear after ceasing working or taking analgesic. The second part of this questionnaire was to identify migraine and photosensitive epilepsy. It included questions about the preference of reading text with colored overlay or tint lenses besides four grating patterns varied in size to be presented to the subjects for 10 seconds, and then they asked to rate the discomfort by Likert scale to describe the responses to these grating patterns charts as no effect, mild, moderate and high. The symptom questionnaire was designed to evaluate visual comfort immediately before and after reading for comparison purpose. The first two questions asked to rate the general visual comfort and the clarity of vision, while the remaining questions were to estimate the severity of eyestrain, glare, headache and dizziness. Visual Analogue Scale (VAS) was used to determine the level of each symptom, which requires indicating the intense of each symptom by placing a mark on a line with extreme limits at each end. The amount recorded as the ratio of the measurement of the line from the left of the line to the subject’s mark to the total line measurement. To find the effect of light on reading discomfort in each lighting condition, the difference between before and after reading was computed by subtracting the amount of severity after reading from that in pre reading questionnaire. Finally, the effect under each situation was compared with the control situation by calculating the difference. These differences were obtained by subtracting the effect of light on reading comfort in AC rated on 50 Hz, 75 Hz, 100 Hz and 140 Hz from that in DC condition. Results A total number of 18 subjects (2 male and 16 female) completed the study. The mean age of the subjects was 23.1 ? 3.9 years. Of them, 9 subjects had normal vision at near without any requirement for visual aids, while the remaining 9 had to wear their habitual glasses or contact lenses to perform the task. Moreover, all the participants reported that they did not have migraine or photosensitive epilepsy, which was confirmed by their responses in the initial questionnaire to the grating patterns and whether tinted glasses and colored overly are preferred during reading. Discomfort Associated with Reading vs. Computer Use 1- Incidence of Discomfort Symptoms The frequency of discomfort symptoms in reading and using computer was studied by analysing the responses toward the initial questionnaire. Figure 1 shows the frequency of discomfort because of prolonged reading or working on computer. In terms of sustained reading, only 1.1 percent of subjects reported continuous frequency of discomfort symptoms, 12.8 percent reported occasional frequency, 23.9 percent stated that they experienced these symptoms rarely, while majority (62.2 percent) of them reported that they had never experienced discomfort symptoms because of continuous reading. Similarly, a minor proportion of the participants (5.5 percent) reported continuous frequency of discomfort symptoms because of prolonged work on computer, 16.1 percent stated that they had these problems occasionally, 26.7 percent stated that they experienced these symptoms rarely, whereas majority (51.7 percent) of them reported that they had never had these symptoms before. As shown in figure 1, similar pattern was observed in both situations where recurrent symptoms of discomfort were not induced. However, using computer was more related to recurrent discomfort than reading but the difference was insignificant. Figure 1. The frequency of discomfort after sustained reading and using computer In spite of overall similarity between reading and using computer regarding the incidence rate of discomfort, each condition probably stimulates different sets of discomfort symptoms. Figure 2 and 3 show the incidence of each discomfort symptom in reading and using computer. In terms of reading, results indicated that apart from the overall complaints, only two symptoms were reported equally as recurrent symptoms; blurred vision at distance and top neck ache (20 percent each). On the other hand, prolonged use of computer was linked to blurred vision at distance and bottom neck ache (20 percent each). Smaller proportion of other symptoms were also described as frequent symptoms, which include eyestrain, tendency to skip words, glare, headache, top neck ache and back ache (10 percent each). Interestingly, glare was the only symptom that neither experienced in reading nor in using computer with a proportion of 13.4 and 15 percent, respectively among other complaints that were described as never been experienced. Figure 2. The incidence of discomfort symptoms after sustained reading Figure 3. The incidence of discomfort symptoms after sustained working on computer 2- Time Taken for discomfort symptoms to Appear The second aspect of previous discomfort experience of the participants was the time taken during reading and using computer to notice symptoms of discomfort. In general, discomfort during the first hour of working in both conditions is reported less compared to longer