Abstract:
A microscopy technique for viewing regions where a sample absorbs, as opposed to emits, fluorescent light. The technique includes illuminating a sample with an exciter light in order to generate fluorescent light from the sample and filtering light received from the sample such that fluorescent light is substantially attenuated. Regions that primarily emit fluorescent light will then be black, but regions where the exciter is primarily absorbed but where there is little emissive fluorescent activity can be viewed as a darker shade of the background color.

Description:
RELATED APPLICATIONS 
   This application claims the benefit of priority of U.S. Provisional Patent Application Ser. No. 60/567,822 filed May 5, 2005. 

   FIELD OF INVENTION 
   The invention generally relates to the field of microscopy, and more specifically to fluorescent contrast techniques and related apparatus. 
   BACKGROUND OF INVENTION 
   An important class of microscopy image contrast techniques is based on the phenomenon of photoluminescence. It well known that when certain molecules are exposed to an exciter light (typically a high power, short wave ultra-violet or blue light), they will emit a longer wavelength light (i.e., fluorescent light). Fluorescence (and/or phosphorescence) may occur naturally in a sample, for example, chlorophyll in botanical specimens, or may be induced through the use of particular binding agents such as dyes or other such fluorochromes. In conventional fluorescence microscopy, an excitation filter is typically employed to generate the exciter light from a polychromatic illumination source, and a sharp barrier or emission filter is typically employed to permit only the fluorescent light to reach the ocular or camera (i.e., the viewer). Thus, for example, a short-wave blue light exciter filter in conjunction with a red barrier filter enables chlorophyll-containing organelles to appear brilliant red over a relatively dark background. 
   One of the shortcomings with conventional fluorescence microscopy, however, is that the information which is provided to the viewer only indicates the area or region where fluorescent light is emitted from the sample. The conventional technique does not inform the viewer as to where the light is absorbed, which may be different from where light is emitted. This difference may arise, for example, due to some type of tunneling mechanism or other unexplained phenomenon. 
   Accordingly, it would be desirable to be able to image a sample in order to ascertain where the exciter light is being absorbed, and to ascertain if fluorescent emitting structure is or is not co-located with the fluorescent absorbing structure. This would enable the microscopist to determine if the sample does indeed exhibit some type of tunneling effect. 
   SUMMARY OF INVENTION 
   A first aspect of the invention relates to a method and related apparatus for viewing the absorption of a fluorescent exciter light. According to this aspect of the invention an image is acquired by illuminating a sample with an excitation light so as to generate fluorescent light from the sample. The light received from the sample is filtered such that the fluorescent light emitted by the sample is substantially attenuated, thereby acquiring a filtered image of the sample. The filtered image is provided as an observable image to a viewer, e.g., a person or an automated vision processing system. The observable image visually indicates areas where the exciter light is predominantly absorbed. 
   The light illuminating the sample preferably consists essentially of wavelengths which generate fluorescence in the sample. Preferably, substantially all light received from the sample is filtered except for the exciter light. 
   Another aspect of the invention relates to a method and related apparatus for differentially comparing a fluorescent image with a fluorescent absorption image. According to this aspect of the invention an image is acquired by illuminating a sample with an exciter light in order to generate fluorescent light from the sample. A first image of the sample is acquired by substantially attenuating the fluorescent light received from the sample. A second image of the sample is acquired by substantially attenuating the exciter light received from or otherwise reflected off the sample. The first image is differentially compared against the second image and displayed to a viewer (e.g., a person or an automated vision processing system). For example, the differential image can be the result of a differential matrix calculation such as I i =|A i −  B   i |, where A i  represents a pixel from the first image, B i  represent the corresponding (i.e., similarly situated) pixel from the second image, and  B   i  is the inversion thereof. (In this example, each pixel is a 24 bit value comprising three discrete 8-bit integers, each integer representing one of the primary colors.) 

