Imaging System With Means for Sensing a Filtered Fluorescent Emission

This application claims the benefit of U.S. Provisional Appl. No. 60/007,086, filed Oct. 31, 1995.

This invention relates generally to the field of imaging systems used in fluorescence microscopy and more particularly to an electro-optical imaging device adapted to be used in combination with an optical system such as a conventional microscope that displays a diffraction-limited, video image which can be used in transmission, reflection, and fluorescence microscopy.

Fluorescence microscopes evolved from ordinary transmitted light microscopes with the addition of an excitation filter in the illumination path and an emission filter in the viewing path. Low contrast and low fluorescence intensity were problems. For these reasons, transmitted light fluorescence has generally been abandoned. Significant improvements came from using reflected light methods to view the fluorescence from the same side as the illumination. Called incident light, reflected light, or epi-illumination, the fluorescent emission does not have to pass through the sample to be detected, which attenuates and diffuses the already weak radiation. The objective lens is also the condenser, eliminating centering and alignment problems. Difficulties remain, however, in using low power objectives because the low numerical aperture is an inefficient collector of the emitted fluorescence radiation and it is difficult to obtain a satisfactory image.

It is known to apply video technology to the imaging of microscope samples. Video cameras, laser scanners, and flying spot scanners have all been applied to microscopy.

Applying video imaging techniques to fluorescence microscopy has been difficult. The weak emission from fluorescent samples is difficult to detect with ordinary video cameras. Expensive, cooled SIT and CCD cameras have been used with some success. However, most systems produce only monochromatic images, and require either high power light sources that can potentially damage delicate samples, or long exposure times to gain sufficient signal for analysis. Typically, camera systems cannot run in real time at full resolution. The maximum resolution is limited by the camera image sensor, which is usually less than the diffraction limit of the optics.

Better video imaging has been achieved with laser scanning confocal microscopes, which may cost up to $300,000. Laser scanning microscopes use a laser to create a spot of light that scans across the sample in a raster pattern. The fluorescent emission is detected in the reflection mode by a photomultiplier tube (PMT) whose sensitivity is much higher than video cameras. The deflection of the laser beam is accomplished by mechanical servo-galvanometers that oscillate x-and y-axis steering mirrors. The mechanical scan size can be varied to allow changing the image field of view, thus changing the magnification of the image. Because the scanning is mechanical, the maximum frame rate is limited to about 4 Hz, and a usable image is achieved in 2-10 seconds. The disadvantages of laser scanning microscopes include high expense, slow image scan rate, monochromatic light source, and others.

This invention relates to the improvement of a transmitted light full color microscope described in U.S. Pat. No. 5,430,807.

It is accordingly an object of the present invention to provide an advanced scanning fluorescence imaging system that produces an output for display on a high resolution video display terminal and provides an electronic digital representation of the image.

Another object of the present invention is to provide a flurorescence scanning imaging system that operates at the diffraction limit of the optics to which it is connected and is therefore an imaging system operating at the uppermost limits of resolution.

Another object of the present invention is to provide a scanning fluorescence imaging system that operates at a variety of magnification powers in both transmitted light and reflected light modes.

Yet Another object of the present invention is to provide a flurorescence scanning imaging system that provides simultaneous reflected and transmitted light images.

Still another object of the present invention is to provide a scanning fluorescence imaging system that includes the ability to enhance or suppress portions of the field of view.

A still further object of the present invention is to provide a scanning fluorescence imaging system that includes an instantaneous magnification zoom feature.

A still further object of the present invention is to provide a scanning imaging system that includes full color reflected and transmitted light images in addition to fluorescent images using a common illumination source.

A still further object of the present invention is to provide a scanning fluorescence imaging system that is reliable and inexpensive.

The benefits and advantages of the present invention are achieved in an imaging system characterized by its ability to produce a diffraction limited output signal representative of a sample positioned on a sample plane. The imaging system comprises a cathode ray tube, an optical lens system and a means for sensing. The cathode ray tube comprises a spot scanner means for generating an illuminated spot that scans in a raster pattern. The raster pattern is composed of a predetermined number of horizontal lines that are spaced vertically from each other a predetermined and equal distance. An illuminated spot scans correspondingly in the raster pattern and wherein the spot is positioned in an object plane. The system images in a diffraction limited manner the object plane on to the sample plane, such that the image of the spot is the smallest diffraction limited size as limited by the optical lens system. Filters inserted before the sample allow only the excitation band of wavelengths to impinge upon the sample, and filters inserted in the sensor path allow only the fluorescence emission wavelengths to impinge upon the sensors.

Some of the features and advantages of the present invention having been briefly stated, others will appear from the detailed description which follows, when taken in connection with the accompanying drawings.

FIG. 1a illustrates schematically a conventional transmitted white light and fluorescence optical imaging system, such as a compound microscope.

FIG. 1b illustrates schematically a transmitted white light and fluorescence imaging system according to the present invention.

FIG. 2a illustrates schematically a conventional reflected white light and fluorescence optical imaging system, such as a compound microscope.

