2-CRT type projection apparatus including a red/green CRT and a blue/green CRT with different green phosphors

A 2-CRT type projection apparatus having a green and blue two-color cathode ray tube and a green and red two-color cathode ray tube and producing a projected picture of high resolution and brightness. The green phosphor of one of the two-color cathode ray tubes is a phosphor which emits green light of high brightness and the green phosphor of the other cathode ray tube is a phosphor which emits green light extending a color reproduction range. Further, stripe directions of the two-color cathode ray tubes are substantially perpendicular to each other.

BACKGROUND OF THE INVENTION 
Field of the Invention 
This invention relates to a 2-CRT type projection apparatus in which two 
cathode ray tubes (hereinafter, abbreviated as "CRTs") are used. 
Description of the Related Art 
FIG. 1 illustrates the configuration of a conventional projection apparatus 
which is of the so-called 3-CRT-1-lens type. In the FIGS. 1, 2 and 3 
respectively designate red (R), green (G) and blue (B) monochromatic CRTs 
which are separated by 90.degree.. The red CRT 1 faces to the blue CRT 3, 
and between them disposed are a dichroic mirror 4 which reflects red light 
only and a dichroic mirror 5 which reflects blue light only. The dichroic 
mirrors 4 and 5 cross with each other at right angles. A projection lens 6 
and screen 7 are arranged in this sequence so that the dichroic mirrors 4 
and 5 are sandwiched between the projection lens 6 and the green CRT 2. In 
the projection apparatus, red and blue pictures projected from the red and 
blue CRTs 1 and 3 are made overlapped with a green picture projected from 
the green CRT 2 by the dichroic mirrors 4 and 5, and these pictures are 
magnified by the projection lens 6 so as to form a color image on the 
screen 7. Since the three monochromatic CRTs are arranged so as to be 
separated by 90.degree., the projection apparatus is bulky in system 
configuration. 
FIG. 2 illustrates the configuration of another conventional projection 
apparatus which is of the so-called 2-CRT-1-lens type. In the figure, 2 
and 8 respectively designate green and blue/red CRTs which are separated 
by 180.degree.. The blue/red CRT 8 has a phosphor screen consisting of 
blue and red phosphors which are arranged in a stripe form or a dot-matrix 
form. Between the green CRT 2 and the blue/red CRT 8 which are opposite to 
each other, disposed are a dichroic mirror 9 which reflects green light 
only and a dichroic mirror 10 which reflects blue light and red light 
only. The dichroic mirrors 9 and 10 cross with each other at right angles. 
The 2-CRT-1-lens type projection apparatus can be constructed in a smaller 
size than the projection apparatus of the 3-CRT-1-lens type shown in FIG. 
1. 
FIG. 3 illustrates the configuration of a further conventional projection 
apparatus which is of the so-called 2-CRT-2-lens type. This projection 
apparatus is provided with two projection lenses 6 instead of dichroic 
mirrors so that a green picture and a blue and red picture overlap with 
each other on the screen 7 to form a color image thereon. 
Next, the structure of the phosphor screens of the green CRT 2 and blue/red 
CRT 8 shown in FIGS. 2 and 3 will be described. FIG. 4(a) shows the 
phosphor screen of the green CRT 2, and FIG. 4(b) the phosphor screen of 
the blue/red CRT 8. In the figures, the x-axis and y-axis indicate the 
long and short axes of the phosphor screens, respectively, and the arrow 
shows the longitudinal direction (hereinafter, referred to as "stripe 
direction s" of striped green, blue and red phosphors which constitute the 
phosphor screens. FIG. 4(c) is an enlarged view of the phosphor screen of 
the green CRT 2, and FIG. 4(d) an enlarged view of the phosphor screen of 
the blue/red CRT 8. In these figures, CS, BS, RS and BB indicate green 
phosphors, blue phosphors, red phosphors and black phosphors (hereinafter, 
respectively referred to as "green stripe", "blue stripe", "red stripe" 
and "black stripe"), respectively. 
