JPS59180941A - Projection cathode ray tube - Google Patents

Abstract

PURPOSE:To provide an improved quality of picture projected from the projection cathode ray tube on the screen, by providing a coating spread over the external surface of the front glass of the tube and thereby decreasing the degree of reflection from the glass external surface of the ray of light radially emitted from the internal surface of the glass plate. CONSTITUTION:A coating layer 13 is spread over the external surface of the glass 4, and on the outside thereof, there exists the air layer 7. The interface between the glass 4 and the coating 13 will be represented by E, the interface between the coating layer 13 and the air layer 7 by F, the angle of incidence of the ray of light by thetai, the angle of transmission, or refraction, by thetat, and the thickness of the layer 13 by d. Then, the difference L in the optical path between the component 14 of the ray 9 which transmits through the interface E, reflects from the interface F, and incidents again on the interface E, and the ray 9a which incidents on the interface E and reflects therefrom as the reflected ray 15 is represented by one side of a right-angled triangle having an angle of thetat, or by the formula, L=2nc.d.costheta. Since there arises a phase difference at the interface E between the ray 14 which has passed the layer 13 and the ray 9a which reflects therefrom as the ray 15, the intensity of the reflected ray 15 can be decreased by interference between these rays.

Description

[Detailed Description of the Invention] [Technical Field of the Invention] The present invention relates to a projection tube, and in particular to light emitted from a phosphor layer of a projection tube used in a projection television receiver (this light is not emitted from a point light source). The screen of a projection television receiver reflects light (equivalent to reflected light) at different angles of incidence at the interface between the windshield and the air in front, producing a large amount of reflected light and a bright flare. The present invention relates to a projection tube that prevents deterioration in image quality, especially contrast.

[Background of the technique of invention]

As shown in FIG. 1, the projection tube of a conventional projection television receiver applies an electron beam 1 to a fluorescent screen 2 of a cathode ray tube, and transmits image light 3 emitted from the fluorescent screen 2 through a front glass 4 of the cathode ray tube. The image is projected onto a screen 6 through a lens 5 disposed in a large screen so that a large screen image is projected.

[Problems with background technology]

However, in a projection tube, it is necessary for an electron beam to hit a phosphor screen coated on the inner surface of the front glass of the cathode ray tube to emit light, and for that light to be sent through the front glass to the air space in front of it. It is configured to emit high-intensity light. For this reason, with conventional projection tubes,
The light emitted by the phosphor screen was reflected at the interface between the front glass and the air layer in front of the vehicle, due to the difference in refractive index, resulting in a large amount of reflected light, creating a bright flare. This flare causes a deterioration in the image quality and contrast ratio on the screen of a projection television receiver, causing brightness even in the pixels of a mature level, resulting in an overall dull image being projected on the screen. Ta.

One possible way to eliminate this flare is to apply coating technology to the glass surface of the projection tube.

However, conventional coating technology applied in fields other than projection tubes has a coating film formed on the front surface, which reduces the reflectance even more when light is incident perpendicularly to the laser beam surface. Since research has been carried out in the past, and progress has been made in the direction of selecting wavelength regions or broadening the wavelength range, conventional coating techniques cannot be directly applied to projection tubes.

Therefore, in order to reduce flare in projection tubes with high efficiency, it is necessary to research coating technology from the perspective of lowering the reflectance for all angles of incidence.

[Purpose of the invention]

In view of the above-mentioned points, the present invention provides a projection tube that can reduce the flare of the projection tube with high efficiency, thereby improving the image quality, particularly the contrast ratio, on the screen of a projection television receiver. The purpose is to

[Summary of the invention]

In the projection tube of the present invention, a coating film is applied to the outer surface of the front glass of the projection tube, and the light emitted from the phosphor layer on the inner surface travels from the glass toward the air side at a certain angle of incidence and is reflected at the interface. The reflectance is lowered by using interference effect, and since the reflectance is high where the angle of incidence of light is about 25 to 40 degrees, interference effect is produced with respect to the angle of incidence of light. The coating thickness is determined so that the reflectance is reduced.

