This invention relates to optical systems for metrological measuring apparatus, and more particularly to an improved telecentric, parfocal optical system having means for selectively indexing any one of a plurality of different magnifications into registry with a video camera that forms part of the measuring apparatus. Even more particularly, this invention relates to an improved optical system of the type described which utilizes a single, fixed entrance lens assembly for all magnifications.
In metrological apparatus of the type described it is customary to employ a worktable defining an object plane in which the workpiece to be inspected is positioned and illuminated. As disclosed for example in U.S. Pat. No. 5,389,774, an image of the workpiece can then be projected by an associated optical system to a video camera, and from there via a microprocessor or CPU control to an associated TV monitor, or the like. In certain such optical systems it has been customary to employ a zoom lens mechanism to vary the magnification of the image that is projected to the associated video camera. While such apparatus is particularly suitable for inspection of selected objects, it does have the disadvantage that the degree of magnification of the inspected object is limited by virtue of the nature of a conventional zoom lens mechanism. Moreover, even where such variations in magnification of an image can be effected, most such prior art devices do not use telecentric, parafocal optical systems for all such magnifications.
Moreover, still other prior art metrological apparatus, such as disclosed for example in U.S. Pat. No. 4,743,771, includes a beam splitter for reflecting an image of a grid or reticle onto the surface of a workpiece that is being inspected. The combined images of the reticle and workpiece are then projected upwardly along a vertical axis (z) to a video cmaera for use in effecting automatic focusing of the apparatus. Such apparatus, however, includes no means for providing different magnifications of the images. Moreover, by using a beamsplitter to reflect the reticle image through the imaging lenses, objectionable stray light reflections off the optical elements, as caused by the illumination of the workpiece surface, reduce the contrast between the reticle image and the workpiece image.
It is an object of this invention, therefore, to provide for metrological apparatus of the type described an improved optical system capable of selectively providing any one of a plurality of different magnifications of the image that is projected to the video camera of the apparatus.
Another object of this invention is to provide an improved optical system of the type described which is always telecentric and parfocal for all magnifications of the system.
Still another object of this invention is to provide an improved optical system of the type described which utilizes a single, fixed entrance lens assembly for each possible magnification of the system.
It is an object also of this invention to provide for apparatus of the type described a telecentric, variable magnification optical system of the type in which each magnification has a different F-stop aperture, and each such F-stop, when in use, is always located at the back focal point of the system's entrance lens assembly.
A further object of this invention is to provide an optical system of the type described including means for injecting a reticle image and normally directed surface illumination between the entrance lens assembly and the system's telecentric stop location, whereby the E-number for the reticle projection means may be less than that of the imaging lens system.
Other objects of this invention will become apparent hereinafter from the specification and from the recital of the appended claims, particularly when read in conjunction with the accompanying drawings.