These small contact areas are adjusted in space to form the aspheric profile during computer-controlled precision polishing, as shown in Figure 5. In precision polishing, small contact areas on the order of square millimeters are used to grind and polish aspheric shapes. Although this process of individually producing machined aspheres hasn’t changed dramatically, significant fabrication technology advancements in precision polishing have elevated the achievable level of accuracy possible from this production technique. This distinctive shape allows aspheric lenses to deliver improved optical performance compared to standard spherical surfaces.įigure 4: Precision glass molding platform Precision Polishingįor decades, machined aspheric lenses have been ground and polished one lens at a time. 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 (see Figure 3). They also reduce the number of terms needed for manufacturing, thereby avoiding unnecessary complications for fabrication, simplifying testing, and reducing cost. These Q-type aspheres described by Q con and Q bfs give designers more control over the optimization of the aspheres. The Q con coefficient describes the sag departure of the aspheric surface from a base conic. This departure can be easily calculated and provides a useful quantification of how easy it will be to test the surface. The Q bfs coefficient describes the RMS slope departure of the aspheric surface from a best-fit sphere. Aspheres described using these coefficients are called Q-type aspheres. Conic ConstantĪspheric surfaces can also be specified using the orthogonal coefficients Q bfs and Q con. The triplet lens with the aspheric surface shows greatly increased imaging performance at all field angles as indicated by high tangential and sagittal resolution values, by factors as high as four, compared to the triplet with only spherical surfaces.įigure 2: Polychromatic light focused through a triplet lens Object Angle (°) The table quantitatively compares the modulation transfer function (MTF) 20% contrast of on-axis and off-axis collimated, polychromatic light rays at 486.1nm, 587.6nm, and 656.3nm. Both designs use the same glass types, effective focal length, field of view, f/#, and total system length. The table below compares an 81.5mm focal length, f/2 triplet lens (shown in Figure 2) consisting of all spherical surfaces, versus the same triplet with an aspheric first surface. Utilizing aspheric lenses in the design, however, improves aberration correction and makes it possible to design high throughput systems with low f/#s, while simultaneously maintaining good image quality. While this may achieve the desired resolution goal, this technique results in a loss of light throughput. To achieve the required performance of an imaging lens, optical designers often have to stop down, or increase, the f/# of their design. The spot sizes from the asphere are several orders of magnitude less than those of a spherical lens. The table compares the spot size, or blur size, of collimated 587.6nm light rays on-axis (0° object angle) and off-axis (0.5° and 1.0° object angles). The table below further illustrates the difference in focusing performance between an aspheric lens and a spherical lens, contrasting the performance of two comparable lenses with 25mm diameters and 25mm focal lengths (f/1 lenses). Figure 1 shows a spherical lens with significant spherical aberration compared to an aspheric lens with practically no spherical aberration.įigure 1: Spherical aberration in a Spherical Lens (left) vs an Aspheric Lens (right) An aspheric lens can be designed to minimize aberration by adjusting the conic constant and aspheric coefficients of the curved surface of the lens. Spherical aberration is inherent in the basic shape of a spherical surface and is independent of alignment or manufacturing errors in other words, a perfectly designed and manufactured spherical lens will still inherently exhibit spherical aberration. Spherical aberration is commonly seen in spherical lenses, such as plano-convex or double-convex lens shapes, but aspheric lenses focus light to a small point, creating comparatively no blur and improving image quality. The most notable benefit of aspheric lenses is their ability to correct for spherical aberration, an optical effect which causes incident light rays to focus at different points when forming an image, creating a blur.
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