This invention relates to focus adjustment assemblies for projection television.
HISTORY OF THE PRIOR ART
Projection television is a popular entertainment medium. Projection television comprises a projection cabinet and a screen. The projection cabinet may contain one or more cathode ray tubes (CRT's) and a system of lenses for each CRT. For a color projection television, the cabinet typically contains three CRT's, one for each of the primary colors--red,blue and green, and three lens systems, one for each CRT.
The lens systems of a projection television generally comprise at least three lenses. A first lens, usually of low power and which may have at least one aspheric surface, corrects for aberrations. A second, biconvex lens provides power to the image projection. A third, concave lens acts as a field flattener and corrects for field curvatures of the first and second lenses. Additional lenses may be added. A huge amount of research has gone into improving lens systems, and many complex systems have been developed.
Each lens system is part of a lens assembly. The lens assembly comprises two barrels, an outer barrel and an inner barrel. The inner barrel is situated to some degree within the outer barrel so that the barrels telescope. Each of the barrels includes at least one lens. The image is focused on the screen by moving the barrels relative to each other.
Obviously, it is important that the image is sharply focused on the screen. If the image is not focused, the most sophisticated lens system in the world is useless. Consumer projection televisions are typically designed with the intention that focusing is required only once upon assembly. However, even if the barrel assembly is accurately set, from time to time the image may move out of focus and require adjustment.
An optimal focus adjustment should be extremely sensitive (i.e.--allow for small variations in the relative spacing of the two barrels). In projection televisions with multiple lens assemblies, the focus adjustment should be sensitive enough that three images may be focused in concert. To avoid recurrent servicing, the focus adjustment should be mechanically simple and easy enough for the consumer to use. The focus adjustment should be inexpensive so as not to add greatly to the cost of the projection unit. Finally, the focus adjustment should include a locking mechanism that engages immediately after an adjustment has been made and that does not deform the barrels so as to degrade the lens performance.
Various mechanisms have been utilized to focus lenses or lens assemblies. U.S. Pat. No. 3,538,832 discloses a distance determining mechanism, for use in a camera having a focusable objective. To focus the objective lens a control lever is raised and a manual dial knob is rotated. The manual dial knob is fixed to a stub shaft having a pinion around its external end. The pinion meshes with a rack formed on an actuator member. Upon rotation of the knob, an actuator member moves about a shaft and adjusts a positioning pin which positions the lens cell of the objective lens. The complexity of this assembly is undesirable from both a manufacturing and servicing standpoint.
U.S Pat. No. 2,285,470 discloses a plural lens mount in which an inner lens box is longitudinally movable in an outer box via a rack, mounted on the inner box parallel to the longitudinal axis of the inner box and the outer box, and a pinion, connected to a rotatable knob. The sensitivity of this device is limited by the longitudinal orientation of the rack.
Current industry designs for focus adjustment include mechanical fasteners, such as wing nuts or plastic knobs, attached to a metal or plastic post. The post is attached to the inner barrel and travels along a slot in the outer barrel. Focusing is accomplished by moving the post along the slot, then tightening down the fastener creating interference between the outer barrel and the inner barrel. Focusing is performed manually and can be awkward because the post must be pushed along the slot. The mechanical fasteners used can result in overstressed lens mounts due to excessive tightening or lenses moving out of focus due to insufficient tightening.
U.S. Pat. No. 5,054,930 discloses a three tube color projection television in which a lens barrel carries protrusions on its sides. These protrusions are passed through inlets formed at two axially symmetrical places of a cylinder portion and are fitted into oblique guide slots in the side surface of the cylinder portion. To focus the image, the lens barrel is rotated relative to a barrel supporting portion to make the guide protrusions slide on side surfaces of the guide slots, so as to vary the distance between the lens barrel and a tube surface. To secure the barrel, a wire band is put on the cylinder portion and fastened with a screw. Here again, accurate focusing is made awkward because the barrels must be grasped. Fastening the wire band requires additional work and applies force between the inner and outer barrels.
Despite the devices discussed above and many additional attempts since the inception of projection television, a need still exists for a simple, inexpensive, easy to use, and sensitive focus adjustment device and a device which provides immediate locking that does not apply significant force between the inner and outer barrels which could deform the barrels.