Aspherical lens : a complete guide

Improvement of production technology has brought a democratization of the use of complex aspherical lenses. What are their benefits, how to manufacture them, see our guide below for answers.

What is an aspherical lens ?

An aspherical lens can be defined as a non spherical lens with at least a one optical face that can be defined following below conical equation.

Z stands for the surface (SAG) of the lens
R is the Radius of curvature
K: Conic constant
Ax: Aspheric coefficient of xth order (can be null)

Advantages of aspherical lens.

Spherical lenses are easy to manufacture, but it has been identified by early opticians that using these lenses comes with aberrations on the image, called spherical aberrations. To compensate these aberration one can use doublets or complex systems of lens but this is costly and will take more space and weight.

The other solution to remove the spherical aberration is to design the shape of the lens to avoid it, these complex shapes are defined by conical equation and are defined as aspherical shapes. Manufacturing of aspheres is more complex, but with recent (last 20 years) digitalisation of the production and testing equipment it becomes easier and cheaper.

How are aspherical lens manufactured ?

Aspheric lenses can be manufactured per different processes:

Production method	Description	Details
Precision molding	raw material blank is first pressed between to metal matrix at high temperature (400 to 650°C) for several minutes , then a controlled cooling is done and the releasing of the final part is done without further polishing	Ideal for high quantities High tooling costs Limited in raw material (low softening point) Impact on the glass index that will be reduced during the process. Thickness ratio limited Outside diameter less than 35mm.
Diamond turning	Using a very small diamond head to work the surface of the lens.	No need of further polishing Rather slow May leave spacial frequency pattern on the surface Can also be used on plastics
CNC (Computer Numerically Control)	Grinding (with ring or disc tools) and polishing (with small polishing pads).	Time consuming as grinding tools needs to be changed from rough grinding to fine polishing. Grinding and polishing CNC are usually 2 different machines. Can be very precise, better than Lambda/10 in surface accuracy. Limitation of concave radius of curvature due to tooling size limitations.
MRF (magneto-rheological finishing)	Special polishing method used after CNC grinding. Polishing medium viscosity is controlled with a magnetic field enabling a custom polishing.	Better precision than normal polishing.

How to measure an asphere ?

There are different methods to measure the surface accuracy of an asphere which can be divided by contact measurements with probes and non contact measurements using interferometers and accessories.

Contact measurement solutions	ADvantages	Disavantages
CMM (Tri-dimension measurement).	Can measure in 3D May measure big lenses Cheap	Low accuracy Cannot measure the roughness Spot Contact with the lens surface
Profilometre	More precise than CMM and measure roughness Quick Cheap	Only 2D measurement on a diameter of the lens Continuous touching of the probe on the surface, may impact surface quality

Non contact measurement solutions	ADVANTAGES	DISAVANTAgeS
Subaperture Stiching Interferometry	Can be used for sharp angle variation Very good precision	Long measuring time Costly
CGH (Computer Generated Hologram)	Good precision Full picture of the surface	Expensive Long lead time
Spherical Null lens	Easier to set up due to similarity with spherical interferometry	Limited to aspheres with mild departure from spherical shape
Verifire (Zygo)	Good precision	Limitations to certain shapes of aspheres

What is the difference between a spherical lens and a aspherical lens ?

The difference between both lenses comes from their shape, while a spherical lens shape can be defined from a virtual center and a fix radius of curvature, an aspherical lens shape is defined from a virtual center point point but with different values of radius of curvature according to their relative angle.

For a simple image comparison, a spherical lens can be seen as a cut of a sphere whereas an aspherical lens can be defined by more complex shapes, for example conic or elliptic.