| 摘要 |
PURPOSE:To optionally select the use with a high contrast ratio and the use with a low contrast ratio by adding a contrast ratio adjusting means changing the contrast ratio to the outside of an output face section. CONSTITUTION:A thick plate glass 20 formed with an output fluorescent screen 14 on its surface is closely stuck and fixed on the inner face of an output window glass 18 with a transparent adhesive, and an additional thick plate glass 22 is closely stuck and fixed on the outer face of the output window glass 18 with a transparent adhesive. A transparent glass plate 24 is removably provided on the image output side of this additional thick glass 22 at a preset distance. Whether multiple sheets of this transparent glass plate 24 with the same thickness are used or one sheet with a different thickness is used is optionally selected depending on the degree that the contrast is decreased. Therefore, if a high contrast ratio is required, this tube may be used without arranging the transparent glass plate 24, if the contrast ratio is required to be lowered, multiple sheets of the transparent glass plate or one sheet with a different thickness may be arranged in response to the degree of reduction.
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