   
     BRIEF DESCRIPTION OF DRAWINGS 
     The foregoing and other aspects of the invention will be better understood with reference to a detailed description of preferred embodiments of the invention, discussed below, in conjunction with the drawings, wherein: 
       FIG. 1  diagrammatically illustrates conventional fluorescence microscopy techniques and the images resulting therefrom; 
       FIG. 1A  is a color-schematic detail view of a portion of  FIG. 1  (wherein colors are schematically represented by different cross-hatching patterns as indicated in  FIG. 4 ); 
       FIG. 2  diagrammatically illustrates the technique of fluorescence absorption microscopy according to a preferred embodiment of the invention and the images resulting therefrom; 
       FIG. 2A  is a color-schematic detail view of a portion of  FIG. 2 ; 
       FIG. 3  is a diagram of an electrically inverted, fluorescence-absorption image acquired using a darkfield illumination system; 
       FIG. 3A  is a color-schematic detail view of a portion of  FIG. 3 , prior to inversion; 
       FIG. 3B  is a color-schematic detail view of the same portion of  FIG. 3 , post inversion; 
       FIG. 4  is a concordance map that correlates various types of cross-hatching patterns employed in the drawings with representative colors; 
       FIG. 5  is a schematic diagram of an epi-fluorescent microscope according to the preferred embodiment; 
       FIG. 6  is a schematic diagram of a transmitted fluorescent microscope according to an alternative embodiment; and 
       FIG. 7  is a schematic diagram of an epi-fluorescent microscope according to an alternative embodiment. 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1  schematically illustrates the conventional fluorescence microscopy technique wherein an exciter light  10  (blue, in this example) is used to illuminate a sample  12 . Amongst other structures  13 , the sample  12  includes structure  14  which emit fluorescent light  16  (red, in this example). All light acquired from the sample which forms the basis for an image of the sample displayed to an observer is filtered by an emission filter  18  having a pass band tuned to the wavelengths of the expected fluorescent light, which in this case is a red filter. Consequently, the resultant image  20  of the sample viewed by the observer shows only the fluorescent regions  15  in the fluorescent color, e.g., red  2 . The background  21  of the resultant image  20  will be dark or black and, depending on the illumination technique employed and the spectrum of emitted light, the other structure  13  will generally not be visible or will be poorly visible. This is difficult to see in the black and white patent drawings but is shown schematically in the detail view of  FIG. 1A . As previously indicated, one of the shortcomings with the foregoing technique is that it enables the observer to view only the regions where the fluorescent light is being emitted, which may or may not correspond to regions where the exciter light  10  is absorbed. 
     FIG. 2  schematically illustrates fluorescence absorption microscopy according to one embodiment of the invention. In the preferred technique, an exciter light  10  is used to illuminate the sample  12 , which will emit fluorescent light  16 . All light acquired from the sample which forms the basis of an image of the sample for display to an observer is filtered by an emission filter  19 . In contrast to the prior art, the emission filter  19  substantially attenuates the wavelengths of the expected fluorescent light. Consequently, the resultant image  22  of the sample viewed by the observer will be black in regions where the fluorescent light is predominantly emitted. 
   In preferred embodiments, the emission filter  19  has a pass band, which allows through the wavelengths of the exciter light as reflected off of the sample. In most cases, the pass band of the emission filter  19  will be tuned to the wavelengths of the exciter light because the reflected light will be the same color as the exciter light, particularly if the sample is substantially transparent and/or is disposed over a whitish background. If desired, the emission filter  19  can be constructed to substantially attenuate potentially harmful ultra-violet light and pass through only the visible component of the exciter light. Thus, for example, filter  19  can be a violet-blue filter. Consequently, as exemplified in  FIG. 2 , the background color  21  of the resultant image  22  will be the (visible) color of the exciter light  10 , e.g., violet-blue, and the fluorescent regions  15  will be black. However, in regions  17  where the exciter light is primarily absorbed but where little fluorescent activity exists, such regions will be viewable as a darker shade of the background color, e.g., a dark blue. This is difficult to see in the black and white patent drawings, but is shown schematically in the detail view of  FIG. 2A . 
   The contrast between the predominantly absorbing regions  17  of the sample and the background color  21  may possibly be significantly improved by employing dark field illumination techniques, as known in the art per se. In this case, as schematically illustrated in the detail view of  FIG. 3A , the background color  21  is black. The predominantly absorbing region  17  of the sample is dark blue and the fluorescing region(s)  15  is black. By inverting the colors of the sample image, the background and fluorescent regions  21 ,  15  of the image can be made to substantially disappear, leaving the emphasis on the predominantly-absorbing regions  17  in the resultant image  24 , as seen in  FIG. 3 . In the foregoing example, regions  17  will assume an orange color when inverted, as schematically illustrated in  FIG. 3B . 
     FIG. 5  schematically illustrates the basic components of a microscope  30  configured to provide fluorescence absorption microscopy as discussed above. The illustrated embodiment is an epi-flourescent microscope wherein the objective  34  acts as the condenser for the exciter light and as an objective for the fluorescent light. Light source  32  is preferably a strong polychromatic illumination source, which is filtered by an excitation filter  36  to provide the desired spectrum for exciter light  38 . For example, a high-pressure xenon arc can be utilized as the light source  32  in order to provide a high intensity UV light. Monochromatic light sources such as lasers can also be employed in the alternative. A color splitter or dichroic mirror  40  reflects the exciter light  38  through the objective  34  to illuminate the sample specimen on stage  42 . Fluorescent light  44  generated by the specimen and the reflected background light is collected by the objective  34  and passes through the dichroic mirror  40  and an emission filter  46 . As discussed previously, the emission filter  46  preferably attenuates the wavelengths of the expected fluorescent light, and passes through the visible portion of the exciter light  48 . In the illustrated embodiment, the filtered light  48  impinges upon a CCD sensor  49  of a digital camera  50 , although in alternative embodiments the filtered light  48  can be viewed through an ocular. A microprocessor  52  is coupled to the camera  50  and a display monitor  54  in order to display the resultant image. The microprocessor provides the ability to easily manipulate the resultant digital image, such as providing a color inversion function. 
