Transcript Subjects. - Pacific University

Viewing Distance & Visual Discomfort Associated With the Use of Handheld Computer Devices
Yu-Chi Tai, James Sheedy, John Hayes , James Kundart, Hannu Laukkanen
College of Optometry Pacific University, Forest Grove, Oregon
ABSTRACT
RESULTS
Handheld computer devices (HCDs) have become widely used in recent years by the general public, from game
consoles for preschoolers to handheldmed for medical profession. In this study we measured the viewing
distance while using HCDs and evaluated its effects on ocular and musculoskeletal symptoms of discomfort.
• Viewing distance at hardcopy reading was a compromised result of text resolution (the
viewing angle of the text), individual’s visual acuity and the resulted accommodative stress.
Reading 12-point font results in viewing distance of 49 cm, closed to the theoretical
accommodative demand of 2 Diopter (50 cm). As font size decreased (10-, 8-, 7-, to 6point), viewing distance became closer (47, 45, 44, 40 cm, respectively), which increased
the accommodative demand. which eventually posted a limit to the visual angle for how
much it could be increased.
METHOD
Subjects. Thirty-four young adults 19 males & 15 felames, age 18-33, mean age = 24.8) participated in this
study. They all have 20/20 or better vision for each eye, no eye strain history, and at least 6 months of
handheld use experiences. A written consent for recording their postures was obtained prior to the test of
the study.
Tasks. The subjects was asked to perform 5 typical handheld-use tasks for 10 minutes each on 3 selective
handheld devices:
• Watching an animated movie on a portable media player (Apple iPod)
• Playing jigsaws on a game station (Sony PSP 2)
• Keying text messages, searching a film database, and reading word documents on a smartphone (HP iPAQ
hw6945).
• For comparison, subjects were aslo asked to read text on hardcopy at font size 6, 7, 8, 10, and 12 point to
examine the effect of text resolution on viewing distance.
Hardcopy conditions
Handheld conditions
HC12pt
HC10pt
50
HC8pt
HC7pt
Viewing distance (cm)
Apparatus. Three handheld devices were chosen to be tested:
•
Apple iPod: 2.5” LED backlight color display, 320x240 pixels, 163 ppi, weight 4.8 oz, One thumbscroll wheel for single-hand operation.
•
Sony PSP: 4.3” TFT LCD widescreen, 480x272 pixels, 10 oz, Two thumb-operating button area.
•
HP iPAQ HW6945 smartphone: 3” TFT LCD touch screen, with LED backlight & stylus, weight
6.33 oz.
55
45
HC6pt
iPod Movie
40
iPAQ Reading
35
iPAQ text messaging
iPAQ database
searching
PSP game
30
25
1
1.2
1.4
1.6
1.8
2
2.2
2.4
2.6
Text size (x-height, mm)
• In contrast, viewing distance at handheld conditions was significantly closer (30.3 ~ 42.7 cm)
than reading hardcopy at similar or smaller sizes (F(1, 75.2)=269.246, p<.0001). Further
analysis suggests that the nature of the task (Table 1) is responsible for the close distance.
When the task was more cognitively (t(132) = -6.646, p<.0001) or visually (t(132) = 6.991,
p<.0001) demanding and required more active motor responses (t(132) = 8.514, p<.0001),
viewing distance was shorter.
• No difference was found for EMG power, though blink frequency was much lower for
handheld use than hardcopy reading.
• Subjects reported higher level of backache/neck pain and eyestrain sensation, especially
when playing games and searching database.
CONCLUSION
With brief use of handhelds, significant difference was observed at viewing distance, blink
frequency, and subjective discomfort sensation, which seem to result from the nature of the
task. Periodic resting is suggested to relieve discomfort symptoms.
Target measurements
• Viewing distance was recorded with a videocamera and measured offline by sampling one frame per min.
• Surface-electromyography (EMG) response of the right orbicularis ocular muscle (0.5~1.0 cm under the
lower eyelid) was measured as an objective index of visual discomfort.
• Subject’s subjective rating of visual and body discomfort was obtained through an analog questionnaire
(Analog Sensation Questionnaire) at the end of each task. The derived measures were compared to that in
reading novels with text at regular (12- or 10-point) or smaller (8-, 7-, or 6-point) font sizes.
ACKNOWLEDGEMENTS
This study was supported by a grant from The Advanced Reading Group of Microsoft Corporation to
James E. Sheedy.
CONTACT INFORMATION
Principal Investigator: Yu-Chi Tai (503.352.2289; [email protected])