FIG. 2b illustrates schematically a reflected white light and fluorescence optical imaging system according to the present invention.

FIG. 3a illustrates schematically a conventional combination transmitted and reflected white light and reflected fluorescence imaging system, such as a compound microscope.

FIG. 3b illustrates schematically a combination transmitted and reflected light white light and fluorescence imaging system according to the present invention.

FIG. 4 illustrates schematically a cathode ray tube (CRT) according to the present invention.

FIG. 5 illustrates a normalized phosphor decay curve showing brightness versus time.

FIG. 6 illustrates the maximum and minimum signal states of a diffraction limited spot according to the present invention.

While the present invention will be described more fully hereinafter with reference to the accompanying drawings in which particular embodiments are shown, it is to be understood at the outset that persons skilled in the art may modify the invention herein described while still achieving the favorable results of this invention. Accordingly, the description which follows is to be understood as a broad teaching disclosure directed to persons of skill in the appropriate arts and not as limiting upon the present invention.

In order for the reader to more fully understand the present invention it will be instructive to define certain terms with reference to the optical diagram of a conventional compound transmitted light microscope as shown in FIG. 1a and a conventional reflected light microscope as shown in FIG. 2a. Whenever possible, like reference numerals will be employed when referring to a conventional microscope and the imaging system according to the present invention which may employ such a microscope as part of the system. In the combined drawing, FIG. 3b, the letter T appended to a numeral refers to the transmitted light path and the letter R appended to a numeral refers to the reflected light path.

In a conventional transmitted light microscope shown in FIG. 1a, generally indicated at 10, the lamp or bulb 12 illuminates the sample S lying in a sample plane 17 through relay and condenser lenses 14 and 16, respectively. An objective lens 18 creates an image S' of the sample S in the intermediate image plane 20. The diameter of the field stop 22, located in this plane, defines the field of view (FOV) surrounding the sample image S'. Lens 24 creates a projected image S" of the intermediate image S' either in the eye, or directly onto a projection image plane 26, typically 25 cm from the projection lens 24.

According to the reciprocity rules of optical systems, an object placed in the projection image plane 26 will be imaged backwards through the optics onto the slide plane 17 with the lens magnification factors reversed.

For fluorescence operation, excitation filter 27 is inserted in the illumination path, generally before the condenser lens 16, and an emission filter 28 is inserted into the viewing path, generally after the objective lens 18. The excitation filter 27 defines the bandwidths of radiation falling on the sample to excite fluorescence, and the emission filter defines the bandwidths of fluorescent emission allowed to be passed to the viewing path. Single or multiple excitation and emisssion bandwidths may be used as determined by the availability of filters from manufacturers in the field.

Referring now to FIG. 1b, the transmitted light fluorescence imaging system according to the present invention is there illustrated and comprises generally a means such as an optical system 10', a spot scanner means generally indicated at 30 and means for sensing 40. In a further embodiment of the invention the electrical output signal is transmitted to a video display device 60. A digital processing device 45 may be used for computation and additional control for display.

Referring now to FIG. 4, the spot scanner means takes the form of a conventional cathode ray tube (CRT) 30. The CRT includes a glass envelope 32 with a means for generating an electron beam or electron gun 34 positioned at one end and a substantially flat screen 36 that is coated with phosphor 38 which is positioned at the opposite end. Individual phosphor spots 39 are excited as an electron beam 35 sweeps in a raster pattern across the screen 36. The deflection system 31 is adapted to scan in a raster pattern which comprises a predetermined number of horizontal lines spaced vertically from each other a predetermined and equal distance. The electron beam 35 is adapted to sequentially excite the phosphor 38 on the screen 36 in order to produce an illuminated spot 39 in the object plane 26 of FIGS. 1b, 2b, and 3b that scans correspondingly in the raster pattern. The phosphor 38 is chosen from among commercially available types emitting the desired wavelengths. Standard phosphor emission spectra are presented in various reference materials, including the Phosphor Resource Manual for Industrial and Military Cathode Ray Tubes published by the Imaging and Sensing Technology Corporation of New York.

For a detailed description of the operation of a CRT 30 the reader is referred to any one of the numerous texts on the topic. Furthermore, the means for generating the spot 30 may also comprise an electron beam such as is found in electron microscopes, or a laser beam such as found in laser scanning microscopes.

The transmitted light imaging system 10' of FIG. 1b also includes an optical lens means or optical lens system 18, 24 for imaging in a diffraction limited manner, the object plane 26 on to the intermediate image plane 20, where a field stop 22 defines the field of view, and then imaging the intermediate image plane 20 onto the sample plane 17 such that the image 39b of the spot 39a is the smallest diffraction limited size that can be created by the optical lens system 10'. In its simplest form, the optical lens system 10' substantially resembles the conventional transmitted light optical microscope 10 of FIG. 1a without the lamp 12.

Also included in the imaging system of the present invention is a means 40 for sensing the interaction of the spot 39b with a corresponding location on the sample S and for producing a modulated output signal proportional thereto. The means 40 generally comprises one or more photomultiplier tubes (PMTs), depending on the number of signal channels to be output.

