Technical Report: A New Method for Determining Image Distance for Stand Magnifiers.

SIGNIFICANCE: Low vision rehabilitation clinicians must sometimes evaluate stand magnifiers to determine their true optical properties. A novel method is described for measuring the image distance of stand magnifiers. PURPOSE: We describe a new method for determining the image distance of stand magnifiers, which has some advantages over previously described methods. METHODS: Diverging light emerging from a stand magnifier is brought to a focus on the ceiling or other flat surface using convex lenses placed on the top of the stand magnifier lens. Knowing the power of this convex lens and the distance from the magnifier lens to the imaging surface, one can calculate the degree to which the emerging light is diverging and, from that, the image distance. Each author evaluated three stand magnifiers using this method and compared our results with each other and with the values for image distance published by the manufacturer. RESULTS: Our method produced measurements that were consistent between three clinicians, with emerging divergence values differing by no more than 0.37 D and image distance differing by 0.6 to 3.1 cm for the three magnifiers evaluated. Our measured values also corresponded favorably with the published image distances listed in the manufacturer's catalog, deviating by a maximum of 3.7 cm and a mean of 1.3 cm. When the manufacturer's image distances were converted to dioptric divergence, our measurements varied by a maximum of 0.48 D and a mean of 0.17 D. CONCLUSIONS: Our method provides an expedient and clinically accurate way to evaluate the divergence and image distance of a stand magnifier. Knowing the image distance is valuable by itself, but the divergence of the emerging light is also critical in determining the stand magnifier's enlargement ratio.

The effect of prism on preferred retinal locus.

BACKGROUND: Whether prism, especially base-up prism, affects the area of the retina used for fixation in a patient with central scotoma has been a controversial subject for 35 years. Our pilot study employed microperimetry to evaluate the effect of base-up prism on the fixation locus, or preferred retinal locus (PRL), in subjects with central scotoma. METHODS: We used a microperimeter to assess the PRL in 13 visually impaired subjects with central scotoma under four conditions: no lens, a lens with no prism (control lens), 6Δ base-up, and 10Δ base-up. The PRL was measured in degrees in horizontal and vertical co-ordinates from the centre of the optic disc using graphical analysis. RESULTS: The PRL with the control lens was not significantly different from the PRL with no lens. The preferred retinal loci with the two powers of prism were compared to the control lens and showed a superior shift in 22 of 26 cases (84.6 per cent). The amount of movement was significantly different from zero (p = 0.001 for 6Δ and p = 0.004 for 10Δ ). The vertical movement with the 10Δ prism (1.73 ± 1.73 degrees) was not significantly greater (p = 0.562) than with the 6Δ prism (1.37 ± 1.08 degrees). The shift was significantly less than the prism powers used (p < 0.001), and the amount of vertical relocation was not significantly different from the amount of horizontal movement. CONCLUSION: In our study, base-up prism appears to shift the PRL in the direction of the prism base most of the time, but our findings do not support the use of prism as a way of predictably relocating the PRL. More study is indicated to evaluate whether such a small shift is clinically or functionally significant.

Comparing LEA numbers low vision book and Feinbloom visual acuity charts.

PURPOSE: This study aims to determine the accuracy and repeatability of visual acuity measurements taken with the LEA numbers low vision book (LNLVB) and the Feinbloom chart in visually impaired subjects. METHODS: Distance visual acuities were taken with the LNLVB, the Feinbloom chart, and the Early Treatment of Diabetic Retinopathy Study (ETDRS) chart, then repeated 15 to 30 min later to evaluate for inter-test repeatability. To determine threshold, participants were encouraged to guess until they could not identify any optotypes. The total number of correctly identified optotypes was recorded and extrapolated to logMAR on all three charts. The Feinbloom chart and LNLVB were evaluated for correlation to ETDRS. Nine visually impaired children and nine visually impaired adults participated in the study. RESULTS: In comparing the LNLVB to ETDRS, the 95% limits of agreement were +0.099/-0.240, and the correlation coefficient (R) was 0.953 (p < 0001). The same comparison for the Feinbloom chart vs. ETDRS was +0.169/-0.322 with r = 0.905 (p < 0.0001). On test-retest comparisons, the ETDRS was highest with the 95% limits of agreement of +0.117/-0.128. LNLVB and Feinbloom were nearly identical at +0.159/-0.200 and +0.184/-0.202, respectively. The R value for ETDRS was 0.976, LNLVB was 0.954, and Feinbloom was 0.942 (p < 0.0001 on all three). CONCLUSIONS: The results of our study indicate that there is a slight advantage, in terms of agreement with ETDRS, of LNLVB over the Feinbloom chart when testing visually impaired patients. The two tests proved nearly identical in terms of repeatability in a low-vision setting. Both handheld charts measured slightly worse acuity than the ETDRS chart, but overall proved to be suitable alternatives when the ETDRS chart is not available or practical in certain clinical situations.