Trade name for an accessory to be inserted in a lens aperture of a phoropter or trial frame used to refine both the sphere and cylindrical components of a refraction. It provides simultaneous viewing of a distant test object through either a plus and minus sphere lens (+0.25 and −0.25D) or through two +0.25 sphere, −0.50 cylinder lens systems with their cylindrical axes perpendicular to each other. Either mode of viewing is obtained by turning a knob, which actually rotates one of the lenses in the system. See phoropter; trial frame.
Millodot: Dictionary of Optometry and Visual Science, 7th edition. © 2009 Butterworth-Heinemann