Imaging tube

Abstract: An imaging tube has a fiber optic plate (FOP) (3) as an output face plate. On one surface of the FOP (3) within the evacuated envelope of the tube is deposited a first transparent conductive layer (61) . On the first transparent conductive layer (61) is deposited a fluorescent layer (62). On the fluorescent layer (62) is deposited a metal back electrode (63). On the other surface of the FOP (3) outside the evacuated envelope is deposited a second transparent conductive layer (7). The first transparent conductive layer (61) and the metal back electrode (63) are preferably electrically connected so that an electrical field is not developed across the fluorescent layer (62) when the metal back electrode (63) is applied with a high positive voltage and the second transparent conductive layer (7) is grounded. Therefore, even if leakage currents flow through the FOP (3), electric charges arrived at the first transparent conductive layer (61) do not cause the fluorescent layer (62) to generate noise spots.

Description: The present invention relates to an imaging tube including an image intensifier, a framing tube, and a streak tube. There has been known an X-ray florescence multiplier tube provided with a photocathode and a fluorescent surface, as disclosed in Japanese Laid-Open Patent Publication SHO-53-67347 and similarly disclosed in NL-A-7508792. The fluorescent surface of this multiplier tube is formed using electrophoretic techniques and is a multilayer structure consisting of a transparent conductive layer, a fluorescent layer, and a metal thin layer which are sequentially deposited in the stated order on the inner surface of a glass plate (an output faceplate)facing the photocathode. To improve optical coupling at the output of the imaging tube, a fiber optical plate (FOP) is generally used as an output faceplate. The fluorescent surface of the imaging tube in which the FOP is used is a two-layer structure. Specifically, the fluorescent layer is directly deposited over the inner surface of the FOP and the thin metal layer is deposited over the fluorescent layer. The thin metal layer prevents light generated at the fluorescent layer from feeding back toward the photocathode, and so is called a metal-back film. Generally, the imaging tubes with FOPs are used in conjunction with a solid-state image pick-up device. In use, the image pick-up device is mounted directly on the FOP. In order to maintain the image pick-up device at ground potential, a transparent conductive layer is formed on the outer surface of the FOP to connect it to ground. On the other hand, because the metal-back thin film is applied with a positive high voltage, a strong electric field is developed between the inner and outer surfaces of the FOP. This strong electric field causes electric charges to appear in the fluorescent layer as a result of leakage currents flowing through the FOP. Due to the electric charges staying in the fluorescent layer, dark spots are locally observed at the output side of the FOP for a brief period of time when light is uniformly applied to the photocathode. The dark spots finally disappear, because the fluorescent layer which normally has electrical insulation properties exhibits conductive properties when the fluorescent layer generates light, so the electric charges are released from the fluorescent layer soon after the imaging tube is operated. Further, due to discharges occurring between the metal-back thin film and the FOP caused by the strong electric field developed across the fluorescent layer or incident electrons into the fluorescent layer from the FOP, bright spots are locally observed at the output side of the FOP when no light is applied to the photocathode. These dark spots and bright spots have a similar pattern because these spots are generated resulting from the fact that some fibers of the FOP exhibit conductivity. While the use of heavily insulated FOPs can prevent generation of dark and bright spots, that is, degradation of image quality, the expense of heavily insulated FOPs creates an additional problem of increasing the total cost of imaging tubes in which they are used. Also, dark spots and bright spots tend to occur easily even when using highly insulated FOPs if the FOPs are slenderized or high voltage is applied thereto. According to the present invention, an imaging tube comprising: a photocathode for producing photo-electrons in response to radiation incident thereon; and, a fiber optic plate having a first side and a second side opposing the first side, said fiber optic plate being arranged so that the first side is oriented in a direction to confront said photocathode, is characterised by; a first transparent conductive layer deposited over the first side of said fiber optic plate; a fluorescent layer deposited over said first transparent conductive layer; a metal back electrode formed on said fluorescent layer; and a second transparent conductive layer deposited over the second side of said fiber optic plate. Preferred features of the invention are set out in the dependent claims, where preferably, the first transparent conductive layer and the metal back electrode are electrically connected so that an electric field is not developed across the fluorescent layer, whereby the cause of the dark and bright spots is eliminated. However, it is generally sufficient for the first transparent conductive layer to be present, even if electrically disconnected from the metal back electrode, since, in this case, the electric field across the fluorescent layer is substantially uniform which eliminates the pattern appearing on the first conductive layer. Particular embodiments of imaging tubes in accordance with this invention will now be described with reference to the accompanying drawings, in which:- Fig. 1(a) is a cross sectional diagram showing an overall arrangement of a proximity type imaging tube according to a first embodiment of the present invention; Fig. 1(b) is a cross-sectional diagram showing a structure of a fluorescent surface formed on a FOP of the imaging tube shown in Fig. 1(a); Fig. 1(c) is a cross-sectional diagram showing a modified structure of a fluorescent surface formed on a FOP; and Fig. 2 is a cross-sectional diagram showing an overall arrangement of an imaging intensifier according to a second preferred embodiment of the present invention. Referring to the accompanying drawings, preferred embodiments of the invention will now be described wherein like parts and components are designated by the same reference numerals to avoid duplicating description. As can be seen in Fig. 1(a), an evacuated envelope is formed from a cylindrical vessel 1 with a generally circular glass faceplate 2 hermetically attached to one opening thereof. At the other opening of the cylindrical vessel 1 is hermetically attached a fiber optic plate (FOP)3 via a support 4. A photocathode 5 is formed on the inner surface of the faceplate 2 from a material such as an alkali metal. A fluorescent surface 6 is formed at the side of the FOP 3 confronting the photocathode 5. As shown in Fig. 1(b), the fluorescent surface 6 consists of three layers; a first transparent conductive layer 61(made from indium tin oxide ITO)deposited over the FOP 3,a fluorescent layer 62 with high insulation properties deposited over the first transparent conductive layer 61, and a metal-back electrode 63 (made from aluminum) formed on the fluorescent layer 62. The edge of the metal-back electrode 63 connects to the edge of the first transparent conductive layer 61 to maintain both the first transparent conductive layer 61 and the metal-back electrode 63 at the same potential. At the surface of the FOP 3 opposing the fluorescent surface 6 is formed a second transparent conductive layer 7 is also made from indium tin oxide. The second transparent conductive layer 7 is connected to ground. The fluorescent surface 6 is applied with a positive potential higher than that of the photocathode 5. Therefore, when the photocathode 5 generates photoelectrons upon being struck by incident light (hν), the generated photoelectrons become incident to the fluorescent surface 6 which fluoresces as a result. Because the second transparent conductive layer 7 provided at the outer surface of the FOP 3 is grounded, a strong electric field is developed across the FOP 3. Therefore, some leakage currents may flow through the FOP 3. However, even if the leakage currents flow therethrough, electric charges arrived at the first transparent conductive layer 61 are released therefrom. Consequently, discharges at areas of the fluorescent layer 62 and charge-ups into the fluorescent layer 62 will not occur. Therefore, after applying voltage to the imaging tube and immediately after start of imaging, no bright and dark spots are generated. Therefore, image quality is improved, especially during the period three to thirty seconds immediately after the start of imaging when dark spots are most likely to occur. The imaging tube shown in Fig. 1(c) is a modification of the tube shown in Figs. 1(a) and 1(b), wherein the first transparent conductive layer 61 and the metal-back electrode 63 are electrically disconnected from each other and the first transparent conductive layer 61 is held in a floating condition. The first transparent conductive layer 61 may be held at a potential differing from that of the metal-back electrode. With such structures, uniform electric field across the fluorescent layer 62 can be attained although the discharges at areas of the fluorescent layer 62 and charge-ups into the fluorescent layer 62 may occur unlike the embodiment shown in Figs. 1(a) and 1(b). Consequently, the dark spots and bright spots do not become notable at the output side of the FOP. The material for the first transparent conductive layer 61 is not limited to indium tin oxide. However, it is desirable that the first transparent conductive layer 61 be a layer thin enough (for example, one hundred to several hundred nanometers for indium tin oxide) to prevent reductions in image quality. Fig. 2 is a cross-sectional diagram showing an imaging intensifier according to a second preferred embodiment of the present invention. The output portion of the imaging intensifier is the same as that shown in Fig. 1(b). In the second preferred embodiment, the faceplate 2 is formed integral with a glass envelope. An electron lens 8 for focusing the electron beam and a microchannel plate (MCP) 9 for multiplying the electrons are provided between the photocathode 5 and the fluorescent surface 6. In the second preferred embodiment, the electric potential between the fluorescent surface 6 and the second transparent conductive layer 7 is generally greater, so that the favorable effects gained by using the present invention become more pronounced. As described above, an imaging tube according to the present invention has a first transparent conductive layer deposited over the inner surface of an FOP. Because the fluorescent layer and the conductive reflective layer are formed on the surface of the first transparent conductive layer, all have the same high positive electric potential. Therefore, even if leakage current is generated partially at the inner portion of the FOP with a structure having a second transparent conductive layer deposited over the outer surface of the FOP and grounded, electric charges do not appear in the fluorescent layer. Because this eliminates any need to use heavily insulated FOPs, providing a high performance imaging tube at low cost becomes possible. Use of thinner FOPs also becomes possible.