For example, the green, blue and red stripes CS, BS and RS are structured 
so as to have a width of 0.35 mm, and the black stripe BB so as to have a 
width of 0.1 mm. In the blue/red CRT 8 which is a two-color tube of blue 
and red, the black stripe BB is formed as a dead space required for 
two-color discrimination in order to improve the contrast. It is not 
necessary to provide the green CRT 2, which is a monochromatic green tube, 
with the black stripe BB. Although disadvantageous in terms of contrast, 
the green CRT 2 may be constructed so as to project a so-called "uniform" 
picture. 
Conventional 2-CRT type projection apparatus have the following problems. 
A first problem is that the structure of the phosphor screens of the green 
CRT 2 and blue/red CRT 8 shown in FIG. 4 cannot provide sufficiently high 
brightness and resolution. Namely, it has been found that the ratio of 
electron beam amounts required for green, blue and red phosphors of the 
green CRT 2 and blue/red CRT 8 is 
EQU P G: B: R=40 (=20.times.2):28: 32 
when pictures of the green CRT 2 and blue/red CRT 8 having the phosphor 
screen structure shown in FIG. 4 are projected on the screen 7, for 
example, so as to realize the condition of 9300.degree. K+27 MPCD which is 
a general criterion for a white picture. 
In the above, the rate of 40 (=20.times.2) of the electron beam amount 
required for green phosphors means that the required electron beam amount 
for one green stripe GS of the green CRT 2 is 40. More specifically, since 
two green stripes GS correspond to one blue stripe BS and one red stripe 
RS in the structure shown in FIG. 4(e), the rate of the electron beam 
amount required for the green CRT 2 is 20. 
Generally, in relation to the luminosity factor of an eye, resolution is 
largely affected by the brightness of green. The rate of 20 of the 
electron beam amount required for the green CRT 2 means that it is 
sufficient for the electron beam to have such a reduced amount, and 
therefore shows a preferable tendency only in terms of resolution. 
However, the ratio of red to green is 32/20=1.6, and namely, the electron 
beam for red must be 1.6 times the amount for green, thereby causing a 
problem that the red image becomes blurred. From this standpoint, it is 
preferable that the rates of the electron beam amounts respectively 
required for green, blue and red are equal to each other as far as 
possible. 
In the structure of the phosphor screens of the green CRT 2 and blue/red 
CRT 8 shown in FIG. 4, the effective luminescence area for green can be 
smaller than that for blue and red, and, namely, the ineffective portion 
of the green CRT 2 is wider than that of the blue/red CRT 8. When improved 
so as to reduce such a wasted portion, the widths of the green, blue and 
red stripes GS, BS and RS are 0.233 mm, 0.327 mm and 0.448 mm, 
respectively. 
A second problem is that, in the phosphor screen structure shown in FIG. 4, 
pictures for three colors overlap with each other and hence color 
reproduction is performed only inside the triangle which is in a CIE 
chromaticity diagram shown in FIG. 5 and defined by chromaticities of 
three colors G, B and R emitted from the phosphor screens, resulting in a 
narrow range of color reproduction. 
A third problem is that the phosphor screens are formed by arranging 
striped phosphors in a fixed direction so that there is a direction along 
which a linear structure notably emerges, with the result that "Moire 
fringes" which are interference fringes caused by such an arrangement 
structure of the phosphor screens may appear in a picture projected on the 
screen 7. The same problem arises in a CRT wherein small circular 
phosphors are arranged in a matrix form and directions t along which 
linear structures notably emerge coincide with each other. 
Measures for preventing "Moire fringes" (which is the defect in the third 
problem) from occurring cause the fourth problem that the manufacturing 
process of the green CRT 2 is different in some steps from that of the 
blue/red CRT 8 and hence it is not possible to commonly use manufacturing 
facilities for both kinds of CRTs. 
SUMMARY OF THE INVENTION 
It is an object of the invention to provide a 2-CRT type projection 
apparatus which can project a picture of improved resolution and 
brightness. 
It is another object of the invention to provide a 2-CRT type projection 
apparatus which has an enlarged color reproduction range. 
It is a further object of the invention to provide a 2-CRT type projection 
apparatus which can suppress "Moire fringes" generated in a projected 
picture. 
According to the 2-CRT-1-lens type or 2-CRT-2-lens type projection 
apparatus of the invention, one of the CRTs is a green and blue two-color 
CRT and the other CRT is a green and red two-color CRT. In one of the 
CRTs, the green phosphor emits green light of higher brightness, and, in 
the other CRT, the green phosphor emits green light extending a color 
reproduction range. The directions, along which the linear structures in 
the phosphor arrangements each constituting the phosphor screens of the 
two CRTs notably emerge, cross with each other at right angles. 