[Embodiments of the invention]

Embodiments of the present invention will be described below based on the drawings.

First, Figures 2 to 4 show how flare is formed in the projection tube.
This will be explained using figures.

Figure 2 is an explanatory diagram showing an enlarged view of the area around the front glass of the projection tube, Figure 3 is a diagram showing the intensity distribution of the reflected wave as a function of the angle of incidence determined by calculation, and Figure 4 is a diagram showing the formation of flare in the projection tube. It is an explanatory diagram for showing a range. However, in Figure 2,
The same parts as in the figures will be described with the same reference numerals.

In FIG. 2, the phosphor 2 emitted by the electron beam 1 at a certain instant transmits the light 3 into the front glass 4 and then to the front air space 7, thereby forming one pixel 8 on the screen. Form. In this case, the light emitting portion of the phosphor layer 2 is considered to be a point light source and has light components 9 that diverge in multiple directions. A is the phosphor layer 2 and the front glass 4
Let B be the interface between the front glass 4 and the front air layer 7,
If θ1 is the angle of incidence of the light component 8 on the interface B, then at the interface B, the refractive index of each of the air layer 7 in front of the front glass 4° is ng
, no is n', = 1.52, no=
1.0, a reflected light 10 is generated, and this reflected light 10 reaches the phosphor layer 2 again and is diffusely reflected there to generate reflected light 11 and form flare light 12.

The power of the reflected wave 10 is determined by the ratio of the following equation, assuming that the incident light is 1.

R,==(CO8θ; -nyoN)'n'(cosθ
, 10 NGON)2 ・・・・・・α)R3= (11,
, C0FIθ, −N )2/(n, ocosθ, +N
)2-...(2) However, n,, = n, /n,
, N = ,7'i - (n, oCO3θθ2, R
p and R are reflectances for TM waves and TE waves with polarization modes, respectively. It is assumed that both components are emitted in the same amount in the phosphor. The reflectance R, , R, given by the above equation (1) varies depending on the incident angle θl, and the incident angle θ determines which position of the bow flare is formed.

This situation will be explained step by step: (1) The phosphor 2 emits light in all directions, and then the light is transmitted through the boundary A. ■After passing through glass 4, it enters boundary B.

Here, reflected light 10 is generated at the rate of the above equations (1) and (2). The zero reflected light 10 reaches the boundary A again. Boundary A is a rough interface. The reflected light at the boundary nB has a small incident angle θ1 and is reflected in a direction substantially perpendicular to the boundary A. The light is incident on the boundary A and passes through it to form a part of the flare, but the proportion is small. The part where the intensity of the flare is large is the reflected light 10 whose incident angle θ2 is within a certain range, and this reflected light 10 is transmitted at the boundary A, reflected by the phosphor 2 almost entirely, and scattered in the front direction. and passes through boundary A,
After passing through glass 4, it passes through boundary B, so boundary B
There is flare in some areas, making them appear bright and shiny.

If the reflectance R of the reflected light 10 with respect to the incident angle θ1 is calculated from the above, it will be as shown in FIG. 3, and the intensity distribution of the flare with respect to the incident angle θ1 can be found. According to this figure, since the reflectance R is high when the incident angle θ is 25° to 40°, it is thought that the flare intensity increases within this range (see FIG. 4).

However, 41° is the critical angle for total reflection at interface B. In addition, in the figure, when θi = 0°, R = 4.2%.

When θ1=−40.8°, R=16.4%.

Next, the effect of forming a coating film on the outer surface of the front glass of the projection tube and reducing reflected waves due to the thin film layer will be explained using FIGS. 5 to 7.

