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.
We describe a new method for determining the image distance of stand magnifiers, which has some advantages over previously described 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.
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.
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.