Method for Making a Stripe Screen on a Face Plate of a Cathode Ray Tube by Rotating Correction Lens

This invention is further explained with reference to the accompanying drawings:

FIG. 1 is a perspective view, partly broken away, showing a color cathode ray tube equipped with in-line arranged three electron guns;

FIG. 2 is a cross-sectional view of an exposure device used in forming a luminescent screen of a color cathode ray tube;

FIGS. 3A to 3H show steps for forming a luminescent screen;

FIGS. 4A-1 to 4A-3 are a plan view, front view and side vide, respectively, for explaining in connection with a pair of side beams an exposure method according to this invention;

FIGS. 4B-1 to 4B-3 are a plan view, front view and side view, respectively, for explaining in connection with a central beam the exposure method according to this invention;

FIGS. 5-1 to 5-3 are a plan view, front view and side view, respectively, for explaining in connection with a central beam the exposure method according to this invention; and

FIGS. 6-1 to 6-3 are a plan view, cross-sectional front view and cross-sectional side view, respectively, of a correction lens used in the exposure method according to this invention.

Let us explain first a method for forming a luminescent screen and then an exposure method according to this invention.

As shown in FIG. 3A a light sensitive layer 20 such as polyvinyl alcohol (PVA) containing, for example, ammonium dichromate is first formed on the inner surface of a face plate 2. Then, as shown in FIG. 3B, the inner surface of the face plate 2 is exposed through a shadow mask 7 by a light source 21. As the light source 21 use is made of a linear light source so as to form light-absorbing stripes 5 without interrupted portions corresponding to bridges 9 provided between mask apertures 8. The exposure is sequentially effected once (three times in all) each time the light source is placed at a position corresponding to a deflection center of each electron beam. In a conventional method, when exposure is effected, one exposure device is used for a central electron beam and the other exposure device is used for a pair of side beams.

After completion of exposure the mask 7 is removed from the face plate 2. The PVA layer is developed and the unexposed portions of the PVA layer are removed to form PVA stripes 22 as shown in FIG. 3C. In this figure the PVA stripes 22 are lengthwise formed in a direction perpendicular to a paper surface. As shown in FIG. 3D, a light-absorbing layer 23 such as graphite pigment is formed on the inner surfaces of the face plate. The PVA stripes 22 are dissolved, for example, by a hydrogen peroxide solution and subsequently the PVA stripes 22 and portions of the light-absorbing material 23 overlying the PVA stripes are flushed with warm water. As a result, the light-absorbing stripes 5 are formed, as shown in FIG. 3E, with the portions of the PVA stripes 22 left as spaces 24 between the light absorbing stripes. Then, a phosphor slurry layer 25 having green phosphor particles dispersed into dichromated polyvinyl alcohol solution is formed as shown in FIG. 3F. A light is projected through the mask 7 to the inner surface of the face plate 2 using a light source 26 located at a position corresponding to the deflection center of the central electron beam which impinges on the green phosphor stripes. After exposure, the mask 7 is removed from the face plate 2 and the face plate is subject to development and the unexposed portions of the face plate is removed to form green phosphor stripes 4G, as shown in FIG. 3H, between the predetermined light-absorbing stripes.

The same processes as in the case of the green phosphor stripes are repeated with respect to blue phosphor stripes and red phosphor stripes.

In the above-mentioned method for forming the stripe screen, the light-absorbing stripes are formed prior to the formation of the phosphor stripes. The reverse processes are also possible. However, it is preferred to form the light-absorbing stripes prior to the formation of the phosphor stripes, since the edges of the light-absorbing stripes shown in FIG. 3E can be trimly formed. In the above-mentioned method two separate exposure devices are used, as in the case of the prior art, in forming light-absorbing stripes and phosphor stripes. Though one exposure device may be used in this case, it is preferred to use two exposure devices taking the productivity of cathode ray tubes into consideration. Where light-absorbing stripes are beforehand formed, the width of the light emitting area -- the width of the portion contacted with the face plate -- of phosphor stripes is defined by the width of the spaces between the light-absorbing stripes and, therefore, the phosphor stripes are allowed to have the edges laid on the light-absorbing stripes. This means that the width of the phosphor stripes is not required to be so strictly formed as the width of the light-absorbing stripes.