periods. Figure 4 shows the time taken for discomfort to appear in both conditions. As shown in figure 4, during the first half hour, the average percentage of discomfort complaints was 5 and 9 percent in reading and using computer, respectively. In addition, discomfort experienced was less at the beginning of work, and then slightly increased after 30 minutes, before increasing rapidly following 2 hours of prolonged reading and working on computer. Furthermore, both conditions showed similar pattern of discomfort incidence along the time. In fact, results indicated that 63 percent of complaints were after 2 hours of prolonged reading and using computer. Thus, there appeared higher chance of discomfort after 2 hours of continuous work. Figure 4. The time taken for discomfort to appear in reading and using computer Figure 5 and 6 show the time taken for each discomfort symptoms to appear in reading and using computer, where x is the period of reading or using computer in minutes. As shown in figure 5 and 6, the results indicated that reading and working on computer tend to stimulate similar symptoms of discomfort during the time. For example, blurred vision at distance was the first detected symptoms during the first hour. This accounted to 24 and 33.3 percent of complaints caused at this period of reading and using computer, respectively. On the other hand, dizziness was the least likely symptom that may appear before two hours of working. The proportion of dizziness after two hours among other complaints was the highest and accounted to 13 and 15 percent in reading and using computer, respectively. Figure 5. The time taken for discomfort symptoms to appear during reading Figure 6. The time taken for discomfort symptoms to appear during use of computer 3- Time taken for discomfort symptoms to disappear The last section in the initial questionnaire analysed the required time for discomfort symptoms to disappear. On average, discomfort symptoms after ceasing reading, using computer or taking analgesic required about 15 to 30 minutes to disappear. Figure 7 shows the time taken for discomfort to disappear in both conditions. In case of reading, about 33.3 percent of the subjects reported 15 to 30 minutes for discomfort symptoms to disappear, while in computer use situation, about 39.7 percent reported the same time. Furthermore, majority of the subjects reported less than 15 minutes time required for discomfort to disappear in reading situation. As shown in figure 7, about 27.4 percent of the subjects reported less than five minutes and about 24.2 percent reported less than 15 minutes for discomfort to disappear in reading situation. In computer use situation, about 25.3 percent of the subjects reported less than five minutes and about 14.4 percent reported five to less than 15 minutes for discomfort to disappear. Therefore, there appears an overall pattern of time required for discomfort alleviation after reading and using computer; however, the time required is lower in case of reading. Figure 7. The time taken for discomfort to disappear in reading and using computer Analysing discomfort symptoms individually in each situation revealed that there were some differences in the nature of recovery despite the overall similarity between the two conditions of working. Figure 8 shows the time required for every symptom to disappear, where (y) is the time taken after ceasing reading or taking analgesic in minutes. About 23.8 percent of the subjects equally reported glare and blurred near vision symptoms that quickly relieved in less than five minute, whereas about 40 percent of the subjects reported backache and top neck ache symptoms that required more than an hour to disappear. Figure 9 shows the time required for every symptom to disappear, where (y) is the time taken after ceasing computer use or taking analgesic in minutes. About 23.8 percent of the subjects reported glare as the symptom that quickly relieved in less than five minutes, whereas about 40 percent of the subjects reported top neck ache symptom that required more than an hour to disappear. Thus, in both situations, glare was the first symptom to relieve in less than five minutes, whereas top neck ache was the last symptom that requires more than an hour to disappear. Figure 8. The time taken for discomfort symptoms to disappear Figure 9. The time taken for discomfort symptoms to disappear Effects of LEDs on Reading Comfort The symptom questionnaire was analysed to investigate the effects of LED light on reading comfort. In general, the noticeable effects were shown for the frequency of 50 Hz compared to others. To find the final change of discomfort, the mean amount of discomfort symptoms under 50 Hz, 75 Hz, 100 Hz and 140 Hz flicker were subtracted from that in the control condition (DC light). Figure 10 shows the changes of individual symptoms compared to the control condition. Of all discomforts symptoms, vision comfort was the most that apparently influenced by 12 percent of increase under 50Hz condition compared to DC situation, while that was 2 percent