Patent Application
20050195483
1. Technical Field of the Invention
The present invention relates generally to the field of imaging systems. More particularly, the invention relates to an imaging system and method for achieving a large depth of field without decreasing the relative aperture of an imaging lens.
2. Background of Invention
A large depth of field is important in imaging systems having an imaging lens position that is fixed with respect to an image receiving surface, but in which the distance along the optical axis from the lens to the object to be imaged may vary significantly. In a typical imaging system, depth of field is inversely proportional to the relative aperture of the imaging lens such that the relative aperture of the lens must be decreased in order to increase the depth of field. Decreasing the relative aperture, however, reduces light sensitivity and resolution; and, as a result, it is difficult to design an imaging system that has a large depth of field while maintaining high light sensitivity and fine resolution.
There is, accordingly, a need for an imaging system that achieves a large depth of field without decreasing the relative aperture of an imaging lens.
In accordance with the invention, an optical imaging system and method is provided for achieving a large depth of field without having to decrease the relative aperture of an imaging lens.
An imaging system in accordance with the invention has a light source for sequentially illuminating an object to be imaged with light of different ones of a plurality of wavelengths, and an imaging lens that has a focal length that varies with the wavelength of the light that illuminates the object. For each wavelength of light by which the object is illuminated, the imaging lens will image a different object plane onto an image receiving unit. If the object remains substantially stationary while the light source is being tuned to the different wavelengths, and if the wavelength range and the design of the imaging lens are chosen appropriately for possible object distance variation, the image receiving unit will capture one well-focused, high resolution image of the object.
Embodiments in accordance with the invention provide an imaging system and method for achieving a large depth of field without having to decrease the relative aperture of an imaging lens.
Imaging lens 14 has a focal length that varies inversely with the wavelength of the light that illuminates the object being imaged during an imaging operation. Thus, for each wavelength of light by which object 20 is illuminated, imaging lens 14 will image a different object plane onto image receiving unit 16. It should be understood that the term “varies inversely” as used herein means that the focal length of the imaging lens increases when the wavelength of the light that illuminates the object decreases, and decreases when the wavelength of the light that illuminates the object increases. The term is not intended to imply that there is a strict mathematical inverse proportionality between focal length and wavelength. In addition, it should be understood that although, in the exemplary embodiments described herein, the imaging lens has a focal length that varies inversely with the wavelength of the light that illuminates the object, the focal length may also vary in other ways relative to wavelength, for example, the focal length may increase as the wavelength increases; and the invention is not limited to any particular manner in which the focal length of the imaging lens varies with the wavelength of the light that illuminates the object.
In the exemplary embodiment illustrated in
fdiff=f0/λ (1)
For a combined refractive/diffractive lens that has both refractive and diffractive properties, however, the focal length (fref) of the refractive component and the focal length (fdiff) of the diffractive component are combined as follows:
1/f=1/fref+1/fdiff (2)
As will be discussed hereinafter, with a properly designed combined refractive/diffractive lens, a well-focused image of an object can be formed by illuminating an object with light of only a few, for example, 3-5, different wavelengths within an appropriate range of wavelengths. In accordance with further exemplary embodiments of the invention, imaging lens 14 can also be a diffractive lens or a refractive lens formed of a dispersive material (a material that changes with wavelength), and can be formed of a single lens or a plurality of lenses, and the invention is not limited to any particular imaging lens.
Light source 18 is a wavelength-tunable light source capable of illuminating object 20 with light of a plurality of different wavelengths. In the exemplary embodiment illustrated in
In the exemplary embodiment illustrated in
Image receiving unit 16 is a photosensor array together with appropriate electronics for converting analog photosensor outputs into digital data. In the exemplary embodiment illustrated in
As shown in
In an imaging operation, light sources 18a-18d are turned on and off one at a time in sequence by controller 30; and image receiving unit 16 captures an image of object 20 each time the object is illuminated by one of the light sources. If the wavelength range of the light sources and the design of the imaging optics are chosen appropriately for the possible variation in object distance, one of the images captured by image receiving unit 16 will comprise a well-focused, high resolution image of the object.
In an exemplary embodiment of the invention, object 20 comprises the iris of a person's eye. Such an imaging application may involve a variation in object distance of from about 5 inches to about 20 inches. For such an application, light sources 18a-18d can comprise commercially available LEDS having different wavelengths in the wavelength range of from about 450 nm to about 980 nm. With an imaging lens having a diffractive component only, this wavelength range will produce a change of over 2× in the focal length of the lens. With a combined refractive/diffractive lens as the imaging lens, the lens will not have to change its focal length by such a large factor. For example, if the refractive surface power is designed to be suitable for forming a well-focused image of an object positioned at about the middle of the anticipated object distance range, the combined lens would only have to change its focal length by about 1.14×, and this can be readily achieved by choosing only a few, for example, 3-5 different LED wavelengths in the 450 nm to 980 nm wavelength range.
Selection of the best image among the plurality of formed images, i.e., the image that is most sharply focused, is accomplished by selector 32. Selection can be accomplished in various ways. One selection procedure comprises a passive autofocus procedure in which the intensity among adjacent pixels of the formed images is examined. For images that are out-of-focus, adjacent pixels will have similar intensities. The image that provides the maximum intensity difference between adjacent pixels will be the image that is most in-focus and that is selected by the selector. In another selection procedure, Fourier transforms of the formed images are taken, and the image with the highest frequency component is the image that is most in-focus. In general, procedures for selecting a sharply focused image are well-known in the art and need not be described in any detail herein, and the invention is not limited to any particular selection procedure.
While what has been described constitutes exemplary embodiments of the invention, it should be recognized that the invention can be varied in many respects without departing therefrom. For example, although exemplary embodiments described herein provide a well-focused image of the iris of an eye, the invention can be used in other biometric applications, such as providing a well-focused image of a fingerprint; and in numerous other applications wherein the object distance may vary. Among such other applications include applications involving triggered surveillance of a defined space. For example, an entomologist can set up an unattended imaging system to be triggered by an insect that enters a defined space along and around the optical axis of the imaging system, but not at any specific object distance. The imaging system will then capture a series of images, one of which is selected for best focus. In another application, an imaging system can be provided on a robot to provide well-focused images of nearby objects to assist the robot in moving around. In a further application, an imaging system can be used by a consumer to record images of valuable possessions to provide an inventory for insurance purposes and the like without it being necessary to remove the possessions from their normal positions within the home. In addition, although exemplary embodiments described herein describe forming an image of an object having an object distance that may vary from between about 5 inches to about 20 inches, the invention can also be used to capture a well-focused image of an object within other object distance ranges, and the invention is not limited to any particular object distance range. Because the invention can be varied in many ways, it should be understood that the invention should be limited only insofar as is required by the scope of the following claims.
