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  • A total of 25 volunteers aged between 25 and 39 years (median 29.5) were recruited into this study. The cohort included male subjects only. Written informed consent was obtained from each participant prior to data collection. The study was approved by the institutional review board of the Medical University of Graz, Austria. All subjects were asymptomatic, did not wear contact lenses, had normal ranges of accommodation and did not have any type of ocular disorder. None of the subjects used eye drops or were taking any systemic medication. The temperature and humidity of the room were controlled within the ranges of 17–20°C and 60–70% relative humidity at an illumination of 200 lux. Two order picking sessions, each lasting over 10 hours, took place on two consecutive days. In one session the subjects used a visually-guided system and in the other they used a voice-guided system. All the subjects took part in both sessions. In the visual-guided condition, the operators wore a head-mounted see-through display and received instructions through the display to their right eyes. Knapp KiSoft Vision: See-through-display (SVGA 800*600 Pixel, up to 23mm distance to the eye), microcamera (800 x 480 Pixel, 30 frames per second). In this version the display was not changeable to the left side. “Fig 1” Figure data removed from full text. Figure identifier and caption: 10.1371/journal.pone.0157564.g001 Visual guided system. In the voice-guided condition, the operators used a wireless, wearable computer with a headset on both ears “Fig 2” and were instructed via the headset. Figure data removed from full text. Figure identifier and caption: 10.1371/journal.pone.0157564.g002 Voice guided system. Subjective dry eye and asthenopic symptoms as well as objective ocular surface and tear function parameters of the participants were measured in a baseline examination and at the end of the commissioning sessions on each of the two days. Subjective dry eye symptoms (i.e. the sensation of dry eye, ocular fatigue, ocular pain, foreign body sensation, irritation, burning sensation, and itchiness) of all participants were assessed using the tests described below. The visual analogue scale (VAS) is a measurement instrument for subjective symptoms. We used a scale of 0–100. Each participant marked their level of ocular discomfort; 0 to indicate no symptoms and 100 to indicate severe discomfort. The VAS score is determined by measuring the distance between the left end of the line and the point marked by the subject. The FACE scorecard scale ranges from 1–9. It shows images of faces with different expressions. For example, the saddest face (score 9) describes severe discomfort, and the happiest face (score 1) represents no symptoms. [1, 2] The effect of working with these systems on asthenopia was tested with a questionnaire based on Stüdeli, 2001. [3] The original questionnaire was designed for visual display unit (VDU) users for investigation into eye discomfort, which is the second-most frequent problem reported by VDU operators. [4, 5] Analyses of the objective parameters were performed non-invasively for both eyes with a Keratograph 5M (manufactured by Oculus GmbH, Wetzlar, Germany) in the order listed below. The tear meniscus height was measured manually with an integrated ruler. Tear film break-up time: the time from a complete blink to the initial tear film break-up was measured with the Keratograph. The pre- and post-task break-up times were recorded. Redness of the conjunctiva: the bulbar redness (nasal, lateral, and total) and the limbal degree of redness (nasal, lateral, and total) were automatically and objectively measured and classified with the R-Scan module of the Keratograph 5M. The R-Scan module detects thin blood vessels in the conjunctiva and evaluates the degree of redness, using the following scoring scale: 0, no redness; 1, single conjunctival injections, 2, minor, scattered injections; 3, enhanced local injections; and 4, enhanced scattered injections. [6]
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