| 摘要 |
In a video projection apparatus including a projection lens, a cathode-ray tube and a projection screen, a layer of a medium whose refractive index np decreases with a temperature rise is disposed between the projection lens and the faceplate of the cathode-ray tube, and the thickness lp of the medium layer is selected to substantially satisfy the following equation thereby compensating the temperature-dependent variation of the focal length of the principal lens: <IMAGE> where DELTA T is a temperature variation; DELTA np is a variation of the refractive index np due to the temperature variation DELTA T; f is the focal length of the projection lens; K is a constant representing the proportion of the power of plastic lenses to the total power when the projection lens is of a hybrid type; DELTA f is a variation of the focal length f due to the temperature variation DELTA T; and M is the magnification of projected images.
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