Abstract
Purpose :
As part of a study to determine the efficacy of head-worn assistive-technology (AT) vision devices, we examined the relationships between nominal and physical magnification of four ATs: eSight, IrisVision Inspire, and Eyedaptic EYE 3 and 4. The nominal magnifications for these devices range from 1–24 for eSight and from 1-10 for the others; however, the ranges of physical magnification are unknown.
Methods :
Because the small displays of these devices do not allow easy direct measurement, we used change in visual field extent (VFE) as a surrogate for magnification. Four control subjects with <20/20 acuity and full VFs participated. For each device, we measured dynamic VFE for each subject at each nominal magnification. Magnification was calculated as the ratio of VFE at each nominal value to that at the nominal value of 1.
Results :
The averaged data for eSight were best fitted by a function of y = 0.90e0.13x; actual magnifications were less than nominal, reaching ~5x at a nominal magnification of 12x. The relationship was logarithmic for IrisVision (y = 0.70e0.20x), reaching ~5x magnification at the highest nominal value of 10. The relationships were linear for the Eyedaptic devices (y = 1.04x - 0.27), where the nominal value equaled the actual magnification.
Next, we compared the relationship between acuity gain and actual magnification for each device. We measured near visual acuity for 15 participants with low vision from various etiologies at baseline and when wearing each device. For each device, participants were asked to select the optimal nominal magnification that allowed them to read the most letters on the chart. The order of device testing was counterbalanced across subjects. LogMAR acuity was determined based on the number of letters read, and equivalent magnification was calculated using the formula 0.99*EXP(-2.25*D), where D is the difference in logMAR acuity between baseline and with a device. We found that the gain in letter chart acuity was always less than expected based on the magnification provided by the devices, with a relative saturation of acuity gains with actual magnifications greater than 5x. This might be due, in part, to magnification-related increased visibility of the anti-aliasing blur effects.
Conclusions :
These results emphasize the need to validate the specifications of AT devices and understand the effects of image manipulation on visual function.
This abstract was presented at the 2023 ARVO Annual Meeting, held in New Orleans, LA, April 23-27, 2023.