Referring again to FIG 1b, according to the imaging system of the present invention if a CRT screen 36 is placed in the projection image plane 26, the image of the scanning spot 39a is demagnified through the projection lens 24 and the objective lens 18 and is focused onto the sample plane 17 so as to trace over the sample S. The light transmitted through the sample S is collected by lens 16 and is directed through the color filters 50 and the filtered light is input to the respective photomultiplier tubes (PMTs) 40. The PMTs output electrical signals on lines 42 that are then input to signal amplifiers 44. The amplifier signals are transmitted by lines 46 to each of the red, green and blue inputs of a color display monitor 60 thereby recreating the image of the scanned portion of sample S on the display monitor 60. A digital processing device 45 may be used for computation and additional control for display. In addition, the number of color detector channels may be any reasonable value and the imaging system may also be operated as a black and white model using one PMT 40 and without the color filters 50 if a particular application so requires

For fluorescence operation, excitation filter 27 is inserted into the illumination path before the objective lens 18, and emission filter 28 is inserted in the sensing path generally after the condenser lens 16. The excitation filter 27 defines the bandwidths of radiation falling on the sample to excite fluorescence, and the emission filter 28 defines the bandwidths of fluorescent emission allowed to be passed to the sensing path. Single or multiple excitation and emisssion bandwidths may be used as determined by commercially available filters.

The color filters 50 can be chosen for several different reasons: 1) to divide the received radiation into color wavelength bands suitable for full color display of white light non-fluorescent images with filters 27 and 28 removed, 2) to define distinct fluorescent emission wavelength bands to be used in conjunction with, or in place of, emission filter 28, and 3) to define color wavelength bands that are compatible with both fluorescent discrimination in conjunction with filter 28 and for white light separation for full-color display of non-fluorescent white light illumination with filters 27 and 28 removed. The last option is preferred for rapid interchange of white light color imaging and fluorescent imaging.

In a conventional reflected light microscope shown in FIG. 2a, generally indicated at 10, the lamp or bulb 12 illuminates the sample S lying in a slide plane 17 through a relay lens 25, a partially transmitting and reflecting surface 29, and an objective lens 18. From the reflected light leaving the sample, objective lens 18 creates an image S' of the sample S in the intermediate image plane 20. The diameter of the field stop 22, located in this plane, defines the field of view (FOV) surrounding the sample image S'. Lens 24 creates a projected image S" of the intermediate image S' either in the eye, or directly onto a projection image plane 26, typically 25 cm from the projection lens 24. For white light viewing, the partially transmitting and reflecting surface can be a simple beam splitter with the same spectral bandpass in both optical channels.

For fluorescence operation, excitation filter 27 is inserted in the illumination path, generally before the partially transmitting and reflecting surface 29f, and an emission filter 28 is inserted into the viewing path, generally between the partially transmitting and reflecting surface 29f and the intermediate image plane 20. The excitation filter 27 defines the wavelengths of radiation falling on the sample to excite fluorescence, and the emission filter 28 defines the wavelengths of fluorescent emission passed to the sensing path. Single or multiple excitation and emisssion bandwidth filters may be used according to commercial availability. The partially transmitting and reflecting surface 29f is generally a dichroic mirror, fabricated to reflect the excitation wavelengths and to transmit the emission wavelengths. Operating in this manner, surface 29f aids the excitation filter 27 and emission filter 28 in separating wavelengths and minimizing cross talk between the excitation and emission bands.

According to the reciprocity rules of optical systems, an object placed in the projection image plane 26 will be imaged backwards through the optics onto the slide plane 17 with the lens magnification factors reversed.

Referring now to FIG. 2b, the reflected light fluorescence imaging system according to the present invention is there illustrated and comprises generally a means such as an optical system 10', a spot scanner means generally indicated at 30 and means for sensing 40. In a further embodiment of the invention the electrical signal from the sensing means 40 is transmitted to a video display device 60. A digital processing device 45 may be used to convert the signal to digital form for computation and additional control for display.

The reflected light imaging system 10' of FIG. 2b also includes an optical lens means or optical lens system 18, 24 for focusing in a diffraction limited manner, the object plane 26 on to a sample plane 17 such that the image 39b of the spot 39a is the smallest diffraction limited size that can be created by the optical lens system 10'. In its simplest form, the optical lens system 10' substantially resembles the conventional reflected light optical microscope 10 of FIG. 2a without the lamp 12. However, the filters 27 and 28 are exchanged because the illumination and sensing paths are exhanged.

Also included in the imaging system of the present invention is a means 40 for sensing the interaction of the spot 39b with a corresponding location on the sample S and for producing a modulated output signal proportional thereto. The means 40 generally comprises one or more photomultiplier tubes, depending on the number of color signals to be output.

