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Joan K. Portello, Olga Kochurova, Mark Rosenfield; Is the 3x rule appropriate for computer users?. Invest. Ophthalmol. Vis. Sci. 2012;53(14):5461.
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When reading from a computer monitor or other form of electronic display, many patients report visual symptoms such as tired eyes, blur or eye strain. In order to minimize these symptoms, it has been suggested that a 3x acuity reserve should be adopted for comfortable reading, i.e., the visual acuity should be 3 times better than that required to read the text on the display. Therefore, prolonged viewing of a 6/18 letter would require visual acuity of at least 6/6. However, there does not appear to be any objective evidence to support the 3x rule for sustained reading. While it does seem reasonable to assume that many patients will not be comfortable reading text which is close to their acuity limit for a sustained period of time, the aim of the present study is to evaluate whether the 3x rule is appropriate, or if an alternative relationship between threshold visual acuity and letter size for sustained reading would be superior.
The study was performed on 25 visually-normal, asymptomatic young subjects who viewed a series of random words on a laptop computer monitor. The furthest distance at which the subject could just resolve the text (Times New Roman 12 point font) was determined. Subsequently, both reading speed and reading accuracy were measured during a 10 min test period performed at both the threshold distance and at 0.5, 0.33 and 0.25 times this threshold distance.
The mean threshold distance was 131.4cm. Mean reading speed at viewing distances of 1.0, 0.5, 0.33 and 0.25 times threshold were 63.4, 117.6, 128.5 and 131.1 words per minute, respectively. While the latter 3 values were significantly different from the threshold distance (p<0.0001), they were not significantly different from each other. The mean total number of reading errors made during the 10 min task at distances of 1.0, 0.5, 0.33 and 0.25 times threshold were 11.3, 1.6, 1.3 and 0.8, respectively. Again, the latter 3 values were significantly different from the threshold distance (p<0.001) but not from each other.
The results indicate that for visually-normal subjects, a 2x rule is appropriate, i.e., for sustained comfortable reading, the text size should be at least twice the individual visual acuity. However, higher values may be necessary for more prolonged or demanding tasks, or in the case of individuals with visual abnormalities.
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