US 3560792 A
Description (OCR text may contain errors)
United States Patent [111 3,5 0,792
[72| Inventors Gerhard W. Goetze  US. Cl 315/11 Elmira, N.Y.:  Int. Cl i H0lj 31/08 Karl-Heinz l-lerrmann: Dieter Krahl,  Field of Search 3 l 5/] l, 12 Berlin, Germany [2| Appl. No 811,283 References Cited  Filed Mar. 26, 1969 UNITED STATES PATENTS 1 Patented -1 E C 3,213,316 lO/l965 Goetz 315 12 est t r ration  Asslgnees g zf g g; cc no 0 p0 Primary Examiner-Rodney D. Bennett, Jr.
a corporation of Pennsylvania; Assistant Examiner-J0seph G. Baxter Siemens Aktiengesellschaft AttorneysF. H. Henson and C. F. Renz Berlin and Munich, Germany, a corporation of Germany  APPARATUS FOR OBSERVING DISPLAY SCREENS OF INSTRUMENTS USING PARTICLE BEAMS 6 Claims, 2 Drawing Figs.
ABSTRACT: Apparatus associated with a device, such as an electron microscope, in which fiber optic coupling is utilized in combination with a secondary electron conduction type camera tube for deriving an electrical signal representative of low light level emitted from the phosphor within an electron microscope.
PATENTEU res 24971 PHOTOELECTRON APPARATUS FOR OBSERVING DISPLAY SCREENS OF INSTRUMENTS USING PARTICLE BEAMS BACKGROUND OF THE INVENTION This invention is directed to apparatus associated with instruments, deriving a low light level output, for amplifying the light while retaining high resolution and displaying the amplified image on a display monitor. In electron microscopy, the high density of beam current required at relatively high levels of magnification to provide a bright image for focusing or short-exposure photography may damage the specimen under examination. Sufficient intensification of the image from an electron microscope permits the beam current densities to be reduced to acceptable levels. One arrangement which has been utilized includes the incorporation of an intensifier type tube to intensify the image from an electron microscope which is in turn viewed by a TV camera for a television display of the intensified image. It is to this general low light level input type of device that this invention is directed. The secondary electron conduction camera tube has a very linear transfer characteristic which describes the relationship between signal current and illumination. This especially good linearity of the transfer characteristics of the secondary electron conduction tube, as compared to other television pickup tubes, makes it possible to conduct intensity measurement, for example, contrasts steps in an electron microscope image with a high degree of accuracy. The secondary electron conduction tube also provides another important advantage in permitting integration of an input light image over a considerable time period. The input image to a secondary electron conduction tube may be integrated for a period of 1 hour or more and stored. This is possible in the secondary electron conduction tube in that secondary conduction electrons are generated within the target of the tube in proportion to the incident primary beam intensity and because of the resistivity of the secondary electron conduction target. The resistivity of a secondary electron conduction target is extremely high that is about so that a leakage current is not present during integration and storage. It therefore becomes possible using this invention to transmit and display, with a high degree of resolution, pictorial information which would otherwise only result in exceedingly dim or even unusable pictures with the prior art type arrangements.
This invention therefore opens the possiblity to further investigate electron optical specimens of a more delicate nature such as biological tissue without danger of destruction by using only extremely low current densities within an electron microscope and time integration within the secondary electron conduction camera tube. In addition, the secondary electron conduction tube provides an extremely high sensitive pickup tube of relatively small mechanical size and reasonable price. In addition, the time lag in the target of a secondary electron conduction tube is extremely short, so that, in addition to the above-described integration operation, transients effects can be studied without image smear. Fiber optic coupling means between the electron microscope phosphor and throughout the observing system insure even further high resolution within the system.
SUMMARY OF THE INVENTION A low light level device such as an electron microscope is provided with suitable apparatus including fiber optics and a secondary electron conduction pickup tube for intensifying and deriving an electric signal representative of the output of the electron microscope which is displayed on a television monitor.
BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic showing of an electron microscope with associated apparatus in accordance with the teachings of this invention; and
i F IG. 2 is a curve illustrating the linear properties of a secondary electron conductive tube.
