Evaluation of stereoacuity with RDS of different size
A novel stereopsis measurement system was developed, using a phoropter (VT-10, Topcon Corp, Tokyo, Japan) and two Sony smartphones (Sony Xperia Z5 Premium Dual E6883, Sony Mobile Communications Inc. Tokyo, Japan) (Figure 1). At a test distance of 65 cm with this system, a 1-px (pixel) disparity of two images equates to an angular disparity of 10” (arcsec, second of arc), which makes it possible to test stereoacuity with a resolution of 10”.
A program written in C# was used to produce all test targets. The test symbol imitated the Random Dot 3 Stereo Acuity Test (Vision Assessment Corporation, Illinois, USA). Lea symbols (house, circle, square and apple) were used as the test targets, solid filled, hidden in random-dot stereograms. The random dots were chosen to be square rather than circular because of the limitations of the screen pixel arrangement of the smartphones. Black dots and white dots constituted test pictures, and the proportion of them was 1:1 (dot density 50%).
Three dot sizes were used to construct the random-dot arrays, including 1-px (1×1 px, equivalent 0.17 min arc at 65cm, similarly hereinafter), 6-px (6×6 px, 1 min arc), and 10-px (10×10 px, 1.68 min arc). Each test pattern included a symbol hidden within it. Two test images were created. The first image included four lines ranging from 8-px disparity to 5-px disparity, while the second image included four lines ranging from 4-px disparity to 1-px disparity. The detail was shown in Figure 2.
After tested with an auto-refractor (KR-8900PA, Topcon Corp, Tokyo, Japan), the subjects were examined with a phoropter to determine the diopter and the best corrected visual acuity ( BCVA), after which the two 4k smartphones were attached to the near vision rod. The test distance was 65cm. With the aid of two 5.5Δ base out Risley prisms, two smartphones could be fused as one image (Figure 1). The subjects were asked to identify the forms hidden in the random dot stereograms, from left to right, top to bottom. The last correct answer for each size of dot was recorded as the stereoacuity of the subject.
A total of 30 subjects were enrolled, 11 men and 19 women, with ages of 20‒31 (25.3 ± 3.8) years. The corrected visual acuity of each eye was at least 0 logMAR, and the stereoacuity was at least 40", as measured by the Fly Stereo Acuity Test. The study was conducted at the Second Hospital of Jilin University in China. The research protocol followed the tenets of the Declaration of Helsinki, and was approved by the ethics committee of the Second Hospital of Jilin University (No. 2017-89).
All data were processed using PASW Statistics 18.0 (IBM SPSS Inc.). Because the data did not satisfy tests for normality of distribution, nonparametric tests were carried out. The Friedman test was used to analyse the differences among the groups, with P<0.05 used as the threshold for statistical significance. The Wilcoxon signed ranks test was used to compare the differences between pairs of groups. Because three comparisons were conducted, P< 0.017 (0.05/3) was used as the threshold for statistical significance.
Measurement of the dot size of RDS utilized in the clinic
TNO stereotest, Randot Stereotest (Stereo Optical Company, Inc. Illinois, USA), Butterfly Stereo Acuity Test (Vision Assessment Corporation, Illinois, USA), Pass Test 3 (Vision Assessment Corporation, Illinois, USA), Random Dot E Stereotest, Random Dot Stereo Acuity Test (Vision Assessment Corporation, Illinois, USA), Random Dot 2 Stereo Acuity Test (Vision Assessment Corporation, Illinois, USA) and Random Dot 3 Stereo Acuity Test were chosen. A scanner (ScanMaker S260, Microtek International, INC. Shanghai, China) was used to scan the pictures in a resolution of 3200×3200. TNO stereotest was scanned directly. Other 7 tests were all based on polarizing technique, which means the tested image actually contained two polarization pictures, for which the polarization direction of them was perpendicular to each other. The test card was scanned twice covering by a polarizing glass plate. The polarizing orientation of the glass was perpendicular during the two scans which keeping the same polarization direction as original images. Two clear images would be decomposed by this manner. A 600×600 frame was used to cut the image 10 times randomly in the random dots background region, and to find the smallest short diameters of the dot in the areas. The size of the dot was measured by counting the number of px occupied in the narrowest diameter.
Specifically analyzed images selected: TNO stereotest, the 19th edition, the first image for plate Ⅴ; Randot Stereotest, the first image in large homogeneous areas; Butterfly Stereo Acuity Test, the random dot pattern for butterfly; Pass Test 3, 60” test card; Random Dot E Stereotest, card with letter E; Random Dot Stereo Acuity Test, the first image in section A page 1; Random Dot 2 Stereo Acuity Test, the first image in section A page 1; Random Dot 3 Stereo Acuity Test, the first circle in section A page 2. The measurement method was relied on subjective judgment. The form of the dots was different from test to test. For instance, the dots in the background of TNO stereotest were irregular, while the examiner should catch the smallest dot in the view window and measure the narrowest diameter.