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02 Oct 2008
Precision moulded glass aspheres have made the transition from being exclusively high-cost components to mainstream optics for a variety of imaging applications. Gregg Fales of Edmund Optics looks at the factors that have changed the asphere's fortune.
Using aspheres in the design of an optical system can reduce the number of optical surfaces while achieving more highly corrected imaging performance. Despite this appeal, designers have been reluctant to include traditional polished aspheres because they have been expensive to fabricate and offer few economies of scale in mass production. Recent advances in glass technology, however, have turned moulded aspheres into a viable option for precision optical designs.
An aspherical lens, also called an asphere (see figures 1 and 2), is a lens whose surfaces have a profile that is not simply a portion of a sphere. The advantages of using glass aspheres in optical designs have long been known. Their complex surface profiles can greatly reduce or eliminate many types of aberrations, resulting in optical systems with better performance, lighter weight, less assembly time and reduced tolerance stack-up compared with systems utilizing spherical lenses.
Traditional polished aspheres are manufactured one at a time. This makes them both inherently expensive to fabricate and eliminates most opportunities to reduce cost through volume production. An alternative approach to machining – precision moulding – has until recently only been practical for producing aspheres of small diameter and in large quantity. This has relegated their use to high-volume applications such as collimating laser diodes.
That situation has started to change. Advancements in commercially available precision grinding platforms, the introduction of low transformation temperature (Tg) glass and availability of advanced metrology equipment for characterizing both the quality of the moulding tool and the finished lens are expanding the practicality of moulded aspheres. These factors are combining to extend the range of options for precision moulded asphere lenses as well as lowering the cost of production.
Glass moulding
Glass moulding, as a manufacturing method for optical components, has existed for centuries. Slump moulding is old but still a common technique for making low quantity aspheres. The process involves raising the temperature of a polished lens to the point that it softens and begins to deform under its own weight.
Slump moulding produces aspheres with poor surface accuracy that are unsuitable for imaging applications. It wasn't until the 1970s that the advent of press moulding techniques began producing precision lenses, primarily for the camera industry.
Precision glass moulding today begins with fabricating a mould from a hard, thermally durable material that can withstand high temperature and pressure. The mould must have an optical quality surface finish as well as a very precise form that accounts for the shrinkage of the glass as it cools.
Glass preforms, which are either specially made gobs of glass or polished spherical lenses, are inserted into one half of the mould, heated to the point where the glass is malleable and then compressed between two moulds. The mould then cools, releases the lens and the process is repeated. Moulding chambers may contain multiple moulds to produce several lenses in a single pressing cycle.
Still, these moulded aspheric lenses had to be small because heating, pressuring and cooling the glass all affected the material's optical properties and resultant transmitted wavefront. The greater the volume of glass, the greater the variation. In order to maintain "precision" elements, glass moulding has traditionally been restricted to lenses with diameters of greater than 10 mm. The process and the metrology of moulded aspheres remained essentially constant through to the early 1990s and was historically limited to high-volume applications.
Creating the mould
One of the main hurdles to the wider use of moulding for fabricating aspheres has been the high cost of creating the mould. Developing a mould only made economic sense when production volumes were high enough for cost-effective amortization. Recent technology improvements, however, have reduced the overall manufacturing cost associated with the glass mould tool.
One improvement has been the development of new mould materials suitable for production operation in the harsh environment associated with near-molten glass. These materials satisfy demanding requirements. First and foremost, tool and mould materials must conduct heat well and have a low coefficient of thermal expansion. Secondly, the materials must be corrosion resistant, as the elevated temperatures required for the moulding process make many materials vulnerable to oxidation. And lastly, the materials must be amenable to CNC machining to sub-micron surface figure accuracies.
Another improvement has been in fabricating the mould using a precision grinding platform referred to as a Deterministic Micro Grinder (DMG). The DMG is a very precise and accurate piece of manufacturing equipment that has nanometre resolution and tens of nanometres repeatability. These attributes are required to achieve the high surface quality levels needed for glass moulding.
