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Accuracy and precision of the Tomey ViVA infrared photorefractor
1Section of Neurobiology and Behavior, Comeli University, Ithaca, New York, USA.
Purpose:
The Fortune Optical (Tomey ViVA) VRB-100 video refractor was tested to determine its accuracy and precision in measuring manifest refractions of human eyes with and without cycloplegia. The specific issues addressed included its accuracy in measuring spherical and cylindrical refractive errors and its precision in refracting near emmetropia.
Methods:
To determine its ability to measure moderate to high (> 4.00 D) myopia, we compared the VIVA's refractions to those taken by a Canon Autorefractor R1 and a retinoscopist. A spherical lens series from -7.00 to + 7.00 D at 1.00 D intervals, or -5.00 to + 5.00 at 0.50 D intervals, was placed over a subject's eye, which was then covered by an infrared (IR) filter, refracted, and analyzed to determine the VIVA's ability to measure spheres. Subjects with refractive errors of -2.00 to + 2.00 DS (diopters sphere) and 0 to 1.00 DC (diopters cylinder) were refracted 7 to 15 times during 1 sitting to determine the VIVA's precision. The instrument's accuracy in measuring cylinders was tested by placing + 3.00 to -3.00 D cylinders (at 0.50 D intervals) over the eyes of subjects at 0 degree, 45 degrees, 60 degrees, 75 degrees, 90 degrees, 105 degrees, 120 degrees, and 135 degrees.
Results:
The VIVA measured spheres of +/- 3.00 D with a root mean squared (rms) error of 0.5 +/- 0.1 D. Beyond this power, its accuracy progressively worsened. In some subjects, irregular intensity profiles compromised the VIVA's accuracy even with low spherical refractive errors. The VIVA was very precise in measuring spheres from + 2.00 to -2.00 D and cylinders from 0 to 1.00 D. Although the ViVA adequately measured all cylinder powers at 0 degree and 90 degrees, the accuracy of cylindrical power measurement decreased with obliquity; only cylinders < or = 1.00 D magnitude were accurately measured at 45 degrees and 135 degrees.
Conclusions:
We conclude that, although the ViVA offers many attractive features for vision screening, it is seriously limited by its inability to property detect and measure oblique astigmatic errors.