Claims: An imaging tube comprising: a photocathode (5) for producing photo-electrons in response to radiation incident thereon; a fiber optic plate (3) having a first side and a second side opposing the first side, said fiber optic plate being arranged so that the first side is oriented in a direction to confront said photocathode (5), characterised by; a first transparent conductive layer (61) deposited over the first side of said fiber optic plate (3); a fluorescent layer (62) deposited over said first transparent conductive layer (61); and a metal back electrode (63) formed on said fluorescent layer (62); characterised by a second transparent conductive layer (7) deposited over the second side of said fiber optic plate (3). An imaging tube according to claim 1, wherein said first transparent conductive layer (61) and said metal back electrode (63) are electrically connected. An imaging tube according to claim 1, wherein said first transparent conductive layer (61) is electrically disconnected from said metal back electrode (63). An imaging tube according to any one of the preceding claims, wherein said first transparent conductive layer is made from indium tin oxide. An imaging tube according to any one of the preceding claims, further comprising first means for connecting said metal back electrode (63) to a positive voltage terminal of a power source, and second means for connecting said second transparent conductive layer (7) to ground. An imaging tube according to any one of the preceding claims, further comprising electron multiplying means (9) for multiplying the photo-electrons produced from said photocathode (5). An imaging tube according to any one of the preceding claims, further comprising means for applying a first positive voltage to said photocathode (5) and means for applying a second positive voltage higher than the first positive voltage to said metal back electrode (63).

Inventors: MORI KUNIYOSHA, MORI, KUNIYOSHA
Applicants: HAMAMATSU PHOTONICS KK, HAMAMATSU PHOTONICS K.K.
Application year: 1993
Main classification: H01J31
IPC codes: H01J31, H01J29