Imaging system with an integrated source and detector array

An imaging system with an integrated source and detector array. A plurality of light detectors are arranged in a detector array and a plurality of light sources corresponding to detectors in the detector array are arranged in a source array in an epi-illumination system so that light radiated from a point on the object illuminated by a given source is detected by a corresponding detector. An optical system is disposed with respect to the source array and the detector array so as to illuminate an object with light from the source array and image the object on the detector array. Ordinarily, the sources and detectors are coplanar and, preferably, are fabricated or at least mounted on the same substrate. One or more sources in the source array may have a corresponding plurality of detectors, and one or more detectors in the detector array may have a corresponding plurality of sources. In one embodiment the Airy pattern of the point response of the optical system encompasses both a detector and its corresponding light sources. In another embodiment, the optical pathway is split by a diffractive element to produce conjugate points corresponding to light sources and their respective detectors. In a further embodiment, the pathway is split by a Wollaston prism. In yet another embodiment where the illumination and image light have different wavelengths, the pathway is split by dispersion. The system is particularly suited for fluorescence imaging, confocal microscopy and array microscopes.

FIELD OF THE INVENTION

This invention relates to illumination for optical imaging systems, particularly to an integrated detector and source array for epi-illumination in an imaging system, and more particularly in an array microscope.

BACKGROUND OF THE INVENTION

In an imaging system, adequate and appropriate illumination of the object to be imaged is essential. There must be enough light provided to the object to permit a viewer or detector to discern features of the object in the image thereof. In addition, the manner in which the light is provided to the object makes a difference in what features can be detected and the contrast with which they are imaged.

The way in which illumination is provided is particularly important in a microscope. If the object is opaque, it must be illuminated so that the light used to form an image of the object is radiated from the same side of the object on which light illuminates the object. This type of illumination is known primarily as epi-illumination. In epi-illumination the light radiated from an object may be in the form of reflection, in which case the illumination light is modulated upon reflection from the object, or it may be in the form of fluorescence, in which case the illumination light induces fluorescent emission by the object at a different wavelength from the illumination light, as determined by the fluorescence characteristics of the object. The latter case is known as epi-fluorescence. The term “radiated” is used throughout this specification and the claims hereof to include reflection, scattering and fluorescence.

One type of epi-illumination is critical illumination. In this case, the light source is imaged into the object plane. This provides efficient illumination and a short illumination system, but requires that the light source provide uniform radiance. The light source is ordinarily disposed actually or virtually on the optical axis of the imaging lens.

In the foregoing it is assumed that the entire field of view of the imaging lens is simultaneously imaged. However, in a confocal microscope only discrete points in object space are imaged. This is accomplished by placing one or more “pinhole” stops at the image plane of the microscope matched to corresponding discrete points in the object plane, and scanning the object laterally, either by moving the object or the microscope, or moving the scanning the beam through the microscope using, for example, scan mirrors. The light passed by the pinhole is detected and related to the object position as the scan occurs, and the output of the detector is used to produce an image of the object as a whole. In this case, light from the light source is focused to the point on the object plane that is currently imaged. This is typically accomplished by placing a beam splitter between the imaging lens and the image plane so as to pass image light to the image plane while reflecting source light from a virtual image plane created by the beam splitter along the optical axis of the microscope toward the object plane.

In classic optical instruments employing critical illumination, the image is detected by the human eye. In many modern optical instruments, the image is detected by a photo-sensitive device, typically an array of photodetectors. In confocal microscopy, the image is necessarily detected by some storage means. While the use of electronic image detection offers electronic capture of an image and the possibility of reducing the size of an imaging system, effective, compact epi-illumination has remained a challenge.

The recent development of array microscopes, also known as miniaturized microscope arrays, presents new challenges for illumination. In array microscopes a plurality of laterally-distributed optical imaging elements having respective longitudinal optical axes are configured to image respective sections of an object and disposed with respect to a common object plane so as to produce at respective image planes respective images of the respective sections. The individual lenses of this array are formed of small optical elements, or “lenslets,” that place severe constraints on providing illumination. Indeed, the multiplicity of lenslets arranged in an array and the small dimensions of the array suggest that prior art epi-illumination techniques cannot be used. Yet, a principal application for array microscopes is to image specimens, such as biological microarrays for protein analysis that are sufficiently opaque that dia-illumination cannot be used effectively.

Accordingly, there is a need for novel systems and methods for providing critical illumination in epi-illumination imaging systems employing electronic image detection.

