Projection type television apparatus

In a projection type television apparatus, a first and a second color selecting device are located on an optical light path passing through a cathode ray tube and a series of projection lenses are disposed in front of the cathode ray tube without a color selecting device therein. The first color selecting device has a high refractive factor to light having a wavelength of 570nm or more and the second color selecting device has a high reflection factor or a high absorption factor to light having a wavelength of 520nm or less. With this color selecting device, the range of color reproduction can be much more improved. This thus causes an increased degree of color purity of green light at the central and peripheral areas of the cathode ray tube. Moreover, with this arrangement, it becomes easy to conserve the characteristic of the color selecting devices and to facilitate the manufacturing operation.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a projection type television apparatus using a 
green cathode ray tube having an improved color reproduction 
characteristic. 
2. Description of the Related Art 
FIG. 8(a), (b) of the accompanying drawings are diagrams showing a 
structure of a projection lens unit for a cathode ray tube, equipped with 
a multilayered interference filter is employed in a conventional 
projection type television apparatus. As shown in FIG. 8(a), (b) the 
projection television apparatus comprises a cathode ray tube 1 and a 
series of projection lenses 2 located in front of the cathode ray tube 1. 
A reference numeral 3 designates an incident pupil position of the series 
of projection lenses 2; 4, a display face plate pane; 5, a multilayered 
interference filter; 6, a fluorescent material; and 7, a vacuum 
evaporation aluminum film. 
The operation of this conventional projection television apparatus will now 
be described. 
When electron beams are incident on a fluorescent material 6 coated on the 
cathode ray tube 1, the fluorescent material 6 shows a characteristic 
curve 9 of a luminous spectrum of a multilayered interference coating with 
respect to transmittivity shown in FIG. 5. In view of the life and 
brightness, any existing cathode ray tube to be used in a projection 
television apparatus has inevitably such a spectrum. As shown in FIG. 5, 
the green fluorescent material 6 includes a spectrum blue-side and 
red-side spectra in addition to an essential green spectral wavelength 545 
nm. Therefore, as shown in a chromaticity diagram of FIG. 6, only the 
chromaticity value at a point G.sub.0 16 remote from the standard green 
chromaticity point G13 can be obtained. Consequently, color reproduction 
is possible only within a triangular range 19 indicated by broken lines. 
To this end, it is a known practice to place between the face plate pane 4 
and the fluorescent material 6 a multilayered interference filter 5 
composed of a plurality of alternately superimposed layers of high and low 
refractive index materials, and then to determine the transmittivity 
characteristic of this multilayered interference filter as indicated by 
the broken lines 11 shown in FIG. 5. Thus, unnecessary light spectrum 
having peak values around 600 nm of the light spectrum emitted from the 
green fluorescent material 6 is reflected. Therefore it is possible to 
improve the color reproduction up to the chromaticity point G.sub.1 17 
shown in FIG. 6. However, since the transmittivity characteristic of the 
interference filter 5 is determined as indicated by the broken lines 11 of 
FIG. 5, it is impossible to reflect unnecessary light spectrum having a 
peak at a wavelength of a slightly less than 500 nm. As a result, color 
reproduction is yet possible in the triangular range 20 indicated by the 
dash-and-one-dot lines in FIG. 6. 
Since the image on the cathode ray tube 1 is disposed at an angle of 
.theta..sub.1 with respect to the incident pupil 3 which is at the center 
of the projection lens series 2 as shown in FIG. 8(a), the difference l in 
the optical path between the layers of the multilayered interference 
filter 5 can be expressed by the following equation: 
EQU l=2d cos .theta..sub.1 (d: layer thickness) 
Consequently, the transmittivity characteristic of, in the direction of 
angle .theta..sub.1, the multilayered interference filter 5 will be as 
indicated by the dash-and-one-dot lines 12 in FIG. 5. This causes even the 
necessary green light spectrum to be reflected, thus impairing the color 
reproduction range. 
Prior to this application, the applicant herein proposed a projection type 
television having the color reproduction and the purity of green highly 
improved by interposing the multilayered interference coating layer 23 
having a high reflective index to light having a wavelength of 520 nm or 
less as shown in FIG. 9(a), (b) in the green projection lens series 
located in front of the cathode ray tube. 
As described above, with this conventional arrangement, partly because the 
range of color reproduction is inadequate for light from the central areas 
of the cathode ray tube, and partly because light from the peripheral 
areas of the cathode ray tube is reflected up to the essential light 
spectrum, the display color would become blue-emphasized green. 
In connection with the above-mentioned existing projection television set 
and the improved projection television proposed by the applicant herein, 
there are still several disadvantages in the process of forming the 
aforementioned multilayered interference coating layer on the inner 
surface of the cathode ray tube with respect to the characteristic of the 
coating layer. 
