Phoropter Product

Overview

The phoropter is the lens-bank instrument the optometrist swings in front of a patient's face and asks "which is better, one or two?". It holds more than a hundred ground glass lenses on rotating discs inside two cast housings, one per eye, and lets the examiner present any combination of spherical power, cylindrical power and axis, and prism in fractions of a second — far faster than swapping loose trial lenses. The design is essentially unchanged since the 1920s and is purely mechanical: knobs, geared discs, and detents, no electronics and no power.

Each eye looks through one battery of the Main Body, with the Sphere Lens Bank and Cylinder Mechanism providing the correction, the Jackson Cross Cylinder Unit and Risley Rotary Prism units providing the refinement and binocular tests, and the Auxiliary Lens Disc supplying occluders and special lenses. The PD & Convergence Mechanism mechanism matches the instrument to the patient's pupillary distance, the Near-Point Rod handles reading-distance tests, and the whole 5 kg instrument hangs from the stand by the Mount Arm Interface.

Sphere and cylinder

Spherical power comes from two stacked discs per eye. The Strong Sphere Disc carries high-power lenses in coarse steps; the Weak Sphere Disc adds 0.25 D increments. Geared together behind the Sphere Control Knob, they sum to any value from +16.75 to -19.00 D, with a lever jumping ±3 D blocks during fogging. Every Sphere Lens Cell is an individually surfaced, coated glass lens, and a Detent Click-Stop snaps each disc so the active lens centres exactly on the viewing aperture — a lens decentred by even a millimetre induces unwanted prism.

Astigmatic correction comes from the Cylinder Lens Disc, holding minus-cylinder lenses from 0.25 to 6.00 D in geared rotatable cells. The clever part is the axis drive: every Cylinder Lens Cell meshes with the Axis Ring Gear, so turning the Cylinder Axis Knob rotates whichever cylinder is in place — and all the others — to the same axis. Power changes via the Cylinder Power Knob therefore never disturb the axis setting. American practice uses minus-cylinder instruments; plus-cylinder versions exist mainly for ophthalmological convention.

Refinement and binocular testing

Cylinder axis and power are refined with the Jackson cross cylinder. The Cross Cylinder Lens combines +0.25 and -0.25 D at perpendicular axes (net zero sphere), mounted on a JCC Flip Axle at 45° to those axes. Flipping it presents two mirrored astigmatic states; the examiner straddles the current axis setting, asks which flip is clearer, and walks the axis toward the preferred side, then repeats with the lens axes aligned to refine power. Because the comparison is two equally blurred alternatives rather than blur versus sharp, the endpoint ("both about the same") is precise to a few degrees.

The Risley Rotary Prism implements the Risley principle: two identical Prism Element wedges counter-rotate through a Helical Gear Pair, so their deviations add anywhere from zero to 20 prism diopters as the Prism Power Dial turns. Combined with the Maddox Rod on the auxiliary disc, this measures phorias — latent misalignments of the two eyes — and vergence reserves. The Occluder blanks the untested eye, the Pinhole Disc separates refractive blur from ocular pathology, the Retinoscopy Lens offsets the examiner's 67 cm working distance during objective retinoscopy, and the Polarizing Filter pairs with polarized charts so each eye sees different lines during binocular balance.

Fitting the patient

The PD Leadscrew, threaded left- and right-hand on opposite ends, spreads both batteries symmetrically across a 48–75 mm pupillary distance set on the PD Adjustment Knob. Vertex distance — cornea to lens plane, reference 13.75 mm — is set with the Forehead Rest and checked through the Vertex Distance Scale, because a strong lens moved away from the eye changes its effective power; prescriptions above about ±4 D must be vertex-corrected. The Levelling Bubble Vial and the Levelling Adjuster on the mount keep the 180° axis truly horizontal, without which every measured cylinder axis is rotated.

For near testing the examiner drops the Calibrated Near Rod, slides the Near Vision Card to 40 cm in its Reading Card Holder, and throws the Convergence Lever so both batteries toe in on their Convergence Hinge pivots toward the near point — letting presbyopic add power be tested with the eyes converged as they would be in real reading.

Role today

Autorefractors provide the objective starting point, but the phoropter remains where the final prescription is decided, because only the patient's subjective judgement integrates optics, neural processing, and preference. Digital phoropters motorize the same lens discs under console control; the optical architecture inside is unchanged.