Characterization of a compact low-cost Stokes lens for astigmatism compensation in optical instruments.

Variable power cross-cylinder lenses (or Stokes lenses) have been widely known in the literature for decades. In this paper, we describe how to build a low-cost Stokes lens and discuss its calibration and its application to two significant cases. The construction is in virtue of a phoropter's Risley prism mount for assembling a couple of equal but opposite sign cylindrical lenses (we have selected $\,\pm 1.50$±1.50 D). Thus, variable astigmatic power is achieved by relative rotation of the lenses in opposite directions, and the resulting astigmatic axis is defined by the global rotation of the device. Calibration measurements are performed using an automatic lensmeter (Topcon CL-300) and an aberrometer (Zeiss iProfiler plus) for low and high order, respectively, aberration characterization. The proposed device has been adapted to a manual Topcon LM-8 lensmeter and to a regular Olympus BX-60 upright microscope for experimental validation concerning astigmatism compensation in a digital microscope and astigmatism cancellation in ophthalmic lenses, respectively. The device can be easily adapted to any ophthalmic/optic instrument for the compensation and/or measurement of astigmatism up to a maximum range of $|3|$|3| D of astigmatism.