Spherical vs. Aspheric Lenses: How They’re Used in Imaging Applications

Aspherical lenses are optical lenses that feature a non-spherical but rotationally symmetric radius of curvature. Unlike Spherical Lenses, they have a radius of curvature that varies from the center to the edge of the lens.

The variation from sphericity is often much too small to detect with the eye. It may be necessary to use specialized optical metrology equipment to determine if an optical surface is a sphere or an asphere. Other times, the aspheric surface is departs so much from a spherical shape that a person can easily see the difference. Some aspheric surfaces even have an inflection point or “gull wing” shape so that the surface changes from convex to concave between the center and edge of the clear aperture.

Computer-numerical-control (CNC) equipment is necessary to shape and polish aspherical surfaces. Since the radius of curvature is not constant over the clear aperture of the lens, purely mechanical equipment is not adequate; the optical technician must program the CNC machine to accommodate the varying radius of curvature.

Since aspherical surfaces must be produced on CNC equipment, they must be made one at a time; they cannot be shaped or polished in batches. This makes aspherical surfaces considerably more expensive to produce than spherical surfaces. Furthermore, the optical designer must carefully consider the specifics of the aspherical surface so that the CNC equipment can accurately follow the complex curve. No comparable design challenge pertains to spherical lens.

While the design and manufacture of aspherical lenses can be challenging, when constructed correctly, they can offer greater optical functionality than a comparable spherical lens. Some of the key benefits of using an aspherical lens in an optical application are:

• Smaller number of elements required in an optical assembly

• Reduced effects of spherical aberration, distortion, and marginal astigmatism

• Sharper focusing

• Larger aperture size

• Improved light focusing and collection efficiency

The figure below shows the performance advantage that makes aspherical lenses worth the extra cost. When a spherical lens focuses an incident beam of light, as shown in the example to the left, not all of the light focuses to the same spot. Light that passes through the the lens closer to the edge of the lens focuses either in front of or behind light that passes through lens closer to the center. This aberration is called “spherical aberration”. Spherical aberration limits how sharp an image can be produced by a spherical lens. Spherical aberration can be minimized by adding additional spherical lenses, but this makes optical systems larger and heavier.  

A properly designed and fabricated aspherical lens focuses light without producing any spherical aberration, as shown in the example to the right in the figure below. Light that in incident on all portions of the clear aperture focuses to the same spot, producing a sharp, higher resolution image with a single aspherical lens instead of with multiple spherical lenses.

For the above reasons, aspherical lenses are used in many imaging applications. They are commonly found in microscope imaging objectives and other image lens assemblies in life science instruments, semiconductor wafer inspection tools, medical devices, and defense and aerospace night vision imaging optics that rely on precision optical components.