| 摘要 |
PURPOSE:To prevent production of a false signal due to reflection at an incident window, so as to improve the contrast of an output image by specifying the thickness of the light incident window while applying a block film for preventing reflection onto the circumferential section of the outer face and the side wall section except the effective area of a light incident window. CONSTITUTION:A light incident window 1 has the thickness (d) larger than 1/3 of the effective diameter (L) of a photoelectric face 2 while a black film 8 is applied onto the circumferential section and the side wall section except the effective area of the outer face. A beam of light reflected with a full- reflection critical angle theta from the photoelectric face 2 onto the outer face of the light incident window 1 is fully reflected, but arrives again at the photoelectric face in such position as separated by a distance of 2d.tantheta from the initial point. Since the critical angle theta is about 42 deg. when the light incident window is made of glass, the reflected light 9 will scarcely be incident again on the effective area of the photoelectric face 2 in case of such image tube as constructed to satisfy the relation 3d.tantheta>L, but arrive at the outer edge section where it will be reflected again to be incident finally on the black film section 8 and absorbed there.
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