Specifically, as it will be explained later, since the multilayered 
interference coating layer is composed of 10 to 20 vacuum evaporation film 
of oxide titanium or oxide silicon, it is not an easy operation to form 
the vacuum evaporation film on the inner surface of the cathode ray tube 
having a cylindrical structure with a bottom. 
Further, as the matter of course, in the cathode ray tube, a consideration 
is necessary for the high level withstand characteristic against X ray and 
electron beams. 
As another consideration, a highly strict requirement of a high chemical 
resistance is also necessary when a fluorescent material is interposed 
between the multilayered interference coating layer and the vacuum 
evaporation aluminum film. 
Accordingly, this invention is made to solve the above-described drawbacks 
of the existing projection type television, and an object of the present 
invention is to provide a projection type television apparatus which is 
capable of improving the color purity at the central and the periphery of 
the green cathode ray tube, whether or not the cathode ray tube has a 
multilayered interference filter interposed between the face plate pane 
and the fluorescent material. 
SUMMARY OF THE INVENTION 
To this end, in one aspect of the present invention, there is provided a 
projection type television apparatus comprising: a cathode ray tube having 
a face plate pane; a display screen of a fluorescent material layer 
disposed on an inner surface of the face plate pane; a series of 
projection lenses located in front of the display screen of the cathode 
ray tube; a first color selecting means disposed on an optical light path 
composed of the series of the projection lenses and a cathode ray tube and 
having a high refractive factor to light having a wavelength of 570 nm or 
more; and a second color selecting means disposed in the optical light 
path composed of the series of the projection lenses and the cathode ray 
tube and having a high reflection factor or a high absorption factor to 
light having a wavelength of 520 nm or more. 
Practically, a first color selecting device may be of a multilayered 
coating formed on a front surface of the foremost one of the series of 
projection lenses, and a second color selecting device may be any of: a 
multilayered interference coating formed over a front surface of the 
second foremost one of the series of green projection lenses, or a 
multilayered interference filter disposed in front of the second foremost 
one of green projection lenses; a lens colored by mixing therein a 
metallic material, a pigment and a dye, or by soaking in a coloring liquid 
to cause infiltration of the liquid to the surface of the lens; a coolant 
interposed between the cathode ray tube and the series of green projection 
lenses and colored by mixing in the coolant a pigment or a dye; or a 
filter disposed in front of the foremost one of the series of green 
projection lenses. 
Specifically, the first color selecting device is a multilayered 
interference filter having a high refractive factor to light having the 
wavelength of 570 nm or more and formed on the foremost one selected from 
the series of green projection lenses by a known manufacturing method. 
Since the required operation is an operation of evaporating glass lenses, 
there is quite few problems in the manufacturing process. Further, and it 
is also preferable to conserve the characteristic of the multilayered 
interference filter liable to be deteriorated by the processing. 
In addition, as the second color selecting device having a high reflection 
factor or a high absorption factor to light having a wavelength of 520 nm 
or less, any of a multilayered interference coating layer formed over a 
lens, made of a glass or a plastic, other than that for the first color 
selecting device in the series of projection lens; a filter; or a glass or 
a plastic lens having the same color selective characteristic as that of 
the multilayered interference layer and colored by mixing metal therein, a 
pigment, or dye, may be used. 
Furthermore, the second color selecting device can be made up of any of a 
plastic lens colored by soaking in a coloring liquid to cause infiltration 
of the liquid to the surface of the lens; or a coolant interposed between 
the cathode ray tube and the series of lenses and colored by mixing 
therein a pigment or a dye. 
With this arrangement, since a first color selecting device has a high 
refractive index to light having a wavelength of 570 nm or more and a 
second color selecting device has a high reflection factor or a high 
absorption factor to light having a wavelength of 520 nm or less, it 
becomes possible to adequately reflect or absorb all the unnecessary light 
emitted from the green cathode ray tube, thereby reproducing a necessary 
light. 
Furthermore, since both the above-described first and second color 
selecting device are disposed in the series of lenses, this arrangement is 
advantageous for conserving a characteristic of a color selecting device 
and greatly facilitates the manufacturing operation. 
The above and other advantages, features and additional objects of this 
invention will be manifest to those versed in the art upon making 
reference to the following detailed description and the accompanying 
drawings in which several structural embodiments incorporating the 
principles of this invention are shown by way of illustrative examples.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A first embodiment in accordance with this invention will now be described 
with reference to the accompanying drawings hereinbelow. FIG. 1(a) is a 
diagram showing a structure of a series of projection lenses using a 
cathode ray tube of a projection television apparatus embodying the 
present invention. Since reference numerals from 1 to 7 designate the 
parts given the same numerals in a series of projection lenses of a 
conventional cathode ray tube shown in FIG. 8(a), the explanation of these 
parts are omitted here for clarity. 