Referring again to FIG. 2b, according to the imaging system of the present invention if a CRT screen 36 is placed in the projection image plane 26, the image of the scanning spot 39a is demagnified through the projection lens 24 and the objective lens 18 and is focused onto the sample plane 17 so as to trace over the sample S. The light reflected or emitted from the sample S is collected by lens 18 and is reflected by surface 29 through relay lens 25 to the color filters 50 and the filtered light is input to the respective photomultiplier tubes (PMTs) 40. The PMTs output electrical signals are transmitted through lines 42 and then input to signal amplifiers 44. The amplifier signals are transmitted by lines 46 to each of the red, green and blue inputs of a color display monitor 60 thereby recreating the image of the scanned portion of sample S on the display monitor 60. In addition, the number of color detector channels may be any reasonable value and the imaging system may also be operated as a black and white model using one PMT 40 and without the color filters 50 if a particular application so requires.

For fluorescence operation, excitation filter 27 is inserted into the illumination path before the partially transmitting and reflecting surface 29f, and emission filter 28 is inserted in the sensing path generally after the partially transmitting and reflecting surface 29f, The excitation filter 27 defines the wavelengths of radiation falling on the sample to excite fluorescence, and the emission filter 28 defines the wavelengths of fluorescent emission passed to the sensing path. Single or multiple excitation and emisssion bandwidths may be used according to the commercial availability of filters.

The color filters 50 can be chosen for several different conditions: 1) to divide the incident radiation into color wavelength bands suitable for full color display of white light non-fluorescent images with filters 27 and 28 removed, 2) to define distinct fluorescent emission wavelengths to be used in conjunction with or in place of emission filter 28, and 3) to define spectral wavelengths that are compatible with both fluorescent discrimination in conjunction with filter 28 and for white light separation for full-color display of non-fluorescent white light illumination with filters 27 and 28 removed. The last option is preferred for rapid interchange of white light color imaging and fluorescent imaging.

Some conventional microscopes can be operated in either a reflected or transmitted light mode shown in FIG. 3a by combining the elements of FIGS. 1a and 2a. The system requires two lamps, one for the transmitted light path 12T and one for the reflected light path 12R and a removable holder for the filters 27 and 28 and the partially transmitting and reflecting surfaces 29 and 29f. There is only one image sensing path through projection lens 24 to the eye or a projection screen.

For transmitted light operation, the reflecting and transmitting surfaces 29 and 29f and the emission filter 28 are removed from the optical path. Lamp 12T is used for illumination and lamp 12R is off. FIG. 3a does not show transmitted light fluorescence components since this technique has been abandoned by equipment manufacturers.

For reflected light operation, lamp 12R is used for illumination and lamp 12T is off. The partially reflecting and transmitting surface 29 is inserted into the optical path. For fluorescence operation, the excitation filter 27 and the emission filter 28 are inserted into the optical path in addition to the partially reflecting and transmitting surface 29f.

Referring now to FIG. 3b, the combination transmitted and reflected white light and fluorescence imaging system according to the present invention is there illustrated and comprises generally a means such as an optical system 10', a spot scanner means generally indicated at 30 and means for sensing transmitted light 40T and a means for sensing reflected light 40R. In a further embodiment of the invention the electrical output signal is transmitted to a video display device 60. A digital processing device 45 may be used to combine the reflected and transmitted path signals and convert the signals to digital form for computation and additional control for display

The transmitted light imaging portion of FIG. 3b also includes an optical lens means or optical lens system 18, 24 for imaging in a diffraction limited manner, the object plane 26 on to the intermediate image plane 20, where a field stop 22 defines the field of view, and then imaging the intermediate image plane 20 onto the sample plane 17 such that the image 39b of the spot 39a is the smallest diffraction limited size that can be created by the optical lens system 10'.

Also included in the imaging system of the present invention is a means 40T for sensing the interaction of the spot 39b with a corresponding location on the sample S and for producing a modulated output signal proportional thereto. The means 40T generally comprises one or more photomultiplier tubes, depending on the number of signal channels to be output.

Referring again to FIG. 3b, according to the imaging system of the present invention if a CRT screen 36 is placed in the projection image plane 26, the image of the scanning spot 39a is demagnified through the projection lens 24 and the objective lens 18 and is focused onto the sample plane 17 so as to trace over the sample S. The light transmitted through the sample S is collected by lens 16 and is directed through the color filters 50T and the filtered light is input to the respective photomultiplier tubes (PMTs) 40T. The PMTs output electrical signals on lines 42T that are then input to signal amplifiers 44T. The amplifier signals are transmitted by lines 46T to each of the red, green and blue inputs of a color display monitor 60 thereby recreating the image of the scanned portion of sample S on the display monitor 60. A digital processing device 45 may be used for computation and additional control for display. In addition, the number of color detector channels may be any reasonable value and the imaging system may also be operated as a black and white model using one PMT 40T and without the color filters 50T if a particular application so requires.

For fluorescence operation, excitation filter 27 is inserted into the illumination path before the objective lens 18, and emission filter 28T is inserted in the sensing path generally after the condenser lens 16. The excitation filter 27 defines the bandwidths of radiation falling on the sample to excite fluorescence, and the emission filter 28T defines the bandwidths of fluorescent emission allowed to be passed to the sensing path. Single or multiple excitation and emisssion bandwidths may be used as determined by commercially available filters.