SUMMARY OF THE INVENTION

The present invention meets the aforementioned need by providing, in an imaging system, a plurality of light detectors arranged in a detector array and a plurality of light sources corresponding to detectors in the detector array, so that light radiated from a point on the object illuminated by a given source of the source array is detected by a corresponding detector of the detector array. An optical system is disposed with respect to the detector array and the source array so as to illuminate an object with light from the source array and image the object on the detector array. Corresponding detectors and sources are disposed in back of the optical system and interspersed among one another. Ordinarily, the sources and detectors are coplanar, and preferably are fabricated or at least mounted on the same substrate. One or more sources may have a plurality of corresponding detectors, and one or more detectors may have a plurality of corresponding sources.

In one embodiment the Airy pattern point response of the optical system encompasses both a detector and its corresponding light sources. In another embodiment, the optical pathway is split by a diffractive element to produce conjugate points coupled to sources and their respective detectors. In a further embodiment, the pathway is split by a Wollaston prism. In yet another embodiment where the illumination and image light have different wavelengths, the pathway is split by dispersion. The system is particularly suited for fluorescence imaging, confocal microscopy and array microscopes.

Accordingly, it is a principal objective of the present invention to provide novel systems and methods for epi-fluorescence imaging.

DETAILED DESCRIPTION OF THE INVENTION

In a modern imaging system having electronic image detection, the image is typically detected by an array of photodetectors disposed in the image plane of the imaging system. The array may be two-dimensional or one-dimensional. In any event, each photodetector is customarily the source of one pixel of data, though in the case of a color imaging system where one photodetector is provided for each color to be detected one pixel may have multiple photodetectors associated with it. The present invention employs such an array of photodetectors, the improvement being that light sources are interspersed in the array among the photodetectors. In this case, each pixel has one or more light sources, as well as one or more photodetectors, associated with it. Ordinarily, the sources and detectors are coplanar and, preferably, fabricated or at least mounted on the same substrate; however, for some applications the sources and detectors may lie in different planes, though laterally interspersed with one another.

FIGS. 1(a),1(b),1(c) and1(d) show exemplary integrated photodetector and light source arrays according to the present invention. InFIG. 1(a) a two-dimensional array10of integrated photodetectors12and light sources14is shown, each photodetector having a light source associated therewith as shown by circle16. The individual photodetectors12may be any practical opto-electonic photo-sensitive device small enough to provide the desired image resolution, such as CMOS photodiodes, as is commonly understood in the art. The light sources14are preferably light-emitting diodes or laser diodes, depending on the type of illumination desired. Vertical cavity emitting semiconductor lasers are particularly suitable for this invention because they emit light perpendicular to their substrate and can produce unpolarized light. However, other light emitting devices small enough to fit within the array may be used, whether they are semiconductors, lasers or not, without departing from the principles of the invention.

InFIG. 1(b) a one-dimensional integrated array18is shown where each photodetector12has only one light source14associated with it, as inFIG. 1(a). However, there may be applications which call for two or more light sources14associated with a single photodetector12in an integrated array20, as shown inFIG. 1(c), or two or more photodetectors12associated with a single light source14, as shown inFIG. 1(d).

Turning toFIG. 2, a first embodiment22of a one-dimensional integrated source and detector array epi-illumination system takes advantage of the diffraction-limited point response function of an optical system to provide both illumination and detection of the light at a point on an object to be imaged. An optical system24has an optical axis26, an object plane28and an image plane30. The optical system may be a single or multiple element system, a refractive element system, a reflective element system, a diffractive element system, or some combination of the foregoing, as appropriate for the particular application. In any case, the optical system produces an image32at the image plane of a point34on the object plane, the image of the point representing the impulse (point) response, or point spread function (“PSF”), of the optical system. The PSF will depend on the wavelength, the aperture of the optical system and the aberrations of the optical system. To the extent the system can be corrected to render the aberrations insignificant, the image will be effectively diffraction limited. In the case of a circularly symmetric aperture, the PSF will then be an Airy pattern, a two-dimensional cross section of which is shown as image32inFIG. 2. The source34and detector36can be positioned so that the central lobe38of the PSF covers both the source and the detector, provided that both the source and the detector are small enough, without spreading a significant amount of energy into an adjacent source and detector pair. In this manner, the source and detector act as a single point to the optics.

The embodiment23shown inFIG. 8is similar to that shown inFIG. 2, except that the source35is in one plane31and the detector37is in another plane33.

