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.


Lens Options At JML Optical

Since 1972, JML Optical has been a leading provider of precision-engineered custom optical lenses for a variety of industries, including aerospace, defense, life science, automotive, entertainment, imaging, and medicine.

Through our expertly trained engineering department and highly advanced equipment, JML Optical offers four types of precision lenses, including cylindrical, aspherical, plano, and spherical. Here, we will discuss each type of lens as well as their various benefits and applications.

Cylindrical

Named for their cylindrical/semi-cylindrical shape, positive and negative cylindrical lenses provide image magnification in only one axis. A positive cylindrical lens focuses a collimated beam of light into a line, rather than into a point. A negative cylindrical lens spreads a collimated beam into a wedge. 

Cylindrical Lens Applications and Industries

Cylindrical lenses are used to focus light, often lasers, into straight lines. These lenses are also used to correct or control astigmatism, which is an optical aberration that produces line foci rather than a single point focus.

Two cylindrical lenses crossed at right angles act much like a spherical lens. When used in a camera, crossed cylindrical lenses produce a bokeh that appeals to many cinematographers. High-end movie and television cameras often use cylindrical lenses. 

Industries that rely on cylindrical lenses include:

  • Semiconductor
  • Industrial
  • Medical
  • Entertainment

Cylindrical Lens Advantages

Cylindrical lenses are often the best way to produce or modify anamorphic images — images that are wider than they are tall (or vice versa). Cylindrical doublets and triplets are used to compensate for dispersion, just as with spherical lenses, so that imaging over a wide color range is possible.  

The beam produced by the ubiquitous semiconductor laser has an elliptical footprint, with divergence greater along one axis than along the perpendicular axis. A pair of cylindrical lenses with different focal lengths can turn the elliptical beam into a collimated “pencil” of rays that can then be focused or shaped by conventional round optics. 

Aspherical

Aspherical lenses have a non-spherical but rotationally symmetric shape. While spherical lenses have the same radius of curvature at every point inside the clear aperture, aspherical lens have a radius of curvature that changes over the clear aperture in a radially symmetric fashion. When manufactured properly, aspherical lenses provide enhanced functionality compared to spherical lenses. This type of lens is primarily used to improve image quality and correct imaging problems that occur with spherical lenses such as aberration, marginal astigmatism, and distortion. 

Because the radius of curvature varies over the surface of the lens, aspherical lenses must be produced by computer-numeric-controlled (CNC) grinding and polishing machines that work on a small portion of the aperture at any one time. 

Aspherical Lens Applications and Industries

Aspherical lenses find uses in industries such as medical, photography, communications, and more. Common applications include:

  • Collimating lenses for laser beam profilers
  • Objective lenses for high-end cameras
  • Eyepieces for military night vision devices
  • Opthalmic instruments
  • Many applications where smaller, lighter, optical assemblies are desirable

Aspheric Lens Advantages

Aspherical lenses can provide many advantages over spherical lenses, including:

  • Better resolution in an imaging system
  • Sharper focusing of collimated beam such as a laser
  • Smaller number of elements required in an optical assembly
  • Reduced weight and size of optical assembly
  • Reduced effects of spherical aberration, distortion, and marginal astigmatism

Plano

Plano optics are flat. The term can refer to the flat side of an optic with one flat and one curved surface (e.g. plano-convex or plano-concave) as well as to an optic that is flat on both sides (plano-plano).

Plano Lens Applications and Industries

Optics that are flat on boths sides are used as windows, mirrors, and filters in optical systems. Optics with one flat side and one curved side are used for many purposes including focusing a collimated beam into a sharp point. Every industry that uses optical components will use components with at least one plano side.

Plano optics are produced on machines that have large, flat, polishing surfaces, and typically in large batches. Depending on the specifications required, it may be possible to polish both sides of a plano-plano optic at the same time. The faster polishing and large batches often allow plano-plano optics to be produced faster than curved optics. 

Plano Lens Advantages

Compared to a curved surface, a planar surface is cheaper to manufacture to a high standard of surface figure and cosmetic appearance. 

Plano optics can be made quite thick if needed to make a sturdy window to protect an optical system from environmental threats. 

Since plano optics can be made extremely flat, they can be used as turning mirrors to fold an optical system into a small volume without compromising wave front quality.

Spherical

Spherical lenses feature a spherical surface with a consistent radius of curvature across the entire lens. A 

A convex, or positive, spherical surface concentrates light — for example, turning a collimated beam into a small spot — producing a real image.  A concave, or negative, spherical surface makes an incident beam diverge, so that a collimated beam is spread out into a cone that appears to diverge from a virtual image.   

Spherical lenses can be made on CNC machines or by purely mechanical means. People have been making spherical lenses for hundreds of years. Many optics shops turn out excellent spherical lenses using pitch tools on spindles in machines that are 30 or more years old! Depending on diameter and radius of curvature, multiple spherical lenses can be polished simultaneously on a pitch tool, greatly reducing manufacturing cost. 

Spherical Lens Applications and Industries

Spherical lenses are used in virtually every optical system that forms an image. Here are just a few examples of applications that use spherical lenses made here at JML Optical:

  • Fluorescence microscope platforms
  • Aerial mapping camera lenses
  • Semiconductor wafer inspection tools
  • Industrial laser machine tools
  • Night vision optics

Spherical Lens Advantages

Spherical lenses are usually quite cost-effective in achieving good imaging performance, especially compared to aspherical lenses. Pitch-polishing techniques can produce spherical surfaces that are very smooth and have few scratches or digs, which is important in high-end imaging or applications that use lasers. The potential for batch polishing means that many spherical lenses can be produced in high quantities rapidly and inexpensively.

JML Optical – Your Source For High-Quality Lenses

At JML Optical, we are an industry leader in the designing, manufacturing, and distribution of lenses for a wide range of industries and applications. To learn more about our lens options, or to get started on your custom optical solution, contact our experts or request a quote today.