under 75 Hz and 100 Hz and 3 percent under 140 Hz. The second affected symptom was eyestrain, and the amount of difference to the control situation was found 8 percent higher under 50 Hz and 75 Hz, and only 2 percent higher under 100 Hz and 140 Hz. Blurred vision and headache was similarly influenced under 50 Hz by 5 percent increase. Under the other lighting condition, the amount of blurred vision was 1 percent lower under 75 Hz and 2 percent higher under 100 Hz and 140 Hz, whereas the amount headache was 1 percent higher under 75 Hz, 100 Hz and 140 Hz. On the other hand, dizziness showed variable pattern among the lighting conditions. It was 3 percent higher under 50 Hz, 1 percent under 75 Hz, no effect under 100 Hz and 3 percent under 140 Hz. Finally, glare was not actually influenced by the light frequencies. The mean amount of glare in the subjects was 1 percent higher under 50 Hz, 1 percent lower under 75 Hz, no effect under 100 Hz and 1 percent lower under 140 Hz. As shown in figure 10, most of the symptoms show individual differences among the subjects. For example, one of the participants reported an increase in blurred vision by about 92 percent in 50 Hz situation than that in the control condition. Another example of variation is the range of differences in uncomfortable vision in 100 Hz situation was from 25 percent increase to 12 percent decrease compared to that in the control condition. Thus, there appears that individual variation of the results is considerable. Thus, the results suggest that flicker at 50 Hz frequency driven from LED lamps may induce discomfort symptoms during reading. The most affected symptoms of discomfort were vision discomfort followed by eyestrain, blurred vision and headache. On the other hand, the mean difference of 75 Hz, 100 Hz and 140 Hz compared to DC lighting situation was not significant. Therefore, despite the individual variations, there appears that LED lamps powered in moderate AC may not have a significant effect on reading comfort. Figure 2. The mean difference of discomfort symptoms over the conditions of flicker compared to light driven from DC Discussion One of the purposes of initial questionnaire was to explore the subjects’ previous experience of discomfort during reading and using computer. Interestingly, the participants were able to distinguish between both conditions and they had the ability that determines the individual symptoms of discomfort under each condition. This association has been a concern for several studies based on the suggestion that working on computer might produce higher level of fatigue than reading from printed material. Based on the responses to the initial questionnaire, the results show insignificant statistical differences between reported discomfort that induced by reading or using computer. Similarly, several studies have shown that there was no difference in discomfort associated with reading or using computer. An earlier study by Knave et al48 compared discomfort symptoms between groups of visual display terminal (VDT) operators and office workers who were not exposed to VDT. They found a similarity in reported discomfort between VDT operators and newspapers office workers, as 58 percent of each group reported visual symptoms. Howarth and Istance also found no difference in visual discomfort symptoms as a result of performing VDT and non-VDT tasks over 5-day period49. A recent study conducted by Chu et al50 found that the level of discomfort symptoms induced by sustained reading from computer is significantly worse than that produced by printed text under same viewing conditions. However, this difference was statistically insignificant except for near blurred vision. In addition, the subjects were aware of the two conditions and not masked, which perhaps influenced their answers by reporting worse symptoms associated with computer. Therefore, there appears that although the present results were based on recalling previous feelings and self-rating symptoms rather than experimental results, the findings were consistent with a number of previous studies that indicated considerable similarities between discomfort caused by sustained reading and computer use. This was important to explore because reading from computer screens has becoming a usual part of learning and working in the current society, and it is more likely to replace reading from printed materials. The main aim of this experiment is to investigate the effects of using LEDs lamps as a source of light during reading. The study hypothesises that when a reading material is illuminated by LEDs lamp powered on AC, produced flicker may influence reading comfort. For reality check, DC was used to supply LEDs lamps as a control situation, since it produces constant illumination that does not produce flicker. The findings suggested insignificant differences in the mean of discomfort symptoms between LEDs powered by DC situation and the AC lighting situations of 50 Hz, 75 Hz, 100 Hz and 140 Hz. One of the main observations is that vision comfort, eyestrain, blurred vision and