Precision double-sided aspheric element

Details Precision double-sided aspheric element Precision double-sided aspheric element

1999-05-20

2003-09-23

Tsai, Henry W. H. (Department: 2183)

Turning

Radially moving rotating tool inside bore

Forming non-circular bore

C082S012000, C082S013000

Reexamination Certificate

active

06622599

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to the field of precision aspheric optical elements, especially lenses, whose surfaces are produced by means of single point machining.
BACKGROUND OF THE INVENTION
Optical imaging systems using conventional spherical lenses generally require a large number of surfaces, and hence a large number of elements, in order to correct for optical aberrations present in the system, and thereby to improve the image quality. In principle, if the number of elements is unlimited, the optical system designer can propose spherical lens assemblies which can, in almost every case, simultaneously correct for all of the common optical aberrations in a lens system of any desired f-number. However, the number of surfaces required to do this may be so high that the resulting lens assembly is excessively large in size and weight, and expensive to produce. Furthermore, because of the residual reflections from each surface, and the bulk absorption in each lens, the transmission of the complete lens assembly may be unduly reduced.
The use of aspheric surfaces, with or without the incorporation of diffractive elements, allows the design and construction of lens assemblies with the same or even better optical performance than an equivalent all-spherical system, but in most cases, with a significant reduction in the number of elements required, and therefore a significant improvement in the overall lens assembly size, weight, cost and optical transmission. In many cases, each aspherical surface in an optical system can be used to replace at least two spherical surfaces. This advantage becomes particularly important in the construction of lens systems for use in thermal imaging systems, such as those which operate in the 8 to 12 micron or the 3 to 5 micron wavelength regions. In order to increase the sensitivity of such systems, the lenses used often have large apertures of the order of several inches. Furthermore, of the materials available for use in these spectral regions, such as germanium, CVD-grown zinc selenide or zinc sulphide, silicon, gallium arsenide, calcium fluoride, and others, some are very expensive, and savings engendered by a reduction in the number of elements, both in material costs and in production and coating costs, are therefore a very significant factor in reducing total system cost. These savings usually outweigh the additional cost of production of the aspheric surfaces.
There are three main methods of producing aspheric surfaces on optical lenses. For the production of low precision aspheric optical elements for use in the visible or near infra-red, aspheric elements are made by casting or molding materials such as glass or optical grade polymers. Because the molds are so expensive to manufacture, such lenses have been used in mass produced optical equipment such as still and video cameras, and in optical disc readers, such as video disc players and.optical memory discs. Such lenses generally contain aspheric elements with one side aspheric and the other side plane or spherical.
A number of patents have recently been granted for inventions which use mass produced lenses with both surfaces of aspherical form. In what is possibly the earliest such patent, U.S. Pat. No. 4,449,792 to N. Arai, S. Ishiyama and T. Kojima, a large aperture single lens is described having both surfaces aspheric. The lens is designed for use as the pickup lens in a video disk reader, and is made of plastic to make it lightweight. A similar lens has been described by M. Koboyashi, K. Kushida and N. Arai, in U.S. Pat. No. 5,475,537. An optical system with improved performance is described, for use in recording and reading information at visible or near infra-red wavelengths on an optical information medium. The system uses a double-sided aspheric objective lens for the imaging function. This lens is described as being made of glass or of “resin”, the resin presumably being a transparent optical grade plastic material. In U.S. Pat. No. 5,583,698 to K. Yamada et al, is described a double asphenric lens for use in a video camera zoom lens. In U.S. Pat. No. 5,642,229 is described a double-aspheric lens for use in a projection lens unit, while in U.S. Pat. No. 5,726,799, a double aspheric lens is described for use in the viewfinder of a compact camera.
All of the above patents describe double aspheric lenses made of glass or plastic materials, and for use in the visible or near infra-red. These are suitable materials from which lenses can be manufactured at low cost, by casting or by molding. Though it is possible to manufacture a mold which will produce high precision cast or molded double aspheric lenses, the system requirements of such lenses are not usually high enough to warrant the cost of such precision. Molding or casting are therefore used, in general, to provide elements with sufficient precision for the requirements of such comparatively low cost, mass produced systems.
Almost all of the optical elements in optical imaging systems for use in the thermal infra-red region are made of materials which cannot be easily cast or molded, if at all. More important, even if they could be manufactured by these methods, the optical precision in these imaging systems is such that the precision afforded by these methods, at a reasonable manufacturing cost, generally falls far short of the system requirements. Maximum surface peak-to-valley irregularities of the order of &lgr;/2 at the red HeNe wavelengths (0.63 &mgr;m) are required in the elements of many such systems to ensure adequate performance. The accuracy of optical surfaces are often measured by using the interference fringes of red HeNe laser light, and for this reason, a description of accuracy in terms of wavelengths of red HeNe laser light is used throughout this specification and is also thus claimed.
The second production method for producing aspheric surfaces are specialized variations of conventional polishing techniques, wherein position dependent pressure is applied to the polishing pad to produce the aspheric form. This method is very labor intensive, and can generally only be used to produce slight asphericity. Recently, automated machines for performing such polishing have been developed.
On an industrial scale, the current almost universally used method of producing the aspheric surfaces of such precision elements, especially those for thermal imaging systems, is by means of turning with a single crystal diamond tool, on a special purpose, ultra-high precision, vibration-free lathe, whose spindle runs at medium to high speeds in air bearings, and which generally uses laser metrology in order to measure the progress of the work. Such diamond turning lathes are capable of accuracies of better than 25 nanometers, and can produce an optical surface of sufficiently high quality for use in the elements of such thermal imaging systems. The cutting tool used is generally a single crystal diamond, specially shaped to provide a smooth cut, though other suitable single point turning tools may also be used. The aspheric profile is obtained by suitable CNC control of the motion of the single point cutting tool relative to the workpiece. Diamond machining can be efficiently applied for small or large production quantities, and for many of the currently used infrared materials, and others. Furthermore, diamond cutting technology can be used for cutting diffractive patterns in addition to the aspheric surface, thereby further increasing the optical performance of the element.
Single point machining of precision optical elements is also used in a fly cutting configuration, wherein the workpiece is substantially static and the cutting tool is rotated at high speed over the element to provide the machining cut. The desired surface profile is obtained by suitable CNC control of the relative motion between the cutting tool holder and the element being produced. Fly cutting is often used in order to produce precision elements without an axis of rotational symmetry, such as precision cylindrical or elliptical surf

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