The color filters 50T can be chosen for several different reasons: 1) to divide the received radiation into color wavelength bands suitable for full color display of white light non-fluorescent images with filters 27 and 28T removed, 2) to define distinct fluorescent emission wavelength bands to be used in conjunction with, or in place of, emission filter 28T, and 3) to define color wavelength bands that are compatible with both fluorescent discrimination in conjunction with filter 28T and for white light separation for full-color display of non-fluorescent white light illumination with filters 27 and 28T removed. The last option is preferred for rapid interchange of white light color imaging and fluorescent imaging.

The reflected light portion of the imaging system 10' of FIG. 3b also includes the optical lens means or optical lens system 18, 24 for focusing in a diffraction limited manner, the object plane 26 on to a sample plane 17 such that the image 39b of the spot 39a is the smallest diffraction limited size that can be created by the optical lens system 10'.

Also included in the imaging system of the present invention is a means 40R for sensing the interaction of the spot 39b with a corresponding location on the sample S and for producing a modulated output signal proportional thereto. The means 40R generally comprises one or more photomultiplier tubes, depending on the number of color signals to be output.

Referring again to FIG. 3b, according to the imaging system of the present invention if a CRT screen 36 is placed in the projection image plane 26, the image of the scanning spot 39a is demagnified through the projection lens 24 and the objective lens 18 and is focused onto the sample plane 17 so as to trace over the sample S. The light reflected or emitted from the sample S is collected by lens 18 and is reflected by surface 29 through relay lens 25 to the color filters 5OR and the filtered light is input to the respective photomultiplier tubes (PMTs) 40R. The PMTs output electrical signals are transmitted through lines 42R and then input to signal amplifiers 44R. The amplifier signals are transmitted by lines 46R to each of the red, green and blue inputs of a color display monitor 60 thereby recreating the image of the scanned portion of sample S on the display monitor 60. In addition, the number of color detector channels may be any reasonable value and the imaging system may also be operated as a black and white model using one PMT 40R and without the color filters 5OR if a particular application so requires.

For fluorescence operation, excitation filter 27 is inserted into the illumination path before the partially transmitting and reflecting surface 29f, and emission filter 28R is inserted in the sensing path generally after the partially transmitting and reflecting surface 29f, The excitation filter 27 defines the wavelengths of radiation falling on the sample to excite fluorescence, and the emission filter 28R defines the wavelengths of fluorescent emission passed to the sensing path. Single or multiple excitation and emisssion bandwidths may be used according to the commercial availability of filters.

The color filters 50R can be chosen for several different conditions: 1) to divide the incident radiation into color wavelength bands suitable for full color display of white light non-fluorescent images with filters 27 and 28R removed, 2) to define distinct fluorescent emission wavelengths to be used in conjunction with or in place of emission filter 28R, and 3) to define spectral wavelengths that are compatible with both fluorescent discrimination in conjunction with filter 28R and for white light separation for full-color display of non-fluorescent white light illumination with filters 27 and 28R removed. The last option is preferred for rapid interchange of white light color imaging and fluorescent imaging.

It has been emphasized herein that the imaging system according to the present invention operates at the diffraction limit of the optics (or more particularly the microscope) to which it is attached. It is believed by the applicant that heretofore no such imaging system has been devised. By operating at the diffraction limit advantages occur, namely resolution is significantly enhanced. In addition, the imaging system of the present invention has the ability to zoom in on the sample and to enhance or suppress the output signal in order to modify the detectability of a portion of the sample. In addition, by combining the transmission and reflection modes by the insertion or removal of combinations of the partially transmitting and reflecting surfaces 29 and 29F, and the filters 27, 28T and 28R, the imaging system of the present invention can operate as:

1) a white light transmission or reflection system,

2) a white light transmission and reflection system,

3) a fluorescence transmission or reflection system,

4) a fluorescence transmission and reflection system

5) a nonfluorescence transmitted and fluorescence reflection system,

6) a transmitted fluorescence and nonflourescence reflection system.

With the foregoing in mind, in the discussion which follows the reader will come to understand the physical parameters of the present imaging system.

In order to achieve diffraction-limited resolution of spot 39 (FIG. 1b), the CRT spot radius is determined by the diffraction limit of the optics. The scanned field size is determined by the diameter of the field stop 22 in the intermediate image plane 20. Table 1 shows the diffraction-limited spot sizes in the three image planes of a conventional microscope for a typical selection of objective powers 18. Column (8) is the number of diffraction-limited picture elements across the field of view.

R.sub.0 is the radius of the smallest resolvable element in the slide plane 17 (FIG. 1b) calculated from the Rayleigh resolution criterion.

where .lambda.=550 nm is the chosen wavelength representing the center of the visible spectrum, and NA is the numerical aperture of the objective lens 18.

