Anatomy of an Aspheric Lens | Benefits of an Aspheric Lens | Precision Glass Molding | Selecting the Right Aspheric Lens
ANATOMY OF AN ASPHERIC LENS
An aspheric lens, also referred to as an asphere, is a rotationally symmetric optic whose radius of curvature varies radially from its center. It improves image quality, reduces the number of required elements, and lowers costs in optical designs. From digital cameras and CD players to high-end microscope objectives and fluorescence microscopes, aspheric lenses are growing into every facet of the optics, imaging, and photonics industries due to the distinct advantages that they offer compared to traditional spherical optics.
Aspheric lenses have been traditionally defined with the surface profile (sag) given by Equation 1:
(1)Where
Z = sag of surface parallel to the optical axis
s = radial distance from the optical axis
C = curvature, inverse of radius
k = conic constant
A4, A6, A8 = 4th, 6th, 8th… order aspheric terms
As they have increased in popularity, there is now a more technically accurate way of describing the surface:
(2)Where
Cbfs = curvature of best fit sphere
ρ = radial distance from the optical axis
u = ρ/ρmax
Qmcon = orthonormal basis of asphere coefficients
am = normalization term
When the aspheric coefficients are equal to zero, the resulting aspheric surface is considered to be a conic. The following table shows how the actual conic surface generated depends on the magnitude and sign of the conic constant k.
| Conic Constant | Conic Surface |
| k = 0 | Sphere |
| k > -1 | Ellipse |
| k = -1 | Parabola |
| k < -1 | Hyperbola |
The most unique geometric feature of aspheric lenses is that the radius of curvature changes with distance from the optical axis, unlike a sphere, which has a constant radius (Figure 1). This distinctive shape allows aspheric lenses to deliver improved optical performance compared to standard spherical surfaces.
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