Projection-lens unit

摘要

The rise in temperature in an optical coupling liquid for cooling a front side of a cathode ray tube alters the refractive index of the liquid and alters the focusing distance of a projection objective, the result being that a picture projected onto a screen becomes blurred. The present invention relates to a countermeasure for compensating the defocusing on the projection screen. An optical coupling lens, which is one lens of the lens elements of the group of the projection objective, is designed for slight movement on the optical axis. The volume expansion caused by the rise in temperature in the optical coupling liquid is transformed into a pressure change. The pressure change is used for controlling the movement of the optical coupling lens. The focusing distance of the projection lens is automatically compensated relative to the rise in temperature in the optical coupling liquid.