what is aspheric lens?

What is an Aspheric lens? How to define aspherical lens?

The ordinary lenses we usually see have two spherical surfaces — just like a piece cut from a perfect sphere, with the same degree of curvature from the center to the edge.

An aspherical surface, however, means:
the surface is not a perfect sphere, and its degree of curvature gradually changes from the center to the edge.

How to design aspheric lens?

I. First Clarification: What Exactly Does Aspherical Design Involve?
Designing an aspherical lens = determining 4 key parameters:
Radius of Curvature R (Base Surface)
Conic Constant k (Conic Section)
Higher-Order Aspherical Coefficients A4, A6, A8… (Aberration Correction)
Thickness, Aperture, Focal Length, Back Focal Length (Structure)
II. Standard Aspherical Formula (Essential for Design)
  The most widely used Rotationally Symmetric Aspherical Equation in optics

III. Complete Design Procedure (From Scratch to Finished Product)
1. Define System Specifications (Must Be Determined First)
Wavelength: UV / Visible / IR (For quartz, specify 193/248/532/1064 nm)
Focal Length f
Incident Beam Diameter D (F/# = f/D)
Working Distance WD
Image Quality Requirements: Spot Size, Wavefront Error, Spherical Aberration
2. Select Material Refractive Index
Common options:
JGS1 / Corning 7980: n≈1.458 (589nm)
JGS3 / 7979: Low-OH for IR, slightly lower n
3. Establish an Initial Spherical Structure
First calculate a reasonable structure using a spherical lens:
Determine R1, R2, Thickness d
Calculate focal length, spherical aberration, spot size
Spherical surfaces inherently produce spherical aberration, which is the target for aspherical correction.
4. Set One or Both Surfaces as Aspherical
Common practices:
Single Aspherical Surface: One spherical surface, one aspherical surface (most commonly used)
Double Aspherical Surfaces: Used for ultra-high requirements (lithography, laser collimation)
5. Optimization: Automatically Fit Aspherical Coefficients with Software
Software used: Zemax / CodeV / Oslo / SYNOPSYS
Operations: Set one surface as Aspherical
Select variables: R, k, A4, A6, A8
Define merit function: Minimize Spot Radius
Minimize Wavefront Error
Spherical Aberration = 0
Auto-optimization
The software will automatically calculate a set of R, k, A4, A6…
6. Verify Aberration Compliance
Key parameters to check:
Spherical Aberration
Coma
Field Curvature / Astigmatism
Wavefront Error RMS (PV)
Typical requirements for quartz UV / laser systems:
Wavefront Error < λ/10
Spot close to the diffraction limit
7. Check Machinability (Critical)
Aspherical surfaces should not be overly complex; they must be grindable:
Aspherical coefficients cannot be excessively large
Slope cannot be too steep
Aspherical Departure cannot be excessive, otherwise machining is unfeasible and precision is poor.
8. Generate Manufacturing Drawings
Final output:
R
k
A4, A6, A8

Aperture, Center Thickness, Edge Thickness
Surface Accuracy PV / RMS

Why Design Aspheric Lens?

In the design of quartz and glass lenses, there are 3 core advantages:
A. Correct monochromatic aberrations such as spherical aberration and coma aberration.
One aspheric surface ≈ the correction effect of multiple ordinary spherical surfaces.
B.Reduce the number of lenses, shrink the volume and reduce the weight.
The optical system is more compact, suitable for micro lenses, laser collimation and lithography systems.
C. Improve the imaging / beam quality.
The light spot is smaller, the wavefront error is lower and the transmittance is higher.