The above and further objects and features of the invention will more fully 
be apparent from the following detailed description with accompanying 
drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Hereinafter, the invention will be described in detail with reference to 
the drawings illustrating the embodiments. 
FIG. 7 is a diagram showing the configuration of a 2-CRT-1-lens type 
projection apparatus according to the invention. FIGS. 7, 11 and 12 
illustrate a green/blue (G/B) CRT and green/red (G/R) CRT which are 
separated by 180.degree. so that their phosphor screens face each other. 
The phosphor screen of the green/blue CRT 11 includes of striped phosphors 
of two colors, green and blue, and that of the green/red CRT 12 includes 
of striped phosphors of two colors, green and red. Between the green/blue 
CRT 11 and green/red CRT 12 which face to each other, disposed are a 
dichroic mirror 13 which reflects only green light and blue light and a 
dichroic mirror 14 which reflects only green light and red light, in such 
a manner that the dichroic mirrors 13 and 14 cross with each other at 
right angles. A projection lens 6 for magnifying a picture and a screen 7 
for displaying the magnified picture are arranged in this sequence along 
the reflection direction of the dichroic mirrors 13 and 14. In the 
projection apparatus, a green and blue picture projected from the green 
and blue CRT 11 and a green and red picture projected from the green and 
red CRT 12 are overlapped with each other by the dichroic mirrors 13 and 
14, and these images are magnified by the projection lens 6 so as to form 
a color image on the screen 7. 
FIGS. 8(a) and 8(b) illustrate the phosphor screens of the green/blue CRT 
11 and green/red CRT 12 shown in FIG. 7. In FIGS. 8(a) and 8(b), the 
x-axis and y-axis indicate the long and short axes of the phosphor 
screens, respectively, and the arrow shows the stripe direction. FIGS. 
8(c) and 8(d) are enlarged views each illustrating the phosphor screens of 
the green/blue CRT 11 and green/red CRT 12 in one embodiment. In the 
embodiment shown in FIGS. 8(c) and 8(d) (hereinafter, referred to as 
"first example"), the green/blue CRT 11 and green/red CRT 12 employ green 
phosphors of the same kind (green stripe GS). 
FIGS. 9(a) and 9(b) are enlarged views each illustrating the phosphor 
screens of the green/blue CRT 11 and green/red CRT 12 in another 
embodiment. In this embodiment (hereinafter, referred to as "second 
example"), the green/blue CRT 11 and green/red CRT 12 employ green 
phosphors of different kinds (green stripes GS1 and GS2). The green 
phosphors (green stripe GS1) of the green/blue CRT 11 include of [Y.sub.3 
Al.sub.5 O.sub.12 :Tb] to give priority to brightness, and the green 
phosphors (green stripe GS2) of the green/red CRT 12 include of 
[ZnSiO.sub.4 :Mn] to give priority to the enlarged color reproduction 
range. 
Widths of stripes which are set in such configurations as first and second 
examples so as to obtain white light of 9300.degree. K+27 MPCD while the 
electron beam amounts respectively required for green, blue and red 
phosphors are fixed, are listed in Table 1 below. In Table 1, widths of 
stripes in the prior art example and ratios of brightness of green light 
to that in the prior art example are also shown. 
TABLE 1 
__________________________________________________________________________ 
Prior Art Example First Example 
Second Example 
__________________________________________________________________________ 
CRT2 GS = 0.233 
CRT11 
GS = 0.269 
GS1 = 0.308 
GS = 0.233 BS = 0.378 
BS = 0.392 
BB = 0.100 BB = 0.100 
BB = 0.100 
CRT8 BS = 0.327 
CRT12 
GS = 0.269 
GS2 = 0.252 
RS = 0.448 RS = 0.431 
RS = 0.448 
BB = 0.100 BB = 0.100 
BB = 0.100 
Ratio of Brightness of Green Light to That of Prior Art 
##STR1## 
##STR2## 
__________________________________________________________________________ 
GS: Green Stripe 
GS1: Green Stripe (Priority to Brightness) 
GS2: Green Stripe (Priority to Color Reproduction) 
BS: Blue Stripe 
RS: Red Stripe 
BB: Black Stripe 
(Unit: mm) 
In the first example, the brightness of green light is improved by 1.15 
times as compared with the prior art example, and, in the second example, 
the brightness of green light is improved by 1.20 times as compared with 
the prior art example. In this way, according to the invention, the two 
CRTs are constructed using phosphor screens which respectively emit 
two-color light of green and blue; and green and red, and therefore it is 
possible to improve the resolution and also to obtain a projected picture 
brighter than that obtained in the prior art example, with the same 
electron beam amount. 