   In the preferred embodiment, the excitation and emission filters  36 ,  46  are mounted on wheels (not shown), which enable the microscopist to vary the particular filter used. This can be carried out manually, or more preferably under computer control. See in this regard PCT Publication No. WO 98/45744, “Color Translating UV Microscope”, the contents of which are incorporated by reference herein, for a description of computer-controlled rotating filters. By varying the filters, it is possible to provide a two or three-segment emission filter  60 , as shown in  FIG. 5A . A first filter segment  62  has its passband keyed to pass through fluorescent light and block the excitation light as in the prior art. This provides a conventional resultant fluorescent image as shown in  FIG. 1 . A second filter segment  64  substantially attenuates the fluorescent light and passes through the visible portion of the excitation light. This provides a resultant fluorescent absorption image such as shown in  FIG. 2 . These first and second images can be simultaneously displayed on the monitor  54  for comparison. Alternatively, the one image can be overlaid over the other image for display. For example, the microprocessor  52  can carry out a differential matrix calculation such as I i =|A i −  B   i |, where A i  represents a pixel from the first image, B i  represents the corresponding (i.e., similarly situated) pixel from the second image, and  B   i  is the inversion thereof. (In this example, each pixel is a 24 bit value comprising three discrete 8-bit integers, each integer representing one of the primary colors.) The image resulting from this calculation highlights the difference between the first and second images. The microprocessor can carry out other types of differential processing calculations such as edge detection algorithms as known in the art per se. These comparisons can be carried out cyclically at a frame rate of 1/30 second or close thereto, providing the microscopist with substantially real time differential images. The differential comparison of a conventional fluorescent image against a fluorescent absorption image is particularly useful if the latter does not readily differentiate between the predominantly absorbing regions and the fluorescing regions. However, the differences between the two regions will likely be highlighted in the differential image. 
   An alternative embodiment, which may eliminate the need for the emission filter, is shown in  FIG. 6 . This microscope has a transmitted light illumination system wherein the exciter light  38 , generated by light source  32  and excitation filter  36 , is collected by a condenser  60  to illuminate a specimen on stage  42 . The exciter light and fluorescent light generated thereby are collected by objective  34  to impinge upon the CCD sensor  49 . The CCD camera provides three output color channels or planes, e.g., red, green and blue. The CCD sensor is sensitive to the visible spectrum and thus substantially attenuates UV information. However, the blue plane is sensitive to the visible component of the exciter light and insensitive to other colors. If the microprocessor is programmed to display only the blue color plane, the camera essentially functions as a blue-violet emission filter and thus can provide a fluorescence absorption image such as shown in  FIG. 2 . Similarly, the fluorescent light, which has a longer wavelength than the exciter light, will be predominantly centered in either red or green, or a combination of both. If the microprocessor is programmed to display only one (or both) of these planes, the camera essentially functions as a red (or red/green) filter and thus can provide a conventional fluorescent image such as shown in  FIG. 1 . Having acquired the conventional fluorescent image and the fluorescent absorption image, the microprocessor can also be programmed to display a comparison or differential of the two images. For example, the monitor can be programmed to display I i =|R i −  B   i |, where R i  represents a pixel from the red plane, B i  represent the corresponding pixel from the blue plane, and  B   i  is the inversion thereof. 
     FIG. 7  shows an alternative embodiment of a microscope which optically enables the fluorescent absorption image to be viewed in conjunction with the conventional fluorescent image. This microscope is substantially similar to the epi-illumination microscope shown in  FIG. 5 . However, filter  46 ′ has a passband designed to permit through fluorescent light as well as significant leakage of the excitation wavelength (or at least the visible part thereof). In this case, depending in part on the strength of the fluorescing light, the primarily absorbing regions should be visible as a darker shade of the reflected background exciter light. 
   While the preferred embodiment has been shown using an epi-illumination system, those skilled in the art will understand that the techniques of fluorescence absorption microscopy and differential fluorescence absorption microscopy can also be applied, inter alias, to transmitted light, darkfield, or confocal illumination systems. Similarly, those skilled in the art will appreciate that a variety of other modification may be made to the methods and apparatus described herein without departing from the spirit of the invention.