The radius of the image of R.sub.0 in the intermediate image plane 20 between the objective 18 and the eyepiece lens 24 is

In the intermediate image plane 20, the field stop 22 defines the diameter of the field of view for the whole system, F.sub.1 =FOV=20 mm in this example. The radius of the image of R.sub.1 in the projection image plane 26 is

At the location of the projection image plane 26, a real image can be shown on a screen. The diameter of the field of view in the projected image plane is

and the diameter of the field of view in the slide plane 17 is ##EQU1##

While this discussion refers specifically to a microscope having a fixed eyelens power and a fixed intermediate image plane field stop size, such conventions are artificial, and not meant to be limiting on the present invention.

The number of resolved elements across the field of view for each objective lens 18, shown in column (8) is ##EQU2## N is therefore a function of the numerical aperture of the objective lens 18 and the diameter of the field stop in the intermediate image plane 20.

From the table, the maximum number of resolved elements is 1785, which occurs for the lowest magnification option. The smallest point radius in the projected image plane is R.sub.2 =111.8 mm from column (7), which also occurs for the lowest magnification option. The projected image field diameter is F.sub.2 =20 cm from equation (4), and is constant for all objective magnification options.

In order to insure diffraction-limited resolution for all optical powers, the scanner CRT 30 must produce a spot radius less than or equal to the smallest value of R.sub.2 in the table. In order to match the field of view of the microscope, the scan area on the CRT 30 must be comparable to F.sub.2. Since video scanning patterns 12 have a rectangular shape (raster scan) and a typical microscope field of view is circular, the diagonal of the scanned pattern should match the circle diameter, F.sub.2. The eyepiece field stop 22 could also be fabricated in a rectangular shape to match the CRT scanned pattern. A random scanning pattern could also be employed for tracing edges of samples and other specialized functions, but the basic geometrical-optical relationships for diffraction-limited resolution and full field display remain the same. It will therefore be noted that the aspect ratios of the scan patterns on the CRT 30 and the display monitor 60 must be the same, whether 4:5, 3:4, 1:1, or any other desired value.

A common manner of determining the contrast of an optical system such as system 10' is to use a sample consisting of a field of parallel black and white bars of the same width. Theoretically, the black bars have 0% transmission, and white (or clear) bars have 100% transmission. As the spot is traced perpendicular to the bars, the modulation of the transmitted light signal may be determined from the formula ##EQU3## which is also referred to as the image contrast. The reader is referred to classical texts, such as Chapter 11 of Modern Optical Engineering, McGraw Hill, 1966, by Warren Smith. The signal modulation is measured for decreasing bar widths and a graph of the modulation vs. bar width, or resolution, can be drawn. For this example, the modulation for a diffraction-limited spot tracing a diffraction-limited bar width is desired. The modulation for these conditions is a function of the scan speed and the phosphor decay characteristics. The relationship will now be derived.

All CRT phosphors exhibit a decrease in intensity after excitation by the electron beam 35. The decay period is normally defined as the time interval in which the phosphor intensity decreases to 10% of the initial intensity. Occasionally, a 1% phosphor intensity is used to define decay time. Decay times and decay curve shapes vary widely between phosphor types and operating conditions. Mathematical representations of decay curves include exponential and power law formulas, but are generally not known. Usually, measured curves are the only representations available.

A specific relationship exists between the scan rate of the flying spot 39 and the contrast of the image. The scan rate determines the pixel dwell time, or the residence time of the optical spot over a single resolution element. The word pixel usually means a "picture element", but as used here means a "diffraction-limited picture element", whose dimension is defined in eqn. (1). If the spot 39a scans rapidly across the screen 36 so that the pixel dwell time is much shorter than the phosphor decay time, then previous resolution elements will continue emitting light after the electron beam 35 has passed. The result is a reduction in contrast of the signal.

The effects can be illustrated by FIGS. 5 and 6. FIG. 5 represents a normalized phosphor decay curve showing brightness versus time. Let us choose the pixel dwell time as 1/4 of the total decay time and divide the curve into four equal time zones. The area in each zone is shown in the table at right and represents the total light flux emitted during that time interval. FIG. 6, case a shows successive positions of the light spot during the four time zones as it scans over a set of parallel black and white bars whose width is the diffraction limit of the optics. Each spot position represents one pixel dwell time. FIG. 4, case b shows case a one pixel dwell time later. Case a and case b represent the maximum and minimum signal states of the position of a scanning spot tracing over the bars for the given pixel dwell time. The illumination of pixels 1, 2, 3, and 4 will be fractions of the decay curve as shown. The total light transmitted through the sample will be the sum of the light flux illuminating each pixel times the transmissivity of each pixel, which was defined as either black (0% transmission), or white (100% transmission).

For this example, the image modulation, or contrast, that follows from the two states is 0.5195, or 51.95%. Any pixel dwell time could be used in the example, and the procedure repeated to determine the resulting contrast. It is common practice in the design of optical imaging systems to specify that the image modulation shall be greater than 50% at the specified resolution. Design values down to 30% may be adequate. The limit of visual detection occurs at image modulation values in the range of 2-3.5%.

