Free-form surfaces will bring more opportunities and challenges to the field of optics

Whether you speak with an optical designer or a fabricator, a special viewpoint may emerge on what could also be considered precisely freeform. In simple terms (and perhaps too simple), a freeform surface is defined as a surface whose surface shape lacks translational or rotational symmetry about axes normal to the mean plane.

Freely designable optical surface profiles offer lighter, simpler, and more compact assemblies versus their spherical counterparts, but they're tougher to manufacture and measure. The classic example of a freeform surface in ophthalmic applications is that the varifocal lens commonly utilized in eyeglasses.

thanks to the traditional aging process, the eye’s ability to adapt between near and much slowly decreases. This results in discomfort, especially at close range. the good advantage of varifocal lenses is that they combine different strengths in one lens.

In contrast to single-vision lenses, they permit for a mixture of three vision ranges: distance, intermediate, and sight. Compared with simple rotationally symmetrical lenses, however, the outline of those freeform surfaces is extremely complex and therefore the calculation costly, as they're individually adapted to every spectacle wearer and every eye for optimal results. 

Freeform surfaces are useful not only in ophthalmic applications but also in precision optics. In systems with complex beam paths, for instance, they will significantly reduce the number of lenses by combining several functions in one lens.

As a result, freeform surfaces can make complex optical systems more compact and may significantly reduce their overall weight. this is often particularly relevant for lenses in precision optics, which are usually made from mineral glass, ceramic materials, or crystals instead of lightweight plastic. Replacing several classic lenses with a freeform surface also can reduce the number of components within the assembly. 

Certain applications, like head-up displays, can even be produced exclusively with a freeform surface. The task of a head-up display is to project a straight image onto a curved glass surface. this suggests that the distortion of the curved projection surface within the lens of the head-up display must be anticipated in order that it's then precisely canceled call at the addition. almost like the varifocal lens in ophthalmic applications, the freeform surface must even be elaborately calculated and designed. 

Manufacturing freeform lenses 

In contrast to the usually fully automated production of ophthalmic lenses, precision optic lenses are usually produced in small batches numbering up to twenty or, more rarely, 100 pieces without the automated linking of processes. the stress on these lenses in terms of accuracy, optical properties, and sturdiness are ~10 to 1000× greater than for ophthalmic lenses.

For this reason, the production of precision lenses relies almost exclusively on high-quality and partly complex glass materials. a number of these hard, brittle materials place high demands on the machining process and therefore the tools used. this is often particularly true for precision-optic lenses with freeform surfaces. The complexity, requirements, and manufacturing costs of the individual lens increase significantly per optically effective area. 

Lenses made from plastic are often produced effectively and economically by milling, turning, and polishing. In precision optics, diamond turning may be a niche application and is typically only possible with metals and certain crystalline materials, like germanium, zinc selenide, calcium fluoride, and silicon.

The advantage to using these materials is that the geometrically defined leading-edge they permit, and therefore the possibility of manufacturing freeform surfaces very flexibly and during a short time. additionally, they will achieve excellent surface qualities directly without the necessity for a subsequent polishing process. In most cases, and for all other materials, grinding produces the essential geometric shape, typically through the utilization of bonded diamond grinding wheels.

To realize high geometric accuracy, the dressing of those tools is significant. Importantly, the wear and tear of the grinding wheels means the form and get in touch with point aren't constant, counting on the kinematics used. 

Ultimately, good surface quality and geometric accuracy are often achieved only through the polishing process. Classic polishing may be a complex process that can't yet be modeled with absolute accuracy. Like grinding, it involves removing material with a geometrically undefined leading edge.

But in contrast to grinding, polishing may be a dwell-time-controlled or time-dependent process. the fabric is removed mainly mechanically between the polishing tool and therefore the workpiece by the abrasive particles within the polishing medium, whereby the mechanical process partly overlaps with chemical processes. 

The amount of fabric removed is extremely low and strongly hooked into various influencing factors. additionally to the dimensions and number of abrasive particles, the quantity of fabric removed is decided by contact pressure, hardness, and pore concentration of the tool, and even the temperature of the polishing medium. Chemical properties, like pH value et al., even have an influence. 

Machining of precision optics requires a closed-loop iterative manufacturing process to realize a high degree of accuracy. The lens is therefore measured to see for quality but also for correction purposes. 

Between the individual production steps of pre-grinding, fine grinding, and polishing, the lens is measured so it is often corrected, if necessary, during the next process step. Process steps may have to be repeated until the lens is within specification.

This also means the lens must be calibrated anew for every measuring process, both on the measuring machine and on the processing machine. the method is often problematic because precision lenses, unlike ophthalmic lenses, don't all share an equivalent basic shape.

Achieving the specified accuracy requires the calculation of a locally resolved deviation from the measured lens. From this deviation — and an algorithm with a selected material removal simulation model — a dwell time on the tool path is then calculated consistent with the deviation. 

