Optical: systems and elements – Lens – Telecentric system
Reexamination Certificate
1999-11-03
2001-11-27
Epps, Georgia (Department: 2873)
Optical: systems and elements
Lens
Telecentric system
C359S364000, C359S419000, C359S431000, C359S433000, C359S631000, C359S727000, C359S740000
Reexamination Certificate
active
06324016
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a telecentric lens for use in viewing terrestrial objects, and more particularly to a reflective telecentric lens and an on-axis type concave mirror to avoid blockage of a portion of the field of view.
BACKGROUND OF THE INVENTION
A conventional lens is incapable of accurately portraying objects that are off axis to the lens or at different distances from the lens. These perspective distortions can create significant problems for optical systems such as, for example, interpretation of data by computer software. A telecentric lens, however, maintains a constant viewing angle at any point across the clear aperture of an associated objective lens. The telecentric lens can therefore be used to accurately portray objects that are slightly off axis or at different distances from the lens. The ability to accurately portray objects in the manner described makes the telecentric lens particularly suitable for use in measurement devices for manufacturing processes. By eliminating perspective distortion, the telecentric lens produces a dimensionally accurate image, which is simple for software to interpret.
In prior art telecentric lenses, the refractive elements are formed by materials such as glass or plastic for visible light; and other materials are employed for use in the infrared region of the electromagnetic spectrum. For any particular type of material used, however, the useable spectral range is greatly limited. Also, as is the case for all imaging devices, as the required size of the device is increased, the weight, complexity, and cost are increased in order to maintain reasonable performance. As the size of refractive imaging devices increase, the imaging devices become problematic more quickly than equivalent performance reflective devices, once a certain size threshold is reached. Such inherent problems are the reasons that larger optical telescopes are made from concave mirrors (reflector telescopes) as opposed to glass elements (refractor telescopes). Also, in general, reflective imaging devices require far fewer optical elements than refractive devices of the same general performance.
Telecentric lenses of the type used to form images of close terrestrial objects, especially at magnifications that are more useful for industrial imaging such as automated inspection or machine vision, have been limited to all-refractive designs. One of the reasons for this limitation is that imaging of near or terrestrial objects can suffer greatly from having a portion of the field of view blocked by a central transfer mirror or lens. The transfer mirror or lens is used with near impunity for astronomical telescopes used for viewing objects that are nearly infinitely far away and thus remain relatively unaffected by the central blocker. U.S. Pat. No. 4,798,450 to Suzuki discloses other multiple mirror designs that overcome the central blocker problem for terrestrial objects, and consist of special and expensive mirror forms such as off-axis parabolas, ellipsoids, or hyperboloids.
An object of the invention is to produce a reflective telecentric lens that may be constructed less expensively than equal performance all-refractive telecentric lenses.
Another object of the invention is to produce a reflective telecentric lens that may be constructed less expensively than multi-mirror reflective telecentric lens designs.
Another object of the invention is to produce a reflective telecentric lens having fewer components than equivalent performance devices of prior art.
Another object of the invention is to produce a reflective telecentric lens that may be easily re-configured to accommodate different magnifications.
Still another object of the invention is to produce a reflective telecentric lens that has improved optical performance.
SUMMARY OF THE INVENTION
The concept of using a concave mirror as part of an optical imaging device is known. Mirror based telescopes are also well known. However, telescopes are designed for viewing infinitely far astronomical objects and thus not subject to the same design limitations as optics designed for viewing close, terrestrial objects.
The present invention uses a simple concave mirror form as a primary collector mirror/lens along with a small aperture and a relatively simple secondary lens to create a telecentric lens system for viewing near or terrestrial objects at magnifications that are useful for industrial inspection or measurements, along with the arrangements that overcome the central blocker problem. These arrangements typically consist of a beamsplitter, a true off-axis concave mirror, or a tilted on-axis concave mirror in a pseudo-off-axis manner.
The design of a concave-mirror based telecentric lens allows for a larger useable clear aperture than is currently commercially available with all refractive optics designs. Additionally, the component cost of constructing such a telecentric lens is inherently less than for all-refractive designs of similar capability in terms of size as well as optical imaging performance. Finally, a mirror-based lens generally allows for a broader useable spectral range than all-refractive designs that will be more subject to image degradation due to chromatic aberration.
The above, as well as other objects of the invention, may be readily achieved by a telecentric lens comprising a concave mirror defining a primary collector lens for receiving light rays from a terrestrial object and for emitting the light, the collector lens having a first and a second optical axis defining a first and second optical path, the light emitted from the collector lens having a focal point along the second optical axis of the collector; an image lens disposed along the second optical axis of the collector lens in the second optical path; and a telecentric stop disposed at the focal point along the second optical axis of the collector lens between the collector lens and the image lens.
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Epps Georgia
Fraser Donald R.
Lester Evelyn A
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