headache were greater in 50 Hz condition than that in the control and other lighting situations. This means that reading under LEDs lamp driving flicker at 50 Hz may increase the level of uncomfortable vision, eyestrain, blurred vision and headache, however, these obvious differences failed to reach statistical significance. Although several lighting conditions were used ranging from constant illumination to gradually increased frequencies for the purpose of comparison, the most important one is 100 Hz that is produced currently for AC. Therefore, the hypothesis was that moderate frequencies of 100 Hz might increase the level of discomfort symptoms. However, the findings showed insignificant differences in the average level of discomfort among all lighting situations of low to moderate frequencies compared to DC condition. Another considerable observation is the individual variation of the results among the subjects. The effects on reading comfort were individually different over the lighting conditions. This was expected since human sensitivity to flicker was found variable within age group25. In addition, early findings44 also showed individual differences among the subjects with relatively small sample size, which indicates the consistency. Thus, it seems that there is a need to increase the sample size in order to draw firmer conclusion. Unexpectedly, the main results are not consistent with the findings of early experiments that linked between intermittent light and reading discomfort. One of the main studies in this field was conducted by Wilkins et al43. Methodologically, their sample size was much larger with 159 office workers over a period of approximately six months. They recorded symptoms of headache and eyestrain against every day of working days to determine the incidence and severity of discomfort symptoms over the week, whereas in the present study the reading period was 30 minutes for each lighting condition. In addition, the responses to the initial questionnaire showed only 5 percent of the subjects reported noticeable discomfort at the first half hour of working. Therefore, increasing reading time is probably required to trigger discomfort symptoms, and it seems more realistic to repeat the reading sessions over a period of time rather than one day. Another possible explanation of the findings is that the illumination was directed to the reading material in the central field, while in Wilkins et al43 experiment, the illumination was peripheral by installing fluorescent lamps in the offices. In contrast, Berman et al29 detected ERG responses when the light was directed to the central field by placing fluorescent lamps in front of the subject one meter away, that substitute an angle of 7 by 53 degrees to emit intermittent light varied in frequencies. Their findings indicated that intermittent light directed centrally had a potential to influence visual system. Therefore, this shows that 100 Hz light in the present study may influence visual system, but could not induce higher amount of discomfort symptoms compared to the control condition. Thus, further studies are required to understand the relationship between reading comfort and the direction of illumination in relation to visual field. In fact, identifying this association would be useful for students and office workers to determine the suitable illumination that probably has less impact on reading comfort. Some possible limitations need to be addressed regarding this study. First, the sample size was relatively small containing only 18 subjects. Increase in the sample size may minimise the effect of variability amongst individuals. Furthermore, the ratio of male to female was 2 to 16 in this study, which may bias the results. Although, evidence shows that no significant differences in the critical fusion frequency (CFF) were found between male and female26, findings of a study conducted by Lindner and Kropf suggests that females have a tendency to experience asthenopic symptoms when working under fluorescent illumination51. Therefore, larger sample size and balance gender ratio would be required to permit generalisation of results to population. Conclusion The present study shows that reading under 100 Hz LEDs illumination has no different effects on comfort than reading under LEDs light powered on DC. In addition, analysis of previous experience of the subjects shows insignificant difference between discomfort induced by reading and computer use in terms of incidence and nature. Further research may be required with larger and balanced sample size. In terms of methodology, it is suggested to involve longer period of reading time or several reading sessions under each lighting conditions. In addition, investigating the link between reading discomfort and direction of illumination is recommended because it has beneficial implications. This suggests that optometrist can play an important role to provide advices related to illumination for individuals who have intense readings as a part of daily life, like students and office workers. Read More