Diamond Turning

Lenses and mirrors for high quality imaging require high precision manufacture. Historically, and to this day, relatively crude  optical grinding and polishing machinery has been employed to generate these very high precision optical surfaces.  However, the overall process is lengthy, labor intensive, and requires  highly skilled personnel. Cptics manufacturing costs are therefore high. Flat and spherical surfaces are routinely achieved but aspheric surfaces which hold the potential to reduce the cost of optics by reducing the number of lens surfaces are very difficult and costly.

Diamond turning holds the potential to not only reduce optics manufacturing costs but also  permits the lens designer to incorporate aspheric surfaces into the lens design to improve image quality. A single point diamond tool is mounted in an air bearing spindle of very high stiffness and virtually non-existent runout.  Adding laser interferometric, computer controlled tool positioning allows the manufacture of highly accurate spherical, aspherical and plane surfaces with minimal residual tool cutting marks. Further processing of the surface is not required for infrared applications. Post-polishing for visible light  applications is desirable.

Examples of some diamond turned optics are shown in the accompanying photo. A high speed scan disc incorporates a circular array of concave spherical  reflectors diamond turned into an aluminum disc substrate. The mirrors are located radially from the  disc axis of rotation with great precision and accurately located  tangentially from one another, as well. The depth of each mirror or “dimple”. and its radius of curvature are inherently  tightly controlled to yield a circular array of focal points equidistant from the disc surface.

A parabolic Collection Mirror and spherical Objective Mirror both incorporate the means to faciltate alignment  and mounting. This advantage further reduces product manufacturing costs.

The Strip Mirror is basically a thin slice of a 90-degree reflective hollow cone  cut  perpendicular to the cone axis. The central 30-degree portion is used to redirect light 90 degrees while the remainder is used for mounting.

The Faming Mirror is a lightweight flat mirror which is required to oscillate about its axis at a relatively high frequency. Its thickness is minimized whiloe maintaining its optical flatness while  in motion by machined in structural ribbing on its rear surface (not visible in the photo)

The Meniscus Lens comprises a germanium subsdtrate and two spherical concentric surfaces. While manufacturable by conventional means, it is readily manufactured using the diamond turning process. An antireflection coating is subsequently applied.

The optical components illustrated above when assembled into a housing become a compact video rate optical scanner, CVROS.

The diamond turning equipment used to machine the scan disc discussed above is illustrated below. Details of its design and application can be found in a technical paper by Barber and Walter at the link, “Dimple Disc Diamond Turning Machine”.

 

A brief history and technical overview of diamond turning can be found at “Diamond Turning at Kollmorgen. This very interesting story of the beginnings of diamond turning optics in the U.S. has been contributed by H. Torberg, a primary advocate and promoter of the technology both within Kollmorgen Corporation and the optical manufacturing community at large.

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