Large screen projection TV's are well known. Typically, such TV sets contain three CRT's, one for each of the primary colors, namely red, blue and green. A lens system is associated with each CRT. Each lens system magnifies the image appearing on the face plate of the CRT and projects the magnified image on the viewing screen of the projection TV. It is, of course, important from the point of view of the user that the projected image be sharp, i.e. the image should not be "fuzzy", and the image should be bright and have natural appearing color and contrast. The history of the design of color TV projection lens systems reflects ongoing efforts to achieve these objectives while, at the same time, endeavoring to avoid undue complexity and cost in the design of the lens system.
The achievement of these objectives has been difficult. Perhaps the best evidence of that difficulty is the fact that room for improvement still exists, notwithstanding the fact those skilled in this art have been working on these matters for more than about 35 years. Some of the difficulties which have impeded progress can be understood by considering the complexities of such a system. For example, the CRT's commonly used in projection TV's have a raster with a diameter of about 5 inches. Commonly, the viewing screen of a projection TV is in the range of 40 to 50 inches. Thus, a remarkably high performance is demanded from a projection TV lens system because the system must not only magnify an image by a factor of more than 8 but, in addition, it must do so while preserving the image quality. Additionally, this performance must be obtained within the confines of the projection cabinet which ideally should be as small as possible. And, once again, the lens system itself should not be unduly complex because, otherwise, the cost of the resulting TV set may become prohibitively high.
Considering such systems in more detail, three CRT's typically have been used in an effort to achieve a bright and sharp projected color image. Specifically, the use of three CRT's, each of which produced a specific color, resulted in a brighter image by allowing the elimination of the color mask needed in single CRT's which produce a full range of colors. The earliest projection TV sets used Schmidt type lens systems that were nearly color corrected. At that time, color was not a problem.
As the art developed, refracting lens systems were introduced and the monochromatic aspects of the three CRT's were exploited, which initially enabled the use of non-color corrected lenses. Thereafter, a demand arose for projection TV's which could be viewed in normally lighted rooms, as opposed to the partially lighted or darkened rooms in which projection TV's were initially used. Also, so-called rear projection TV sets were then introduced which were aimed at usage in normally lighted rooms. As a result, there was an ever increasing desire for a brighter image. This desire was accommodated by improvements in the CRT design. However, as a consequence of the these "improvements" the resulting CRT's and especially the "green" CRT's, i.e. those which produced nominally green light, produced light of other wavelengths. Consequently, the non-color corrected aspects of the projection lens system became a problem because of the resulting chromatic aberration and color correction of the lenses was needed to produce sharper, clearer pictures having acceptable contrast. Such color correction increased the cost and complexity of the lens system.
The problem of chromatic aberration was recognized very early in this art. In fact, the art reflects that at about the dawn of the age of television, some workers in the art had some awareness of this problem. This awareness is reflected in U.S. Pat. No. 2,336,134, which refers to the desirability of using particular materials to correct for "chromatic error". Notwithstanding this early awareness, when color projection TV initially became a reality, those skilled in the art did not pay particular attention to the problems which are caused by chromatic aberration. Thus, early designs of projection TV lens systems did not reflect a concern about the need for color correction. Examples of such early designs of lens systems are seen in U.S. Pat. Nos. 4,300,187 and 4,526,442. However, as progress in the art occurred and systems were improved, concern arose with respect to the need for "color correction", i.e. the need, within the lens systems, to correct for chromatic aberration caused by the lack of color "purity" in the output of the individual CRT's.
Considering the approaches to solving this problem which are evidenced by the prior art, most are centered around the use of ever more elaborate lens systems. Some examples of prior art attempted solutions of the chromatic aberration problem are the use of lenses which have different Abbe numbers. In this regard, U.S. Pat. No. 4,838,670 is illustrative of such attempts to deal with the problem of chromatic aberration. Specifically, in this patent it is pointed out that in supposedly monochromatic CRT's currently used in projection TV's "there occurs a color deviation caused by spectral distribution of fluorescence". In an effort to deal with this problem and thereby provide a truly color corrected or achromatic lens system, the patent teaches that lenses of widely varying Abbe numbers must be used, together with other lenses which must satisfy numerous other optical conditions. Although such lens systems do provide a substantial solution to the problem of chromatic aberration, it will be appreciated that the resulting complexity is highly undesirable, both from a cost and manufacturing point of view, and they do not improve chromaticity.
A different approach to solving this problem, which was tried by at least one prior art worker, sought to utilize the cooling liquid which is typically employed to dissipate the heat generated by the CRT. As is known to those skilled in this art, such liquids are used for the following reasons. In an effort to get an ever brighter picture, CRT's have been operated at increasingly high voltages. However, as a consequence, more heat is generated and therefore must be dissipated in order to avoid damage to the CRT and to improve phosphor life. For some time, the art has achieved this dissipation by enclosing the face of the CRT and filling the enclosure with a liquid, which serves as a heat sink and facilitates dissipation of the heat. The aforementioned prior art worker endeavored to use this liquid to ameliorate the chromatic aberration problem by the mechanism of including a dye in the cooling liquid. Such an approach is shown somewhat schematically in FIG. 1, wherein lens element 40 is the lens closest to the CRT 43 and 46 represents other lenses in the system which, for clarity, are not shown.
Referring to FIG. 1, it will be seen that an enclosure is formed by mounting a glass plate 42 in front of the CRT 43 and the space 49 therebetween, defined by the face plate 41, the collar 45, and the plate 42, is filled with a cooling liquid which includes a dye. Several significant problems resulted from this approach, although this approach is still in use today. For example, the need to provide the glass plate 42 increased both the cost and weight of the lens system. Additionally, and perhaps more significantly, the two surfaces of the glass plate inevitably resulted in transmission losses and scattering because of reflections at these surfaces and, consequently, the brightness and contrast of the picture were reduced. Some effort was made to address this problem by providing a seal between the glass plate and the so-called "C" element 40 of the lens system, i.e. the element closest to the CRT, and filling that space with fluid. However, as will be appreciated, this approach further increased the manufacturing cost and the complexity of the lens design.
The prior art recognized the difficulties caused by the provision of such a glass plate and some designs were evolved which eliminated the plate. In these designs, an enclosure was formed with the "C" element 40 in the front and the CRT in the rear and this enclosure was then filled with a liquid. Typically, this liquid was transparent, i.e. a dye was not added to the liquid. A problem associated with any contemplated addition of a dye to the liquid of such a construction was the fact that the surface or face plate 41 of the CRT 43 generally is concave rather than being flat. Thus, if a dye had been added to the liquid, the light emitted from the CRT would have been attenuated as a function of the radial distance from the center of the CRT. It was thought that such a result would only exacerbate the ever present problem of reduced corner illumination on the screen of the projection TV.
Another recent prior art endeavor to address these problems is reflected in U.S. Pat. No. 4,679,069. This patent includes an exposition of the fact that light emitted from present CRT's is impure, i.e. such light contains undesirable sidebands. In an effort to address this problem, this patent teaches the use of wavelength-selective reflectors which preferably are in the form of dichroic mirrors. Although such devices are in use, there are numerous disadvantages which attend this approach. For example, such mirrors are expensive. (The mirror shown in said patent has ten alternating layers of silica and titania.) Additionally, the filter effect of a dichroic mirror varies as a function of the angle of the incident light. Thus, notwithstanding the expense of this approach, there is an inevitable variation in the picture as a function of the position on the screen. Also, dichroic mirrors reflect light, which reduces contrast significantly.