Fig. 5 is an explanatory diagram showing the principle that reflected waves are reduced by a coating film, Fig. 6 is a diagram showing changes in reflectance R' (λ) with respect to wavelength λ at normal incidence in a single layer coating, Fig. 7 is the wavelength λ0 by converting Fig. 6.
FIG. 3 is a diagram showing a change in reflectance R"' (θI) with respect to an incident angle θ1 of light. FIG.

In FIG. 5, a coating layer 1 is formed on the outer surface of the glass 4.
3 and there is an air layer 7 on the outside. In this figure, the interface between the glass 4 and the coating layer 13 is E,
The interface F between the coating layer 13 and the air layer 7 is defined as the angle of incidence of light, θt is the angle of transmission, and d is the thickness of the coating layer. In addition, glass 4. Coating layer 13.
The refractive index n, , nC, no of the air layer 7 is n, =
1.52, nc=1.3B, n==1.0. After the incident light 9 passes through the interface E, it is reflected at the interface F and enters the interface E again, including the light component 14 and the incident light 9a.
The optical path difference between the point at which the wave is incident at the interface E and the reflected wave 15 is generated is replaced by one side of a right triangle having a hypotenuse of 2d and a -angle θt, and is given by L = 2nccl Cm0t. Therefore, the light 1 that passed through the coating layer 13
4 and the reflected light 15 of the incident light 9a generate a phase difference at the interface E, so the reflected light 15 can be reduced by the interference effect between the two. In other words, the respective phase differences are πtJ
) When they are multiples of integers, they cancel each other out. Therefore, the conditional expression for the coating film thickness for reducing the reflected light at the interface E is % (3) where λ is the wavelength of light.

Now, in the case of k=1, the relationship between the wavelength λ and the reflectance R'(λ) for normal incidence in a single layer film has a characteristic as shown in FIG. However, the coating film thickness d is the wavelength λo (=
0.55 μm) so that the reflectance R'(λ) is the lowest. The reflectance I('(λ) is the lowest when the refractive index nc of the coating layer is given by the following formula in relation to the refractive indexes nl and no of the glass and air layer.

nc=fu no=sE 工方口,,O=1.23
(4) In reality, there is no substance with a low refractive index as given by equation (4), so a similar substance such as magnesium fluoride (My F2) n=1.3
8 and crystal stone (cryolite, 3NaF *
AlF2) n =1.33 is used as coating material.

Next, in the case of the single-layer film coating shown in FIG. 6, what kind of reflectance it has with respect to the incident angle θ1 will be explained. Now, the reflectance R obtained by the coating effect
is determined by the phase difference ψ between the lights at interface E and interface F. From the optical path difference, L = 2ncdcosθ+ ---
(5) R: R(ψ)...
It can be expressed as (7).

The characteristic curve R'(λ) in Fig. 6 is θ=0°, d−λ0/
This corresponds to R in the case of 4nc. Therefore, (6) Equation 1
From the formula %, ψ=4πnC"y・(-;)=π・a, so R
/(λ)=R(ψ) 10 ・・・・−・・・・(8)=R(π
・T) Characteristic curve R″ for the incident angle θ1 that we are about to find
'(θI) is d=λo/4nc When yλ=λ0, the corresponding reflectance R″(θt) is R″(θr) = R(ψ) = R(π θI) = l(/(5)・・・・・・・・・－・(9)
nil sin wide = nc cosθt from sinθt
=π74π1πY ・・・・・・−・・・(10)R
・”(0+)=1ν(λ・/E possible, sit+&Q”)
...0D, where nic is the refractive index of the coating layer with respect to glass.

Actually, the characteristic curve R"' (
θ・) is shown in FIG. Regarding Fig. 7, when considered together with the intensity distribution with respect to the flare incidence angle θl shown in Fig. 3, a coating film designed with a normal incidence wavelength λO has a reduced reflectance region near the normal incidence angle, so it is not effective in preventing flare. not good.