The direct interaction between the scanning spot 39b and the sample S is the only interaction that relates the phosphor decay time to the pixel dwell time. More sophisticated calculation methods may be employed, but adequate values can be achieved with this simple method. The contrast is also affected by scattering or other effects away from the sample S.

The horizontal and vertical scan times of the raster on the CRT 30 are determined from the pixel dwell time. The horizontal scan period is

where t=pixel dwell time,

Nh=number of horizontal pixels,

and t.sub.h =horizontal blanking time.

The horizontal scan frequency is then

The vertical scan period is

where T.sub.h =horizontal scan period from eqn. (7),

N.sub.v =number of vertical pixels,

t.sub.v =vertical blanking time.

The vertical scan frequency is then

At this point, the following parameters have been defined: the phosphor spot size that will yield the smallest diffraction-limited spot 39b on the sample plane 17 for all objective powers; the scan rates that will yield a predetermined contrast for a diffraction-limited pixel; and a scan raster size that will illuminate a full field of view.

The next component of the imaging system requires the conversion of optical information into electronic information. The procedures above insure that the pixel data is being generated at maximum resolution with good contrast.

The light that is transmitted, reflected, or emitted from the sample that falls on the photomultiplier tubes is characterized by a rapidly varying intensity level resulting from the varying absorptivity, reflectivity, or emissivity at each point on the sample. In order to preserve the resolution, the means for sensing or PMTs 40 and associated amplifiers, must be able to create an electrical signal that faithfully reproduces the amplitude and frequency of the rapid fluctuations in intensity of the light from the sample. The minimum frequency response must match or exceed the inverse pixel dwell time. Electronic circuit frequency response is usually stated in terms of a graph of frequency vs. modulation amplitude in a manner very similar to the optical modulation/contrast definition given above. Typical specifications give the frequency response at 3 dB roll-off, or 50% modulation. If the optical modulation at the slide is 50%, and the electrical amplifier modulation is 50%, the resultant signal has a modulation of 25%, which would approach a design minimum. The frequency response could be specified at 1 dB (79%), or 0 dB roll-off for better modulation and higher fidelity to the incoming signal. The reader will note that PMTs are well known in the art and therefore it is not necessary to describe specific PMTs or amplifier circuits, but rather to describe the conditions for the proper operation thereof in the particular image system of the present invention. Frequency bandwidths of 20 MHz and up are typical, exceeding the 3.75 MHz bandwidth of ordinary television standards by a significant amount. According to the conditions outlined above, standard television scan rates and amplifier bandwidths cannot approach the diffraction-limited requirements of an conventional microscope image.

The signals from the PMT amplifiers 44 on lines 46 contain all the information available from the imaging system. The amplified output signals can be displayed directly on a video display 60, or digitized and output to a digital processing device or other permanent storage media for display, analysis, or telecommunication. The specifications of the display monitor 60 determine the final visual quality of the image. If the monitor 60 can display the number of color pixels contained in the signal from the video amplifiers 44, then the resolution and the field of view of the ordinary microscope image can be reproduced. The monitor's video amplifier bandwidth and phosphor characteristics will determine the monitor's resolution and contrast, and hence its ability to faithfully represent the original electronic output signals from the microscope video amplifiers 44. For monitors with fewer pixels than contained in the video signal from the imaging system, a trade-off between field of view and resolution can be made, resulting in full resolution in a smaller field of view.

The display screen of the monitor 60 represents a constant size window showing the contents of the scanned area on the slide. If the scanned area on the slide is made smaller, it is still displayed full size on the monitor 60, giving a magnification zoom effect. In other words, varying the size of the raster pattern on the spot scanning device causes the raster on the sample to be correspondingly changed thereby causing a larger or smaller portion of the sample to be contained within the output signal providing a variable magnification to the display device or other signal processing device. When used with lower power objectives 18 that have a large number of pixels in the field of view, any area can be zoomed in magnification so that the display resolution matches the diffraction limit of the optics. For example, if there are 1700 pixels across the microscope image, and only 1200 pixels across the monitor screen, then the zoom feature can be used to shrink the raster down to cover only 1200 pixels on the sample so the resolution of the display 60 matches or exceeds the image resolution at the reduced field of view.

Another characteristic of the zoom feature is constant brightness in the image for any zoom magnification setting. The brightness of the raster on the scanner CRT 30 is given by the relation

where I is the electron beam current, A is the area of the scanned raster, and k is a constant factor. The subscript p refers to the phosphor 38. Equation (12) is linear over a wide range of beam current settings. When the scanned area is reduced for constant beam current, the brightness increases in inverse proportion to the area change.

If the magnification of an optical system is changed, such as changing objective lenses 18 in a microscope, then the light passing through the lens system, and hence the brightness of the image is changed in direct proportion to the area change of the image. The brightness change can be represented by the relation

where B.sub.0 is the initial image brightness. If B.sub.0 comes from a phosphor satisfying eqn. (12), then eqn. (13) becomes

Equation (14) shows that a change in magnification due to changing the raster size has no effect on the brightness of the image. The variation in phosphor brightness exactly compensates for the variation in area. Since the mechanical system is not altered, the focus does not change. The zoom feature therefore produces a change in magnification with no change in brightness or focus.

