| 摘要 |
PURPOSE:To compact a projection lens and to reduce the cost of lens working by setting the ratio of focal lengths of the 1st and the 2nd lenses to a small value and equalizing radii of the curvatures of both surfaces of the 2nd lens which has positive refracting power is absolute value to each other. CONSTITUTION:When the ratio f1/f2 of the focal length f1 of the 1st lens and the focal length f2 of the 2nd lens exceeds 1.1 as an upper limit, the value is disadvantageous to the compacting of the lens and when the value decreases below 0.55 as a lower limit, the value is advantageous to the compacting, but it becomes hard to compensate a coma aberration because the optical power of the 1st lens L1 becomes too large and the manufacture also becomes hard because the 1st lens L1 is too thick. Both surfaces of the 2nd lens L2 which has positive refracting power are spherical, and r3=-r4, where r3 is the radius of curvature of the screen-side spherical surface of the 2nd lens and r4 is the radius of curvature of the CRT-side spherical surface of the 2nd lens. Consequently, when a glass lens is used as the 2nd lens L2, a polishing jig and a Newton primary standard are required by only one kind, and it is not necessary to make a head/tail decision on lenses during assembly, so the cost is reduced.
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