The aim is to dwell as briefly as possible on surface sections with a little error to attenuate material removal and, conversely, to dwell longer on sections with larger errors to scale back the deviation accordingly. Polishing is administered as evenly as possible across the lens to avoid transitions. 

The production of freeform surfaces, versus spherical lenses, is far more complex. Freeform surfaces aren't rotationally symmetrical or described by an easy geometric curve. The lens, therefore, is typically not created via surface or line contact.

Instead, its production proceeds via some extent contact between tool and workpiece (i.e., the sub-aperture method). a particular overlap is important to make sure minimum removal. The more complex the geometry and therefore the smaller the minimum local radius of the lens, the smaller the important point contact must be. 

For various reasons, this results in a significantly more complex manufacturing process and, counting on the method kinematics used, requires between three and 6 simultaneously moving machining axes. The movement of the axes isn't always uniform, but it's subject to strong fluctuations counting on the geometry.

The resulting accelerations place significantly higher demands on the processing machine and therefore the necessary dynamics since surface defects occur more frequently at the points where acceleration parameters change. The smoother the movements and therefore the greater the overlap between tool and workpiece, the better it's to realize optimal surface quality.

The machining program is additionally far more complex. rather than an easy curve, the geometry is represented by some extent cloud. Finer point spacing more accurately reproduces the freeform surface, but it also demands a more complex calculation of the trail and correction courses. 

Smaller overlap between workpiece and gear increases the potential for transitions to possess a negative impact on the surface quality and therefore the accuracy of the geometry. thanks to the purpose contact, the lens should be machined during a raster mode almost like milling or coordinate grinding.

Raster mode grinding requires some care to make sure that the machining direction or the individual lines aren't reflected within the workpiece. the foremost demanding freeform surfaces require removal of the remaining structures during a subsequent polishing step with greater overlap. Otherwise, geometric accuracy can suffer within the littlest inner radii. 

Measuring freeform surfaces 

The measurement of freeform surfaces is additionally far more complex. An interferometer is enough to live spherical lenses quickly and effectively, though an alternate is to live discrete points with a surface profiler. This relatively quick and straightforward approach also applies to measuring aspheres. counting on the specified accuracy, just one line on the surface must be measured for rotationally symmetrical surfaces. 

The same processes can sometimes apply to freeform surfaces, but they involve considerably greater complexity. Freeform surfaces basically require the whole lens to be recorded and measured point by point employing a suitable procedure, like scanning several lines in equidistant sections then merging them into a 3D graph.

counting on line spacing and lens size, this will take an extended time. Ideally, the use of a 3D measuring machine in conjunction with grinding enables the scanning of all points of the machining program and thus determines the deviation at each point. 

This accuracy is sufficient for grinding. But the interferometer provides even more precise results for polishing. The use of a computer-generated hologram to get the specified shape of a freeform surface’s wavefront can help speed interferometer measurement.

However, the use of a hologram only applies to a selected lens, making it suitable just for series production thanks to the high acquisition costs. Other interferometric methods involving special setups, stitching, or tilted-wave techniques can reduce this disadvantage. 

Alternatively, wave deformation is often determined within the optical system. Following surface measurement, calculating the particular deviation from the target geometry at each point involves superimposing the first and target geometry. a replacement machining program is often created from the difference as a correction program. 

Polishing tools 

A wheel tool typically is a polishing instrument with a point contact. It allows a relatively high removal rate, and dressing permits the tool shape to be maintained or the spherical shape to be restored if it's deformed by wear. 

One major disadvantage of wheel tools, however, is that the pores create linear structures on the lens, which isn't acceptable in optical applications. The high removal rate simultaneously limits the achievable surface qualities. As a result, these tools aren't suitable for the assembly of micro-optics, because the minimum tool radius is restricted. Wheel tools also are unwieldy and may cause collisions.

These drawbacks have led to the event of special polishing tools supported by deterministic adaptive polishing technology. Designed to supply finer surface qualities and improved imaging accuracy, these tools can repair the littlest geometrical deviations with pinpoint accuracy. Areas that correspond to the target geometry see less — and more locally limited — material removed. 

Another advantage is that these tools are often variably equipped with different polishing medium carriers and offer various degrees of hardness. there's no need for a special polishing medium. this enables the tool to adapt to the individual needs and knowledge of the customer and their specific process and polishing kinematics. additionally, the tool is often used on a typical machine to cost-effectively produce aspheres and spheres if required. 

One potential drawback is that the low and unevenly distributed removal that these tools can cause. Knowledge of the precise distribution of the removal, however, offers how to correct for this, and special software has been developed for this purpose.

The software model enables removal with sufficient accuracy to individually determine the tool and workpiece geometry, the defect geometry, and a system-dependent stock removal coefficient. From this, it calculates a dwell-time-controlled program to realize the optimal surface quality and therefore the smallest deviations from the nominal geometry.

To make sure the method is as efficient as possible, it's best to first polish the surface as closely as possible to the nominal geometry with the wheel tool before correcting it with the adaptive polishing tool. 


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