An area of mould fabrication that has recently become substantially more efficient is the methods used to test and qualify the mould. In the past, testing required the use of a computer-generated hologram (CGH) of the mould. Test equipment used the hologram to form a wavefront that represented an ideal lens, then compared that result with a wavefront formed from a moulded lens. This provided a measure of how well the lens conformed to the ideal shape. Every asphere surface requires a unique hologram. This means that there is additional costly tooling that must be amortized over the lens production.
New stitching techniques provide 3D aspheric metrology solutions that can measure many aspheric surfaces, rather than a single design. The SSIA manufactured by QED is a sub-aperture stitching interferometer, which maps a lattice of sub-apertures across the surface of the lens. The size of the sub-apertures and lattice density depend on the aspheric departure of the lens. The greater the departure of the asphere, the smaller the sub-apertures and the denser the lattice. This technology enables direct testing and measurement of both the mould and the resultant lenses rather than relying on holography.
Glass types
Another area of technology that has helped to bring precision glass moulding to the masses has been the development of low Tg glass types for the lens material. Tg is the transformation temperature of glass, and refers to the temperature at which glass transforms from a lower temperature glassy state to a higher temperature-cooled liquid state. The higher the Tg, the higher the cost of finished moulded lenses because of the energy and time required to first heat and then cool the glass, thus the low Tg glasses reduce manufacturing costs.
Perhaps most importantly, though, these glasses are available with indices from below 1.43 to above 2.00 (see figure 3). Most common high-index flint materials have transformation temperatures between 600 and 700 °C. Crown glasses, such as P-SK57, have relatively lower transformation temperatures near 550 °C but also have a lower index.
Because of the high transformation temperature of flint glasses, high-index materials have historically not been available for moulding. As a result, high numerical aperture components were very difficult to make. Because they were constrained to low-index materials, they were rather thick and had large aspheric departures, making mould fabrication and creation of the CGH difficult. This, in turn, led to expensive, long lead-time components. With the range of materials available today, however, just about all conceivable configurations of an optical element are readily within the manufacturing capabilities of an experienced mould designer.
This wide range of glass types thus provides more freedom to the optical system designer seeking to use moulded aspherics. In addition, the advent of low Tg glasses has provided a number of other benefits that have helped the design of moulded optical systems immensely. Because these glasses have been designed to be moulded, care has been taken in their make-up to prevent devitrification. The glass manufacturers have also taken care to develop reliable moulding parameters to ensure customer success with these glass types.
Conclusion
All these improvements mean that glass moulded optics are no longer the exclusive domain of high-volume applications requiring small lenses. Advances in the production of mould tools, development of new optical glass types, and improvements in metrology equipment used to qualify the tools and optics have significantly reduced both recurring and non-recurring costs. Today, applications demanding production as small as 500 lenses may very well benefit economically from introducing a moulded lens into the design. And, almost certainly, applications requiring production volumes of several thousand lenses will benefit in both cost and performance terms by being able to replace multiple spherical elements with a single moulded asphere.
Imaging applications are certainly an area where the advances in glass moulding are realizing benefits today. Aspheres have allowed the newest generation of night-vision goggles to become higher resolution and lighter weight, contributing to the safety of our combat troops. Likewise, imaging optics for the newest generation of CCD sensors are being designed and manufactured with aspheres. This allows the lenses to meet the strict resolution requirements of the small pixels, while simulta-neously allowing the lenses to be used on larger format sensors, at low F-numbers without vignetting.
Whatever the application, working with the optics provider can help to optimize the design of the moulded aspheres. Minor changes in geometries not critical to performance can have dramatic effects on tooling charges and part costs. Likewise, the manufacturer can be very helpful in selecting the lowest-cost appropriate glass type and determining appropriate tolerancing.
• For more information, see www.edmundoptics.com or e-mail gfales@edmundoptics.com.
• This article originally appeared in the October 2008 issue of Optics & Laser Europe magazine.
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