A reference numeral 8 designates a first color selecting device having a 
high refractive index to light having a wavelength of 570 nm or more and 
formed over the surface of a foremost glass lens of a series of projection 
lenses 2, namely a multilayered interference coating layer which is 
evaporated by a known method. A reference 22 designates a multilayered 
interference coating layer or filter embodying a second color selecting 
device formed on one of the other projection lenses 2, and, like the 
transmittivity characteristic 10 indicated by the dash-and-two-dot lines 
of FIG. 5, the multilayered interference coating layer or filter has a 
high reflection factor or a high absorption factor to light having a 
wavelength of around 520 nm. The above-mentioned multilayered interference 
coating 22 includes a vacuum evaporation film composed of a plurality of 
(about 10 to 20) alternately superimposed layers of high and low 
refractive indices. The high-refractive-index layers comprise titanium 
oxide (TiO.sub.2) or tantalum oxide (Ta.sub.2 O.sub.5), and the 
low-refractive-index layers include silicon oxide (SiO.sub.2) or magnesium 
fluoride (MgF.sub.2), for example. 
In operation, when electron beams are incident on the fluorescent material 
6 of the cathode ray tube 1 of the projection television apparatus with 
the above-described structure, the light spectrum indicated by the solid 
lines in FIG. 5 is radiated. Because of the transmittivity characteristic 
11 of the multilayered interference filter 8 formed on a front surface of 
the foremost lens, light having peaks near 600 nm is reflected. In the 
meantime, unnecessary light having peaks at a wavelength of slightly 
shorter than 500 nm is not emitted from the series of projection lens 2. 
In other words, since the multilayered interference coating or filter 22 
formed on the front surface of the other one of the projection lenses 2 is 
determined so as to have the transmittivity characteristic 10 indicated by 
the dash-and-two-dot lines of FIG. 5, light having a wavelength of 520 nm 
or less is reflected and is not emitted forwardly out of the series of 
projection lenses 2. More specifically, unnecessary partial light of the 
light emitted from the cathode ray tube 1 is not emitted forwardly and 
therefore not projected on a screen by being reflected or absorbed by 
means of the multilayered interference coating and layer 8, 22 interposed 
among the projection lenses 2. As a result, it becomes possible to improve 
the chromaticity point up to the point G.sub.2 18 in FIG. 6. Consequently, 
the color reproduction range can be expanded up to a triangular area 
indicated by the dash-and-two-dot lines 21 in FIG. 6. Regarding light of 
an image on the peripheral areas of the cathode ray tube 1, because light 
having a wavelength between 520 nm or less and 570 nm or more is not 
forwardly emitted, all light obtained is only light having a peak 
wavelength around 545 nm, which is the essential green spectrum, thereby 
reproducing only green color. This would improve the color purity at the 
central areas as well as at the peripheral areas with respect to the 
single green light. 
Next, in accordance with this invention, as the second color selecting 
device, instead of the above-stated multilayered interference coating 
layer 22, there may be used a glass or a plastic lens 22a colored by 
mixing therein a metallic material, a pigment and a dye in order to have a 
transmittivity characteristic of reflecting or absorbing light having a 
wave length of 520 nm or less as shown in FIG. 2. 
Alternatively, the colored lens 22a can be a plastic lens 22b having the 
same level of color selectivity characteristic and colored by soaking in a 
coloring liquid to cause infiltration of the liquid to the surface of the 
lens as shown in FIG. 3. 
In another alternative form, as shown in FIG. 4, the second color selecting 
device may be a coolant 22c interposed between the cathode ray tube and 
the series of green projection lenses and colored by mixing in the coolant 
a pigment or a dye to have the same characteristic. 
As a coloring agent for the glass lens, iron oxide or chromium oxide is 
usually used. For the plastic lens and the coolant, phthalocyanine 
pigments or anthraquinone pigments are used as needed. 
In a further alternative form, as shown in FIG. 7, the second color 
selecting device may be a filter 23 having a high refractive index to 
light having a wavelength of around 520 nm and disposed in front of the 
series of projection lenses 2 instead of the multilayered interference 
coating layer 22, providing the similar advantage as mentioned above. 
As has been explained above, in accordance with the present invention, 
because the first and second color selecting devices, having an absorption 
factor or a high reflection factor to light having a wavelength between 
570 nm or more and 520 nm or less, are located among the series of 
projection lenses arranged in front of the cathode ray tube, the color 
reproduction can be much more improved. Furthermore, although the cathode 
ray tube is equipped with the multilayered interference filter, it is 
possible to improve the color purity of green light at the central and 
peripheral areas of the cathode ray tube. 
Additionally, since both the first and second color selecting device 
mentioned-above are disposed among the series of lenses which are located 
outside of the cathode ray tube, it becomes advantageous for conserving a 
characteristic, and greatly facilitates the operation of manufacturing the 
selecting device.