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.


At JML Optics, we rely on a precision grind-and-polish approach to craft custom cylindrical, spherical, aspherical, and flat lenses. In this technique, we use a progressive series of abrasive media to shape glass blanks. This allows us to refine the shape in stages, improving uniformity and tolerance. Other common techniques include molding and diamond-turning, which have their own distinct benefits and limitations.

Optical Lens Manufacturing Process

White paper PDFBlank Production

JML normally purchases bulk optical glass in a form known as a “strip”. Glass manufacturers provide companies like JML with a very predictable and reliable product, and provide us detailed information about the optical and mechanical properties of any glass that we purchase. 

JML has a glass cutting shop that is somewhat like a machine shop for glass. We use saws, core drills, grinders, and other machine tools to make blanks. A blank for a spherical or aspherical lens will be a disc with flat top and bottom – a form factor like a hockey puck. A blank for cylindrical lenses will be a bar with a rectangular cross section. The “hockey puck” blank is made into a single component. The bar is normally made into multiple identical components.

Steps to shape a blank into a lens 

Components with a constant radius of curvature (spheres, cylinders, and flats) can be produced through all-mechanical means in the three steps of generating, grinding, and polishing. Alternatively, these components can be made using CNC processes in two steps: generating and polishing, which between them accomplish the grinding function. 

For components with a non-constant radius of curvature (aspheres and cylinders), the two-step CNC method is used.

The comments below assume a round lens, i.e. a spherical or aspherical lens. Cylindrical lenses are made from bars of glass in an analogous process but not exactly as described here. Please contact us if you’d like to know more about how cylindrical lenses are manufactured.

Generating

The generation step uses solid abrasive tools to remove material from the blank. After generation, the workpiece has the correct radius of curvature, but is slightly thicker and has a larger diameter than specified for the finished lens. The workpiece also has acceptable tilt or runout between the two surfaces. It is important that the generating process gets these parameters right because the later grinding and polishing processes are not practical ways to fix the shape of a lens that was not generated correctly.

Depending on the size of the blank, the hardness of the glass, and how much material must be removed, the generating step can take from tens of minutes to a few hours. Whether the generating is done via CNC or mechanical means, only a single blank at a time is generated.

The texture of the generated glass surface causes it to be visibly grey and mostly opaque. The glass surface looks frosted or acid-etched. Making the surface optically smooth are the functions of the grinding and polishing steps.

Grinding

When a lens is not being produced via CNC equipment, it is common to use successively finer liquid abrasive slurry solutions to gradually make the glass surface somewhat smoother. The appearance of the glass surface changes as the texture gets finer but the surface is still noticeably gray, not transparent, after grinding.

Done properly, grinding does not change the curvature of the lens surface. Grinding does make the lens slightly thinner, which is why the previous generating step leaves a little extra thickness.

Grinding is a fast process, usually requiring only a few minutes per surface. Grinding uses a rotary table with a manually operated swing arm to position the grinding tool against the lens surface.

Polishing

Polishing uses liquid abrasive slurry to smooth out the “frosted” glass texture until the surface is optically smooth and transparent. The polishing pad in a CNC polisher is typically made from a material that looks like a very fine mesh sponge or a very coarse cloth. The polishing pad in a spindle polisher is made from pitch that is scored to allow the liquid abrasive slurry to infiltrate between the pitch and the glass surface.

When a lens “grays out” – all the gray appearance is gone, with the entire clear aperture being visibly polished –  the optical technician can check surface figure using a test plate or interferometer. The tech can then adjust the polishing process to slightly alter the surface curvature and to improve figure to bring the surface into specification. Polishing uses up a small amount of center thickness of a lens, so the generating step must take this into account. 

The polishing step takes considerably longer than generating or grinding. A len that spends one hour in the CNC generator may spend four, six, or even more hours in the CNC polisher. How nearly perfect the final surface is required to be has a major effect on polishing time. Tighter specifications for scratch-dig, power,  irregularity, and surface roughness all require longer polishing time. However, reaching even modest specifications requires multiple hours of polishing. A potential advantage of pitch polishing is that some lenses can be mounted in multiples on a curved block and all polished at the same time. 

Centering

After polishing is completed, the lens meets the following specifications: radius of curvature and surface figure on both sides; center thickness; wedge or runout; and surface roughness. The outer diameter has to this point been left slightly oversized.  

Centering is the process of reducing the outer diameter of the lens to the correct size while ensuring that the optical axis of the lens passes through the center of the diameter.

A centering machine is a fancy lathe. The machine uses optical means to determine the optical axis of the lens. The lens is then spun about this axis as a diamond-abrasive tool reduces the lens diameter. The end result is ready for coating and final inspection, unless it is slated for further shaping.

Optional Shaping

If our customer wants a round lens, which is the most common case, then the lens is ready to be coated. 

Sometimes a customer wants a shape other than round. The lens can return to our glass cutting shop to receive one or more flat edges; get cut into a square, hexagon, oval, or other shape; or even to get cut up into wedges like pieces of a pie. 

Coating

After centering, the lens receives any necessary coatings. JML offers a full range of standard and custom coatings, including options for reflective, anti-reflective, and beam splitting properties. See our coating page for a description of how we coat lenses and what types of coatings we offer.

Inspection

Finally, the finished lenses are subjected to rigorous inspection to ensure they adhere to all quality and performance standards. Every lens JML produces has been checked by the optical technicians who manufactured it; by an independent metrology technician who has to approve cosmetic measurements; and by our Final Operations team who prepare the lens for shipment. If the lens is going into an assembly, the assembly team also checks every lens for cosmetics, coating integrity, and cleanliness before putting the lens into an assembly..

Applications and Industries

Optical lenses are used within diverse industries  – anywhere someone needs to produce an image or project light. JML Optical addresses design challenges for many industry clients:

  • Medical
  • Industrial
  • Life Sciences
  • Semiconductors
  • Entertainment
  • Imaging
  • Aerospace & Defense

Why JML?

Optics manufacturing is an intensive process that requires extensive expertise to get right. JML Optical stays on the cutting-edge of optics manufacturing, delivering innovative solutions to meet complex design challenges. To learn how our process can benefit your product, contact us today.