Optics: measuring and testing – Focal position of light source
Reexamination Certificate
1998-12-30
2001-02-27
Font, Frank G. (Department: 2877)
Optics: measuring and testing
Focal position of light source
Reexamination Certificate
active
06195159
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to a system for testing properties of a lens, and more specifically to a system for testing lenses that are used in scanners or the like. In particular, the system determines a lens' through-focus modulation transfer function, distortion, focal length, and color registration, and then outputs these values to a user.
2. Description of the Related Art
Scanners typically employ a lens to focus light from an image (e.g., a document image, a graphics image, etc.) onto a detector, such as a charge coupled device (hereinafter “CCD”). The detector, in turn, converts light from the document image into electrical signals, from which data corresponding to the document image is generated. As might be expected, the accuracy of this data is based on the quality of the lens used to focus the light from the document image. In particular, the accuracy of the data is dependent, at least in part, upon the modulation transfer function (hereinafter “MTF”) of the lens.
The MTF of a lens defines how sharply the lens is able to focus. That is, for a line pattern having a particular frequency, such as 35 line-pairs-per-millimeter (hereinafter “lp/mm”), the MTF corresponds to the contrast of an image formed by the lens relative to a contrast of an original image. A high MTF value is, of course, preferable, since it means that the lens will have high contrast and, therefore, will provide more accurate document imaging. Accordingly, systems have been devised to test a lens' MTF values at discrete spatial frequencies. Specifically, such systems focus light from a target having a known line pattern of a particular frequency in lp/mm, through the lens, and onto a CCD. Thereafter, the MTF of the lens at the frequency of the target line pair pattern is determined mathematically based on the light received from the lens. One problem with this method is that in order to determine the lens MTF at a plurality of frequencies, a plurality of line patterns, each having a lp/mm frequency of a desired MTF frequency to be measured, is required to be placed on the target.
Other problems also exist with conventional lens testing systems because those systems are not able to obtain a through-focus MTF for the lens, and do not take into account the MTF of the CCD, among other things, when determining the lens' MTF. That is, the MTF of the CCD affects the image data generated from the light incident on the CCD. Thus, conventional lens testing systems, which do not account for the MTF of the CCD, are unable to determine a lens MTF accurately.
Another problem which exists with conventional lens testing systems is that a lens may not be tested over its full field of view. Systems which test a lens in the center of the field of view are known and it is known to translate a test apparatus from the center of the field of view to the extreme edges of the field of view and to repeat the test at each location of the field of view for which the lens performance is to be tested. It is desirable, especially in scanning systems to determine the lens performance characteristics e.g. MTF, distortion and color registration as well as other lens characteristics at a plurality of field positions along the lens field of view and to reduce the time and expense of performing a separate lens test at each of the plurality of field positions along the lens field of view for which test data is required.
Moreover, conventional lens testing systems do not test other properties of a lens which can affect light transmission and thus document imaging. Examples of these other properties include color registration, focal length, and distortion each of which may vary along the field of view of the lens and through the lens depth of field. Distortion refers to the amount that an image is distorted when light is transmitted through the lens. As might be expected, the less distortion that a lens produces, the better. Color registration refers to differences in light transmission for different wavelengths (i.e., colors) of light. That is, a lens may transmit different colored light to different locations, e.g. each color light may have a unique focal plane, thereby leading to registration errors between different colors of the same image. These registration errors can significantly affect the way in which a color image is perceived. Last, but not least, a lens' focal length affects its ability to transmit light accurately.
For at least the foregoing reasons, conventional lens testing systems are inadequate, particularly with respect to testing lenses for use with document scanners or other types of imaging systems. Accordingly, there exists a need for a lens testing system which addresses the foregoing drawbacks of conventional lens testing systems. In particular, there exists a need for a lens testing system which is able to provide an accurate, through-focus measurement of a lens MTF. In addition there exists a need for a lens testing system which is able to quickly provide a measure of a lens MTF, and other lens properties, such as distortion, color registration, and focal length at several field locations.
SUMMARY OF THE INVENTION
The present invention addresses the foregoing needs.
Specifically, in one aspect, the invention is a system for determining one or more properties of a lens, which includes a light source and a target pattern mounted relative to the light source so that the light source illuminates the target pattern and so that light transmitted through the target pattern is transmitted through the lens. In the system, a detector detects the light transmitted through the lens, and outputs analog electrical signals corresponding to features of the target pattern. Preferably the target pattern is positioned at a first conjugate position of the lens and an active surface of the detector is positioned at a second conjugate position of the lens, corresponding to a position of an image of the target pattern formed by the lens.
In order to determine a depth of focus of the lens under test and in order to select an ideal operating position of a lens within a scanner system, the detector is movably mounted so as to be movable along an optical axis of the lens under test. The detector is capable of movement to a plurality of discrete positions on either side of the second conjugate position of the lens such that the detector moves relative to the image formed by the lens. A generator (e.g., an electronics interface box) includes an analog to digital, (A/D) converter to convert the analog electrical signal from the detector to a digital signal representative of analog electrical signal output from the detector in response to the image on the active surface. A memory stores computer-executable process steps and a processor executes the process steps stored in the memory so as to obtain an MTF value of the lens based on the digital signal.
Thus, unlike its conventional counterparts, the present invention is able to obtain a relatively accurate through-focus MTF for a lens. In preferred embodiments, the invention may also obtain a focal length of the lens, a distortion of the lens, and/or a color registration error of the lens, based on processing of the digital signal. Thus, these embodiments of the invention are able to test a lens' MTF in addition to other relevant lens properties. As a result, the invention is able to provide more comprehensive lens testing than its conventional counterparts.
In preferred embodiments of the invention, the processor executes process steps stored in the memory so as to perform one or more corrections on the MTF value of the lens. Such corrections may include corrections to take into account MTF values of the detector. By correcting the lens MTF in accordance with the MTF of the detector, the invention is able to obtain more accurate lens MTF measurements than the conventional systems described above.
In some embodiments of the invention, the foregoing corrections include correcting the lens MTF to account
Berven Danielle M.
MacDonald Robert E.
Agfa Corporation
Font Frank G.
Kelley Edward L.
Nguyen Tu T.
Pysher Paul
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