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Asthenopia (aesthenopia) from the Greek word "asthen-opia : ασθεν-ωπία" or eye strain is an ophthalmological condition that manifests itself through nonspecific symptoms such as fatigue, pain in or around the eyes, blurred vision, headache and occasional double vision. Symptoms often occur after reading, computer work, or other close activities that involve tedious visual tasks. When concentrating on a visually intense task, such as continuously focusing on a book or computer monitor, the ciliary muscle tightens. This can cause the eyes to get irritated and uncomfortable. Giving the eyes a chance to focus on a distant object at least once an hour usually alleviates the problem. A CRT computer monitor with a low refresh rate (<70Hz) or a CRT television can cause similar problems because the image has a visible flicker. Aging CRTs also often go slightly out of focus, and this can cause eye strain. LCDs do not go out of focus but are also susceptible to flicker if the backlight for the LCD display uses PWM for dimming. This causes the backlight to turn on and off at slower intervals as the display becomes dimmer, creating noticeable flickering which causes eye fatigue. A page or photograph with the same image twice slightly displaced (from a printing mishap, or a camera moving during the shot as in (this image )) can cause eye strain by the brain misinterpreting the image fault as diplopia and trying in vain to adjust the sideways movements of the two eyeballs to fuse the two images into one. ==Causes== Sometimes, asthenopia can be due to specific visual problems, for example, uncorrected refraction errors or binocular vision problems such as accommodative insufficiency or heterophoria. It is often caused by the viewing of monitors such as those of computers or phones. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「asthenopia」の詳細全文を読む スポンサード リンク
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