According to the second example, as shown in the CIE chromaticity diagram 
of FIG. 10, the color reproduction range is extended to the range defined 
by G1, G2, B and R. In this way, since the green phosphors of one of the 
two CRTs are formed by those emitting green light of higher brightness and 
the green phosphors of the other CRT are formed by those emitting green 
light which can extend the color reproduction range, it is possible to 
obtain a further brighter projected picture and also to extend the color 
reproduction range. 
FIGS. 11(a) and 11(b) show phosphor screens of the green/blue CRT 11 and 
green/red CRT 12 in another embodiment of the invention, respectively. In 
this embodiment, the stripe directions of the CRT 11 is different from 
that of the CRT 12. More specifically, the stripe direction s of the CRT 
11 corresponds to the y-axis, and the stripe directions of the CRT 12 
along to the x-axis. The aspect ratio of the CRTs 11 and 12 is 4:3. In 
this way, directions along which the linear structures of the phosphor 
arrangements each constituting the phosphor screens of the two CRTs 
notably emerge are perpendicular to each other, thereby suppressing the 
generation of "Moire fringes" which may appear on a projected picture due 
to the structure of a phosphor screen of a CRT. 
FIGS. 12(a) and 12(b) show phosphor screens of the green/blue CRT 11 and 
green/red CRT 12 in further embodiment of the invention, respectively. In 
this embodiment, the stripe direction s of the CRT 11 is perpendicular to 
that of the CRT 12 in the same manner as the embodiment shown in FIG. 11, 
and the phosphor screens of the CRTs 11 and 12 have a square shape or a 
shape similar to a square. According to this configuration, the same 
manufacturing facilities are allowed to be employed in both the 
manufactures of the CRTs 11 and 12, simply by changing the kind of the 
phosphor to be used. 
Although the embodiments in which the CRTs 11 and 12 have a stripe 
structure of a shadow-mask type CRT have been described, dot-matrix type 
CRTs may be used in the invention. When dot-matrix type CRTs are used, 
directions t along which the linear structure of a phosphor arrangement 
notably emerges (refer to FIG. 6) are dealt with in the same manner as the 
stripe directions s. 
Although the embodiments in which both the CRTs 11 and 12 are of the 
shadow-mask type have been described, both the CRTs may be of the 
beam-index type, or alternatively one of the CRTs (the green/blue CRT 11) 
may be a beam-index type CRT which is excel lent in utilization efficiency 
of electron beams and the other CRT (green/red CRT 12) may be a 
shadow-mask type CRT. This combined use of CRTs of two types allows the 
advantages of the two types to be effectively utilized so as to obtain a 
higher resolution and a brighter projected picture. 
Although the embodiments of 2-CRT-1-lens type projection apparatus have 
been described, it is needless to say that the invention can be applied 
also to a 2-CRT-2-lens type projection apparatus in the same manner. 
Furthermore, the invention can be also applied in the completely same 
manner to a 6-CRT-6-lens projection apparatus which projects the same 
picture in an overlapped manner onto a large screen of 120 to 200 inches, 
or to the construction of a 4-CRT-2-lens type or 4-CRT-4-lens type. 
FIGS. 13(a), 13(b), and 13(c) illustrate 4-CRT-2-lens, 4-CRT-4-lens, and 
4-CRT-6-lens projection apparatus, respectively. 
As this invention may be embodied in several forms without departing from 
the spirit of essential characteristics thereof, the present embodiment is 
therefore illustrative and not restrictive, since the scope of the 
invention is defined by the appended claims rather than by the description 
preceding them, and all changes that fall within metes and bounds of the 
claims, or equivalence of such metes and bounds thereof are therefore 
intended to be embraced by the claims.