While this first embodiment does not provide optimal light efficiency, it is simple, compact, and straightforward to manufacture. It can be implemented with either a one-dimensional array, as shown inFIG. 1(b) or a two dimensional array, as shown inFIG. 1(a). To increase light efficiency, multiple detectors surrounding the light source within the central lobe of the image could be used. Also, the optical system can be designed to have desired aberrations so as to produce a non-symmetric PSF and maximize the light irradiating the detector area. As will be understood by a person skilled in the art, there are various ways of accomplishing this, including, for example, forming lenses with aspherical surfaces and decentering the elements of the optical system.

A second embodiment40of a one-dimensional integrated source and detector array illumination system, shown inFIG. 3, uses a diffraction element to separate the illumination light from the image light at image plane. As inFIG. 2, the system has an optical system24, with an optical axis26, and object plane28and an image plane30. A source34and detector36, which are part of a linear array, are preferably disposed symmetrically about the optical axis at the image plane30. In this case, a diffraction element42is also included. The diffraction element, which may be, for example, a grating or hologram, is preferably optimized to maximize the diffraction efficiency of the +δ and −δ first diffraction orders, while minimizing the diffraction efficiency of all other orders. The source and detector are then placed in the respective paths of those two orders, that is, coupled thereto, so that the source and detector are conjugate to one another and thereby provide optimum use of light.

In a third embodiment, conjugate points on the image plane can be formed by a Wollaston prism. As shown inFIG. 4, a quarter wave plate44may be placed in front of a Wollaston prism46at an angle to the two eigenaxes thereof so that the optical pathway is split into two pathways having respectively orthogonal polarizations and respective angles of refraction, as indicated by the dot48and arrow50. This requires either that the source36produce light that is linearly polarized in the direction represented by dots48, or that a linear polarizer49be used to produce such linear polarization. The source light is then circularly polarized in one direction by the quarter wave plate, circularly polarized in the opposite direction upon reflection from the object, then linearly polarized in the direction of arrows50by the quarter wave plate. Thus, this arrangement creates two conjugate points in the image plane that correspond to a light source34and photo-detector36, respectively.

In the case of fluorescence imaging, the dispersive qualities of optical elements can be employed to produce conjugate points in image space. In fluorescence imaging the light source has a first wavelength, or more generally a first energy spectrum, that excites the object to fluoresce and thereby emit light at a different wavelength, or more generally a different energy spectrum. In this case, the light sources34emit light at one wavelength, typically an ultra-violet wavelength, and the photo-detectors36either are sensitive to a different wavelength or associated with filters that limit the spectrum received thereby to a different wavelength. For example, a direct vision prism52splits the optical pathway54into two branches corresponding to the excitation and fluorescence emission wavelengths, respectively, as shown in the embodiment ofFIG. 5. Thus, it creates two conjugate points in the image plane that correspond to a light source34that emits light at one wavelength and photo-detector36that is responsive to another wavelength. A number of detectors can be used as well to detect light corresponding to a corresponding number of different wavelengths, such as red, green and blue light.

Generally, any device that conjugates spatially-separated points corresponding respectively to light sources and photo-detectors in image space for epi-illumination may be used without departing from the principles of the invention.

While the light source array and photodetector array are ordinarily coplanar for producing critical illumination, they can be disposed in axially separate planes. This may be desirable, for example, to compensate for axial dispersion in fluorescence imaging. In that case, the array of light sources is placed at the image plane for the excitation light, while the photodetector array is placed at the image plane of the wavelength of light to be detected.

The embodiments ofFIGS. 2–5can also be used in a confocal mode, as shown with respect to the second embodiment inFIG. 6. In this case, a stop is provided with an array of pinhole apertures55, one for each detector34, and with conjugate apertures for the light sources36. The image of each source, which is essentially a point source, is conjugated with the object plane. After reflection from the object, the light is imaged onto a corresponding pinhole aperture54. The amount of light that passes through the aperture is closely related to the focus of the image and can be used to gauge the distance of the object surface to the focal position. If the object and the light beam are then moved with respect to one another, the profile of the object can thereby be determined. By providing a linear array of source-detector pairs and scanning the object in a direction perpendicular to the array, rapid confocal scanning can be achieved.

The embodiments ofFIGS. 2–6can be employed in an array microscope, as shown inFIG. 7. An exemplary embodiment of an array microscope56comprises pluralities of lenses58, corresponding to individual microscope elements, disposed on respective lens plates60,62and64, which are stacked along the optical axes of the microscope elements. An array66of linear, integrated source-detector arrays68resides above the last lens plate. The array microscope66is typically employed to scan a sample on a carriage70as the carriage is moved with respect to the array or vice versa. Each set of corresponding lenses58and respective lens plates60,62and64images a section of the object onto a corresponding source-detector array58as the object moves by on the carriage70.