Therefore, the thickness of the coating layer is determined by selecting an optimal central incident angle θ+best t, and the same reflectance reduction effect is provided for incident angles before and after this to prevent flare with high efficiency. That is, the film thickness d is controlled so that the incident angle θ+best corresponds to λ0 in FIG. In other words, d-λl/4nc, λ=λ0 (6)
, λ from equation (8). λ R'(λ)=R(πd,)2R(ψ)=R(πtclJ
sθ2) as the input of R'(λ). 2/(λ, CO
Substitute 3θ[). θ is two θzbest (however, θ
tbest is the transmission angle corresponding to the central incident angle θ・best t), and if input 2/(λ, COSθt)=λ0, then λ to λ. / field θ+ best = λ.ノ 1 砺 Shoulder n Work a ... It is found by (12). From the formula (12), θ*bes
It can be seen that when applying a coating that provides the lowest reflectance at t, the V thickness may be controlled so that the reflectance is lowest at λ1 at normal incidence. In Figure 7, θ+be
An example of conversion when st=30° is shown. However, θ! =41
° indicates the critical angle of total reflection from the glass to the air layer, 65
．． 2° indicates the critical angle from the glass to the coating layer.

As described above, the coating layer designed with the optimum central incident angle θ+best has a strong reflectance reduction effect, that is, flare reduction effect.

In the above example, the W1 coating is described in which the outer surface of the front glass of the projection tube is coated directly, but a glass plate coated on one side is coated on the front surface of the projection tube using an adhesive with a refractive index close to that of glass. A similar effect can be obtained by attaching it to the outer surface of the glass.

〔Effect of the invention〕

As described above, according to the present invention, a coating film is applied to the outer surface of the front glass of the projection tube, and the thickness of the coating film is selected according to the incident angle of light that increases flare, thereby emitting light diffusely from the inner surface of the glass. By reducing the reflectance of the projected light on the outer surface of the glass, it is possible to reduce the flare of the projection tube with high efficiency, thereby improving the image quality, especially the contrast ratio, on the screen in projection television receivers. can. In particular, it prevents the surrounding pixels from shining brightly due to flare, which stabilizes the black level.
Conventional brightness.

An image that is difficult to focus and has a dull outline can be sharpened into an image that is easy to see.

[Brief explanation of the drawing]

Fig. 1 is an explanatory diagram showing a conventional projection tube, Fig. 2 is an explanatory diagram showing an enlarged view of the vicinity of the front glass of the projection tube, and Fig. 3 is a diagram showing the intensity distribution of reflected waves with respect to the incident angle determined by calculation. , FIG. 4 is an explanatory diagram showing the range in which flare is formed in the projection tube, FIGS. 5 to 7 are diagrams for explaining the present invention in detail, and FIG. An explanatory diagram showing the principle of wave reduction. Figure 6 is a diagram showing the reflectance of a single-layer coating with respect to wavelength at normal incidence. Figure 7 is a diagram showing the reflectance of light with respect to the incident angle by converting Figure 6. FIG. 4...Front glass 7...Air layer 9.9a...
・Incoming light 13...Coating film 14...Light passing through the coating layer 15...Reflected light
θbi...Incidence angle θt...Transmission angle d...Coating film thickness E...Interface between glass and coating layer F...Interface agent between coating layer and air layer 4FWI
i-1-Norichika Noriia 1C1 or 1 person) Figure 1 Figure 3 Figure 4 Now Figure 5

Claims (2)

[Claims] (1) A coating film having a predetermined thickness is formed on the outer surface of the front glass of the projection tube, and the light emitted diffusely from the inner surface of the glass is reflected at the interface near the outer surface of the glass. A projection tube characterized in that the light is reduced by interference with light that has passed through a coating film. (2) The coating film has a broadband characteristic with respect to the wavelength of light, and is selected so that the angle of incidence of light from the glass toward the air in front of the glass is set at about 25° to 40°. The projection tube according to claim 1, characterized in that the projection tube has a film thickness.