A New Way to Mold Lenses with Freeform and Structured Surfaces
William T. Plummer
WTP Optics, Inc
The thermal infrared spectrum is attracting interest now because of new digital image-capturing devices, such as uncooled bolometer arrays. But these devices still need lenses to form their images. If we want to use an aspheric surface here and there in an optical design, conventional wisdom only offers us relatively expensive diamond-turned components or molded chalcogenide glasses.
A newly patented approach offers a way to manufacture lenses in high volume for the infrared, out to 20 or even 50 microns wavelength, with excellent transmission and a variety of available material properties, for an attractively low cost. We can now make spherical, aspheric, or freeform infrared lens components, diffractive infrared optical elements, and even lens and prism arrays and more general structured surfaces, with the same versatility, convenience, and precision that we enjoy when molding optical plastics for use at visible wavelengths.
The new technique uses a finely divided powder, preferably nanocrystals, of a material such as Potassium Bromide or Cesium Iodide, to fill a mold. These nanocrystals are firmly compressed at room temperature to produce the optical part. A conventional hydraulic press can be used. The mold is constructed much like the molds used for making plastic lenses, but can be simpler because heating and cooling passages are not needed, and the lens material is not injected through a sprue or runners. The optical mold surfaces are made of ground and polished stainless steel, or diamond turned or electroformed nickel, just as for plastic molding.
The resulting optical part does not require secondary polishing. If desired, the optical part can be molded directly into position in its final mounting cell, made of plastic or metal, to avoid all edging or trimming and even the assembly step.
The most effective lens materials for this application are hygroscopic to some degree, but the manufactured part can be protected from moisture in a variety of ways. These include a thin conformal polymer coating, a polymer film sleeve, a spin coating, or merely sealing the part in a mounting cell between non-hygroscopic optical components.
Information about this new manufacturing technology and the much older spectrometric laboratory art on which it is directly based can be found in U.S. Patent #7,591,557 B2, žSolid State Method and Apparatus for Making Lenses and Lens ComponentsÓ.
I will present sample lens designs, and will show photographs of inorganic polycrystalline transparent parts formed by cold powder molding, to demonstrate the capability to mold both freeform and structured surfaces.