In all current scanning type microscopes, the spot brightness is constant as it travels over the sample S. Modulating the spot brightness as a function of image content can be a powerful tool for image detailing. Spot modulation requires that the scanner CRT 30 circuits include a video amplifier and driver capable of modulating the electron beam at a frequency bandwidth no less than the PMT amplifier circuits 44.

If the spot brightness is decreased as it traces over a dark object, the resultant signal will show an enhanced dark swing. The feature may eliminate in many applications the need for conventional microscope contrast enhancement accessories (such as "differential interference contrast", darkfield illumination, and other phase contrast components) while maintaining full image resolution. This mode is called contrast enhancement, or positive feedback, or normal signal mode.

Conversely, if the spot brightness is increased as it traces over a dark object, the signal will show a reduced dark swing, or possibly no dark swing at all. This mode is called the contrast suppression, or negative feedback, or inverted signal mode. In the contrast suppression mode, portions of images can be made to disappear, or become much less prominent.

The signal used to modulate the scanner CRT 30 brightness is derived from the actual sample image signal from one or more of the PMT channels. The signal from a single channel may be used in either the normal or inverted mode, or several channels may be mixed and used in normal or inverted mode. In this manner, colored objects in the image may be selectively enhanced or suppressed.

For example, in the examination of a blood smear, the combination of colors for red blood cells could be fed into the scanner CRT video amplifier in a negative feedback mode to suppress or eliminate red blood cells from appearing in the image. This is not a data processing feature that works on the output image data, but a feature that is operative during creation of the image. Any color channel, or combination of color channels could be combined and used as positive or negative feedback to the scanner CRT.

The foregoing spot intensity modulation can also be applied to laser and electron beam imaging systems.

To illustrate the previous description of the invention, a specific example will now be described.

a) CRT spot size and raster size. The information in Table 1 and surrounding equations describe a conventional microscope optical system. The information in that section defines the following:

Smallest CRT spot size radius=111.8 .mu.m,

CRT raster size=20 cm diagonal.

The CRT envelope, gun structure, and high voltage are specified in combination to meet the specification. Many combinations are available to one skilled in the art. This example assumes a 3:4 raster aspect ratio.

b) Phosphor decay time and raster scan rate. The faster the phosphor decay time, the faster the scan rate. In general, a very fast phosphor is required for a scan rate fast enough to allow direct viewing on a video monitor. Phosphors in the P46 family have 1% decay times of approximately 100 ns. Assume the pixel dwell time is chosen to be 1/3 the phosphor decay time, or 33 ns. for a 1024.times.768 pixel monitor, the horizontal period from equation (7) is ##EQU4## where the blanking time was chosen to be 6 .mu.s. The horizontal frequency is 25 kHz. The vertical period from equation (9) is ##EQU5## where the vertical blanking time is chosen as 10 ms. The vertical frequency is 25.9 Hz. In order to avoid objectionable flicker in a directly viewed image, a display monitor with long persistence phosphors may be required.

c) Video amplifier bandwidth. For a pixel dwell time of 33 ns, a minimum amplifier bandwidth of 30.3 MHz.+-.1 dB is required. The video amplifiers in the display monitor must have at least the same bandwidth to maintain the resolution of the signal.

From table 1 it can be seen that the field of view and resolution of the display monitor match direct viewing in the conventional microscope above about 40x objective power. For the 10x objective, the image must be zoomed to approximately 1024/1785=57% of full field for the monitor to display the full resolution of the optical system. At 100x objective power, the monitor can display more pixels than the optical system can resolve.

d) Fluorescence. P46 phosphor contains blue and green spectral bandwiths suitable for excitation of common fluorescent bands useful in biomedical applications. This example refers to the popular fluorochrome systems using excitation in the 515-560 nm range, with fluorescent emission above 590 nm. This excitation region is used for TRITC (tetramethlyrhodamineisothiocyanate), Evans blue, Rhodamine B 200, Pararosaniline, Thiazine red R, Xylene orange, and Acid fuchsine. The samples were existing laboratory slides of rhizomes and a mouse embryo section approximately 10 .mu.m thick. The slides and had no special treatment for fluorescence, so autofluorescent emission was observed. In the transmitted light mode, the excitation filter was placed above the objective lens and the emission filter below the condenser. The red channel of the three color sensing path was used. The three full color filters 50 were left in place. The radiative power from the CRT screen in the excitation band entering the projection lens was 45.59 .mu.W. Fluorescent Images of the 10 .mu.m thick mouse embryo slide could be achieved at 12 Hz, suitable for focusing and scanning. Averaging 12 frames yielded a usable image.

The foregoing embodiments and examples are to be considered illustrative, rather than restrictive of the present invention, and those modifications which come within the meaning and range of equivalence of the claims are to be included therein.