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SPHERICAL VS. ASPHERIC LENSES
HOW THEY’RE USED IN IMAGING APPLICATIONS
Optical Lenses are often categorized by their shape. Two common surface shapes are spherical and aspherical. Each offers unique characteristics that make it suitable for different use cases.
Spherical lenses are so named because the surface is a section of a sphere. Thus, the radius of curvature is constant across the entire lens. The spherical surface causes light to converge (from a convex surface) or to diverge (from a concave surface), with the amount of focusing power proportional to the index of refraction of the glass..
The main advantages of using spherical lenses in optical systems are their simpler surface design and lower manufacturing cost. Spherical lenses can be fabricated using purely mechanical rather than computer-controlled apparatus. Depending on the lens size and the quantity desired, spherical lenses can often be polished in batches, saving considerable time in setting up the polishing process.
Spherical lenses can be further categorized by the details of their shapes. Plano-convex and plano-concave lenses each have one flat and one spherical surface. Meniscus lenses have one concave, one convex surface. Finally, bi-convex or bi-concave lenses have two spherical sides.
Spherical lenses have been used for hundreds of years. They are suitable for many imaging applications in a diverse set of markets. Almost every imaging system manufactured today – binoculars, camera lenses, tube lenses, relay lenses, microscope objectives, and many more – use at least some spherical lenses.
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.
Use in Imaging Applications
Both spherical and aspherical lenses find application in a wide range of imaging applications in a variety of end markets. They enable engineers, researchers, and scientists to use equipment—such as advanced microscopes, laser scanners, and other imaging devices—to make very precise measurements.
Some examples of the applications where spherical and aspherical lenses are found include:
- Fluorescence microscope platforms: used by researchers to facilitate the identification and examination of specific sections of a specimen (e.g., decoding DNA sequences and imaging individual cells and tissue samples)
- Cameras and laser-based ophthalmic tools for vision correction: used by trained clinicians and surgeons to diagnose and treat eye diseases and correct vision
- Semiconductor wafer inspection tools: used by computer chip engineers to map defects and probe cards
- Industrial laser machine tools: used by manufacturing companies to create and inspect products before, during, and after manufacturing
- Night vision optics: used by defense and law enforcement personnel to operate under cover of darkness