Let us now explain an exposure method using in forming light-absorbing stripes.

As shown in FIGS. 4A and 4B, the linear light source 21 is so disposed that the lengthwise direction thereof is coincided with the lengthwise direction of stripes to be formed on the screen. The linear light source 21 is used to prevent appearance of the shadow of bridges of the shadow mask 7 on the screen and deposit phosphor material in the form of stripe on the face plate 2. As a correction filter 29 there may be used one having the same light transmitting characteristic as a conventional correction filter. For example, the correction filter has its light transmission factor radially graded so that the transmission factor is increased toward the peripheral portion thereof.

A correction lens 28 is fundamentally required to have radial correction and degrouping correction capabilities for a pair of side electron beams. Since, however, a correction lens used in forming light-absorbing stripes elongated in a direction perpendicular to a plane on which electron guns are arranged requires no degrouping correction capability in a direction of the length of the stripes, radial correction component 31 and degrouping correction component 33 of the correction lens 28 are respectively formed symmetrically with respect to a axis 32, as shown in FIGS. 6-1 to 6-3. In practice, however, the correction lens may have a slight degrouping correction component formed in a direction Y-Y perpendicular to the direction X--X in which the degrouping correction component 33 is formed.

At the exposing stage for forming light-absorbing stripes corresponding to one side beam, the correction lens 28 is so disposed that, as shown in FIGS. 4A-1 to 4A-3, the degrouping correction component forming direction X-X or the symmetrical axis 32 is aligned with the plane of in-line arranged three beams. The light source 21 is located in a position corresponding to one side beam. Where light-absorbing stripes corresponding to the other side beam are formed, the optical system including the light source 21, correction lens 28 and correction filter 29 is rotated by an angle of 180.degree. about the center axis 16 of the face plate 2, as indicated by an arrow 30 in FIG. 4A-1, with respect to the face plate 2 and mask 7. Consequently, the light source 21 has its position shifted to a position corresponding to the other side beam. In this case, the face plate-mask 7 assembly may be rotated by an angle of 180.degree. with respect to the optical system.

Where light-absorbing stripes corresponding to the central beam are formed, the optical system is rotated by an angle of 90.degree. with respect to the face plate 2 and mask 7 from a position for one side beam as shown in FIGS. 4B-1 to 4B-3 and only the light source 21 is further rotated by an angle of 90.degree.. As a result, the light source 21 is located at a position to correspond to the central beam as indicated in a broken line. However, the position of the light source 21 is not symmetrical with respect to the center of the face plate and the stripes are likely to be gradually varied in width from one end to the other end and it is therefore desirable to slide the light source 21 in a direction of the stripes as indicated by an arrow 35 in FIG. 4B-1. At the exposing stage corresponding to the central beam, it is possible to rotate the face plate 2 and mask 7 by an angle of 90.degree. as shown in FIG. 5-1, from a relative position in which the face plate 2, mask 7 and light source 21 are occupied at the exposing stages corresponding to side beams and to rotate the light source 21 by an angle of 90.degree. about its axis 36. A desired positional relation is also obtained between the optical system and the face plate-mask assembly when the correction lens 28 is rotated by an angle of 90.degree. about the axis 16 and the light source 21 is shifted towards the central axis 16 of the face plate 2.

According to this invention, where light absorbing stripes corresponding to the central beam are formed, the the symmetrical axis of the correction lens is aligned perpendicularly with the plane of in-line arranged three beams. This does not present any problem because the light-absorbing stripes require accuracy in width only and require no accuracy in length at all. It is therefore possible to form, in desired positions, those light-absorbing stripes having a predetermined width which correspond to each electron beam by the same exposure device. After the formation of the light-absorbing stripes, therefore, respective phosphor stripes having a predetermined width can be formed in a well-balanced pattern over the entire surface of the screen. Consequently, it is possible to provide color cathode ray tubes having excellent color reproducibility.

Though the above-mentioned method has been explained in connection with color cathode ray tubes having phosphor stripes and light-absorbing stripes, this invention can also be applied to cathode ray tubes having tricolor phosphor stripes only.