BRIEF DESCRIPTION OF THE PREFERRED EMBODIMENTS i Referring in detail to FIG. I. an electron microscope is shown in part. The electron microscope includes a vacuum type enclosure I4 with an electron-generating means 16 provided for generating an electron beam which is directed through the specimen 18. The fiber optic window I2 forms a part of the vacuum envelope I4 of the electron microscope. The fiber optic window 12 is sealed vacuum-tight into an enclosure 13 which is in turn connected to be vacuum-tight with the electron microscope table 15. The fiber optic window 12 consists of a plurality of fibers 17. Each of the fibers 17 are surrounded by a jacket of glass of an index of refraction different from the fiber core. The fiber optic window 12 is provided with a phosphor coating 25 on the inner surface. The phosphor layer 25 forms the actual display of the electron microscope image and may also be covered with an antireflecting coating 26 which reflects the light emitted from the phosphor layer 25. An intensifier tube 27 is positioned so as to view the output from the electron microscope. The intensifier tube 27 is provided with an input faceplate 29 in which a fiber optic window 30 is provided. A photocathode 32 is provided on the inner surface of the window 30 of a suitable material which emits electrons in response to input radiation. The electron image emitted by the photocathode 32 is focused by suitable means not shown onto an output phosphor 34 layer provided on an output window 36. The phosphor layer 34 emits light in response to electron bombardment. The tube 26 compressed of the photocathode 32 and the output phosphor 34 makeup an electron optical image 'ntensifier. In this specific embodiment the intensifier is in a separate envelope. The output phosphor 34 emits light through the fiber optic window 36.
The optical image generated by the phosphor layer 34 is transmitted through the optical fiber plate 36 and then through a second optical fiber window 38. The windows 12 and 30 are of similar diameter and windows 36 and 38 are of similar diameter and may be coupled together by using a thin layer of immersion oil 40 and 42 respectively. The window 38 is the input window for a secondary electron conduction tube 46 of the type described in U.S. Pat. No. 3,213,316. A photocathode 44 is provided on the inner surface of the fiber optic window 38 which is the photocathode for the SEC camera tube 46. The optical image amplified by the image intensifier 27 is transmitted through the fiber optic windows 36 and 38 to the photocathode 44 which emits electrons in response to the radiation and which are directed onto a target 48. The target 48 may consist of a support layer 50 of a suita: ble material such as aluminum oxide, an electrically conductive layer 52 and a porous layer 54 of a suitable secondary electron emissive material such as potassium chloride. The layer 54 is a very porous material having a density of less than 10 percent of the normal bulk density of the material and generates secondary electrons from the solid material of the layer 54 into the voids of the layer 54 in response to electrons generated by the photocathode 44. In this manner, a charge is generated on the opposite surface of the target 48 and on the surface of the layer 54:
An electron source 56 of suitable means provides a pencillike electron beam. The beam is scanned over the target 48 and normally stabilizes the surface of the layer 54 at the cathode potential of the electron beam source. The target backplate layer 52 is normally held a positive potential of about 40 volts, with respect to ground The effect of the incoming primary electrons from the photocathode 44 is to a large number of secondary electrons within the layer 54 charge areas of the target 48 which have been bombarded by primary electrons. In this manner, an amplified positive charge image is generated on the scan surface of the target 48. The reading electron beam 56 scans the target 48 again. The missing charges will be replenished so that a video current flows through a resistor 60 and the scan surface of the target is stabilized again to cathode potential. The video information generated across the resistor 60 in terms of current fluctuation is coupled through a capacitor 62 to the first stage of a video amplifier 21 which in turn feeds a signal to the cathode ray dis play monitor.
It is obvious that the secondary electron conduction tube 46 could be positioned directly against the fiber optic faceplate 12 in the electron microscope. This arrangement of two fiber optic discs offers a number of advantages for operation and maintenance. For generation of a television picture containing electron microscope information. the two fiber optic plates 12 and 38 could be brought into intimate contact using immersion oil for good optical coupling.
