bispheric

A bispheric surface has two distinct radii of curvature: one for the central zone and a different one for the peripheral zone. This creates a transition between regions rather than the smooth, uniform curve of a standard spherical lens. The boundary between zones is typically sharp, occurring at a defined diameter across the optical surface.

Bispheric lenses solve a specific problem in imaging: spherical aberration. A single spherical surface focuses light from the centre and edges at different points along the optical axis, degrading image sharpness across the field. By flattening the periphery relative to the centre, or steepening it, a lens designer can bring marginal rays into better focus without sacrificing centre performance. The technique is especially valuable in fixed-focal systems where lens adjustment is not an option.

Manufacturing and variants

Bispheric surfaces are manufactured by grinding and polishing, or by injection moulding in plastic optics. Creating the transition zone precisely is critical: a poorly defined edge causes a visible ring or halo in the image. Some designs use a gradual aspheric transition rather than a sharp step, blending the two radii over a small band. The central radius typically ranges from 10 to 100 mm depending on the lens focal length and application.

Bispheric design appears frequently in camera lenses, particularly in compact telephoto and wide-angle designs where space constraints rule out multiple elements. It is also common in eyeglass lenses for correction of peripheral aberrations, and in LED and laser collimators where off-axis light must be controlled tightly. The term itself reflects the description of two spherical surfaces working together, though only one physical surface is involved.

Because bispheric surfaces depart from the ideal spherical form, quality control during manufacture is demanding. Deviation of more than a few micrometres from design can degrade performance noticeably. Cost rises above that of a simple spherical lens, but remains well below the cost of a fully aspheric or diffractive surface, making bispheric design a practical compromise in cost-sensitive applications where spherical aberration correction is required.