Aspheric Projection Lens System Utilizing Concave Image

1. Field of the Invention

This invention relates generally to optical projection systems, and more particularly, to a simple lens system capable of high-quality performance that utilizes only a single element aspheric projection lens and appropriate coupling optics for projecting an image from one or more cathode ray tubes or the like having concave face plates onto a suitable projection screen.

2. Description of the Prior Art

Projection lens systems usable for projecting cathode ray tube images onto a suitable projection surface are known. Such systems are relatively complex and are generally catadioptric in configuration or use multiple lens elements in order to achieve the necessary transmission efficiency and resolution.

While the prior art systems do achieve satisfactory optical performance, the catadioptric systems and the multiple lens systems tend to be complex, expensive and bulky, particularly the catadioptric systems.

Accordingly, it is an object of the present invention to provide an improved projection lens system that overcomes many of the disadvantages of the prior art systems.

It is yet another object of the present invention to provide an improved projection lens that is much simpler and less expensive than the prior art projection lens systems.

It is still another object of the present invention to provide a projection lens system that utilizes only a single element objective lens.

In accordance with a preferred embodiment of the invention, a single element objective lens having hyperbolic aspheric surfaces is used in conjunction with three cathode ray tubes, each having a concave face plate optically coupled to the objective lens by means of a beam splitter. The curvature of the face plate is optimized for the system, and such optimization of the image shape greatly reduces the complexity of the resulting lens system, and permits high quality performance to be achieved with a single element thick objective lens having identical hyperbolic aspheric surfaces.

These and other objects and advantages of the present invention will be readily understood upon consideration of the following detailed description and attached drawing wherein:

FIG. 1 is a diagram of the lens system according to the invention used in a color television projection system; and

FIG. 2 is a more detailed diagram of the objective lens of the system.

Referring now to the drawing, with particular attention to FIG. 1, there is shown a projection system according to the invention generally designated by the reference numeral 10. The projection system designated by the numeral 10 utilizes a single aspheric objective lens 12 and a beam splitter 14 that is optically coupled to three cathode ray tubes 16, 18 and 20, each having a concave face plate and a different color phosphor, for example, red, blue and green. The beam splitter 14 is of conventional design except for the three convex surfaces that mate with the concave surfaces of the cathode ray tubes 16, 18 and 20, and serves to combine the three images from the cathode ray tubes. The combining of the images is achieved by reflecting the images from the cathode ray tubes 16 and 20 from a pair of planar surfaces 22 and 24, respectively formed at the point of contact of four prisms 14a, 14b, 14c and 14d forming the beam splitter 14. The image from the cathode ray tube 18 passes through the surfaces 22 and 24 directly to the objective lens 12.

In accordance with an important aspect of the invention, each of the cathode ray tubes 16, 18 and 20 has a concave face plate 26, 28 and 30. The use of such a concave face plate reduces the constraints on the design of the objective lens 12, and by optimizing the radius of curvature of the concave face plates 26, 28 and 30, the objective lens can be fabricated from a single thick lens element having two aspheric surfaces, which in the present embodiment are two identical hyperbolic surfaces.

In the present embodiment, the cathode ray tubes 16, 18 and 20 are optically coupled to the beam splitter 14 in order to form an immersed system. The coupling is accomplished by bringing the cathode ray tubes 16, 18 and 20 into physical contact with the beam splitter 14 and placing the immersion medium at the interface between the cathode ray tubes 16, 18 and 20 and the beam splitter 14. Since the optical properties of the face plates 26, 28 and 30 are similar to those of the optical material forming the beam splitter 14, the interface between the beam splitter 14 and the face plates 26, 28 and 30 disappears in an immersed system, particularly when the optical coupling is achieved by the use of a viscous fluid or glue between the beam splitter 14 and the face plates 26, 28 and 30. However, the system need not be an immersed system and the system can be made to work satisfactorily by simply bringing the face plates 26, 28 and 30 into close proximity or contact with the beam splitter 14 without the use of an immersion medium. The only disadvantage that occurs when this is done is a slight reduction in transmission efficiency due to reflections at the interface between the cathode ray tube face plates 26, 28 and 30 and the beam splitter 14.

The use of curved images permits the design of the objective lens to be greatly simplified, and in the present embodiment, the objective lens is formed from a single aspheric lens 12, whose design has been so simplified by the use of concave images that the lens can be fabricated with two identical hyperbolic surfaces. In the specific embodiment illustrated in the drawing, the optical system according to the invention is used to project images from three five-inch diagonal measure cathode ray tubes onto a screen ranging from 52-inch diagonal measure to 72-inch diagonal measure. The distance between the objective lens and the screen ranges nominally from 62 to 83 inches. In the illustrated system, the lens and prisms are fabricated from optical acrylic, poly(methyl methacrylite) having a refractive index, N.sub.D, of 1.490 and an Abbe number, V.sub.D, of 57.8; however, any suitable optical material including various glasses or plastics may be used. The particular design illustrated in the drawing has a speed corresponding to F/1.8 and a focal length of 5.53 inches. In the aforesaid design, the radius of curvature, R, of the cathode ray tube face plate, and beam splitter entry surfaces and beam splitter optical path, D, are as follows for the green optical path:

The blue optical path is 1.6 mm. shorter, and the red optical path is 0.5 mm. longer. The radii of curvature, R.sub.1 and R.sub.2, of the objective lens are:

Prior to aspherizing, with the actual hyperbolic surface of the left-hand surface of the lens (FIG. 2) being defined by: ##EQU1## where the aspheric constant K equals -2.148 and y ranges from 0 to at least 42 mm. A similar equation defining the right-hand surface of the lens is obtained by replacing R.sub.1 with -R.sub.2. An aperture stop in the form of a slot having an inside diameter of 78.6.+-.0.555 mm. is cut in the objective lens at a distance of 29.4.+-.0.5 mm. from the projecting surface. The distance D.sub.1 (FIG. 1) between the objective lens 12 and the beam splitter 14, is variable and depends on the distance to the projection screen and the image size. Typical values for D.sub.1 are:

The aforesaid parameters may be renormalized to accomodate various refractive indices, focal lengths and tube sizes, with all radii, thicknesses and spaces being made proportionately larger to accomodate higher refractive indices, larger cathode ray tubes and longer focal lengths, and proportionately smaller to accomodate lower refractive indices, smaller cathode ray tubes and shorter focal lengths. The asphericity constant K is not affected by normalization.

Obviously, many modifications and variations of the present invention are possible in light of the above teachings. Thus, it is to be understood that, within the scope of the appended claims, the invention may be practiced otherwise than as specifically described above.