During those periods of operation when no television presentation is desired, one can easily remove the television camera and now it is possible to generate a photographic record of the information presented on the phosphor screen 25 of the electron microscope using contact photography. The photographic emulsion or Polaroid film can be pressed against the outside of the fiber optic disc 12 attached to the electron microscope. This method of operation is characterized by its good resolving power since diffusion of the light generated by the phosphor screen 25 is eliminated or at least greatly reduced compared to the commonly used arrangement using a homogeneous plane parallel glass disc of finite thickness. Depending on the format of the photographic emulsion the diameter of the fiber optic plate 12 attached to the microscope may differ from the diameter of the adjacent fiber optic disc 30 or 38 which forms the input to the SEC camera tube.
It is also within the spirit of this invention to use a third fiber optic disc between the disc 12 and the disc 38' when the SEC camera tube 46 is positioned adjacent the disc 12. Such an arrangement affords the possibility of electrically separating the potentials required for proper operation of the electron microscope and the SEC camera tube 46 and which are normally applied to the photocathode of the SEC camera tube 46 and to the phosphor screen 25 of the electron microscope.
The possibility of contact photography can be provided by a different arrangement in which at least one electron optical image intensifier 27 is positioned between the fiber optic window l2 and photographic material. In this particular embodiment, the image input to the intensifier 27 and the image output of the image intensifier 27 are again comprised of fiber optic windows 30 and 36. The fiber optic input 30 of the image intensifier 27 is brought into intimate contact with the fiber optic disc 12 attached to the electron microscope using immersion oil while the fiber optic output 36 of the image intensifier is brought into contact with the photographic material or with the input fiber optic disc 38 of the SEC camera tube 46. It is obvious that more than one image intensifier with fiber optics input and output can be cascaded and inserted between the first fiber optic 12 on the electron microscope and the input fiber optic 38 of the SEC camera tube, or the photographic emulsion. Normally one would choose equal diameter for the fiber optic discs in such an arrangement. In the case of using an electron optical image intensifier such as 27, this does not necessarily mean that the diameters of all the fiber optic discs have to be equal. Since image intensifiers are known which employ fractional demagnification from input to output as shown in the device 27. it has been found that it is practical to use approximately one-half the diameter on the exit side of the image intensifier 27 compared to the input side of the image intensifier. Such an arrangement results in a brightness increase by a factor of four independent on the electron gain of the image intensifier.
As has been pointed out above. considerable advantage of this invention is to be found in the integration capability of an SEC camera tube over prolonged periods of time. For the television display of very dim pictures, it would be advantageous to use the technique of turning the reading beam of the SEC camera tube off for a sufficient duration of time to allow integration of the input image and then subsequently pplsing the reading beam on the SEC camera tube for a bri ht display on the television monitor screen. It IS obvious that t [5 process of pulsing the reading beam could be automatized over a wide range of integration periods necessary dependent upon the light level available.
It is obvious that other modification may be made within the spirit of this invention.
1. Apparatus comprising an electron microscope having an output phosphor deposited on the inner surface of a fiber optic output window, a secondary electron conduction camera tube having its input photocathode surface optically coupled through at least said fiber optic window to said output phosphor.
2. The apparatus set forth in claim 1 which said fiber optic assembly includes a first optic disc serving as the window in said electron microscope and a second fiber optic disc serving as the input window in said secondary electron tube.
3. The apparatus set forth in claim 2 in which said first and second fiber optic discs are coupled using immersion oil.
4. The apparatus set forth in claim 2 in which an electron optical image intensifier is inserted between said first and second fiber optic disc so that said first fiber optic disc is coupled to the input side of said image intensifier and said second fiber optic disc is coupled to the output of said image intensifier.
5. The apparatus set forth in claim 4 in which fiber optic windows are provided in said image intensifier which are coupled to said first and second fiber optic windows by using immersion oil.
6. The apparatus set forth' in claim 1 in which said secondary electron conduction camera tube provides integration over a first time period and read out in a second time substantially less than said first time period.