Abstract
Purpose:
Smart phone based magnification can be a convenient and helpful tool for the elderly and visually impaired people for reading distant text, such as street signs, but usually a telescope would need to be added on to increase the magnification power. Otherwise, the images on phone screens are usually smaller than the objects directly viewed when a reasonable image resolution has to be maintained. With combination of optical and digital magnification, image shaking due to small hand movement can make it difficult to read distant text. An image stabilization function has been implemented in an iPhone magnifier app to reduce the effect of shaking in magnified images. This study investigates the effect of stabilization on reading performance of distant letters.
Methods:
Randomized triplet was shown in each trial for 0.2, 0.47 or 0.93 second on a computer screen. An iPhone 5 attached with an 8x telescope was hand-held by subjects to read the triplet from 20 feet away. In combination with digital zoom, the actual magnification of the triplet was about 7.3x for a viewing eye 15.7 inch (40cm) from the iPhone screen, and the letter size on the phone screen was about 20/100 at that distance. One young subject with central vision loss (visual acuity: 20/200, age: 29, read the letters at her habitual short distance) and 4 normally sighted old subjects (visual acuity: 20/15 to 20/25; age: 53-73) were tested, with and without the image stabilization function activated.
Results:
Overall, the reading accuracy for all the subjects was improved from 36% to 63% when image stabilization was activated (the visually impaired subject improved from 51% to 82%). On average, the accuracy without and with stabilization for 0.2, 0.47 and 0.93 second presentations was 19% vs 40%; 47% vs 69%, and 51% vs 75%, respectively. Significant effects were found for both duration (p=0.001) and stabilization (p=0.036). The interaction between the two factors was not significant (p=0.97).
Conclusions:
When viewing text in the distance through a hand-held device, such as a smart phone, image shaking problem at high magnification levels due to small hand movement reduces reading speed or reading accuracy. The image stabilization may potentially help visually impaired people and old people to improve reading performance.
Keywords: 584 low vision •
754 visual acuity