MXPA00006810A - Exposure device for phosphor screen of cathode-ray tube panel - Google Patents

Abstract

In an exposure device for exposing the photosensitive phosphor coated on a CRT panel 7 with the exposing light similar to the electron beam in the case of operating a CRT so as to form a pattern on a phosphor screen 20 of the CRT panel 7, a light source 1 for emitting the exposing light is provided, and a main lens 11 for forming the exposing light from the light source 1 into the exposing light similar to the electron beam is provided between the light source 1 and the CRT panel 7. The main lens 11 is divided into a fixed main lens 11A having a curved surface and plane lens 11B, and when landing errors are generated, a sub lens for correcting miss landing is replaced in the plane lens 11B.

Description

APPARATUS FOR MODELING A FOSFORESCENT SCREEN, WHICH HAS ONE MAIN LENS, WITH FLAT ELEMENTS SUBSTITUTE FOR SUB-LENSES BACKGROUND OF THE INVENTION The present invention relates to an improved apparatus for exposing a photosensitive coating at the bottom of a cathode ray tube to light to form a phosphorescent screen at the bottom, the improvement being related to a means for causing the light to follow simulated trajectories of electron beams with greater precision. Cathode ray tubes (CRT) are widely used today to display images on television sets, computer monitors and the like. The image displayed is generated when the phosphorescent screen at the bottom of the CRT is scanned by an electron beam emitted from an electron gun. In a color CRT, the electron beams of different electron guns scan the screen simultaneously, striking the phosphor of different colors. To ensure that each beam falls on the phosphor of the appropriate color, the phosphorescent screen is formed by exposing photosensitive phosphorus materials to the light through color separation masks slots through which the electron beams will also pass, using a Main lens with a specially shaped surface that refracts the light in trajectories that simulate the trajectories of the electron beams. If the electron beams do not affect the correct positions, problems such as color blur may occur. The causes of error in the incidence of the beams include environmental factors such as the ambient temperature and factors in the manufacturing process such as a misalignment of the electron guns. The manufacturing process factors are difficult to eliminate completely, but fortunately they can be compensated. A conceivable method of compensation would be to modify the surface figure of the main lens or polish a new lens, provided that the manufacturing process has been found to lead to an error in beam incidence. This solution is impractical. A more practical method is to insert a corrective lens, called a sub-lens, into the light path, either in the front or behind the main lens. A set of sub-lenses with different surface configurations can be prepared in advance and can be inserted alone or in combination to compensate for various types of beam incident error. A known set of sub-lenses is described in Japanese Unexamined Patent Application No. 10-83161.

However, the present inventors have found that the sub-lenses of this known set do not compensate for certain patterns observed in the incidence of beams. In addition, the insertion of additional sub-lenses in the path of light interferes with the simulation of the trajectories of the electron beams. The details will be provided later. The general result is that the prior art beam error offset compensation in the CRT manufacturing process lacks the desired degree of precision.

BRIEF DESCRIPTION OF THE INVENTION An objective of the present invention is to compensate with precision factors in the manufacture of a CRT that could lead to an error in the incidence of beams in the phosphorescent screen of the CRT. The invention provides an apparatus for exposing the bottom of a cathode ray tube to light, whereby a phosphorescent screen is modeled on the bottom. The apparatus includes a light source, and a group of lenses placed between the light source and the background. The group of lenses comprises a fixed lens having a curved surface that refracts light in simulated electron beam paths, and at least one replaceable optical element. The replaceable optical element is usually a flat transparent plate, but the flat transparent plate can be replaced by a sub-lens having a curved surface, to compensate for inaccuracies of the simulated electron beam trajectories. The flat transparent plate and the sub-lens have substantially the same thickness. Due to its substantially equal thickness, the effect of replacing the flat transparent plate with the sub-lens is essentially limited to the effect of the curved surface of the sub-lenses; there is no unwanted displacement of the light paths by the flat body of the sub-lens. Accordingly, the sub-lens can compensate for an error in the beam incidence pattern without adversely affecting the total simulation of electron beam trajectories by the main lens. The invention also provides two specific types of sub-lenses. One has a cosine surface profile with bilateral symmetry. The other has a surface described by a mathematical function that includes a product of a linear factor in one coordinate and a sine factor in another coordinate. These sub-lenses compensate beam incidence error patterns and resolve any inadequate condition by voiding it, of the prior art.

BRIEF DESCRIPTION OF THE DRAWINGS In the attached drawings: Figure 1A is a schematic sectional drawing illustrating the prior art; Figure IB is a schematic sectional drawing illustrating one embodiment of the invention; Figure 2A is an enlarged sectional drawing illustrating the assembly of the main lens in Figure IA; Figure 2B is an enlarged sectional drawing illustrating the assembly of the main lens in Figure IB; Figure 3A is a schematic sectional drawing illustrating the insertion of a sub-lens in Figure IA; Figure 3B is a schematic sectional drawing illustrating the replacement of part of the main lens in Figure IB with a sub-lens; Figure 4A illustrates the bottom of a CRT with a black-stripe phosphorescent screen; Figure 4B is an enlarged view of part of the phosphorescent screen in Figure 4A; Figure 5A illustrates the simulation of an electron beam path; Figure 5B illustrates a beam incidence error; Figures 6, 7, 8 and 9 illustrate beam incidence error patterns in a CRT background; Figures 10 and 11 illustrate two sub-lenses used in the prior art; Figures 12 and 13 illustrate compensation patterns obtainable with the sub-lentps in Figure 10; Figure 14 illustrates a compensation pattern obtainable with the sub-lens in Figure 11; Fig. 15 is a graph illustrating compensation error in Fig. 1; Figure 16 illustrates another beam incidence error pattern; Figure 17 illustrates a sub-lens that compensates for a beam incidence pattern in Figure 16; Figure 18 illustrates the compensation pattern of the sub in Figure 17; Figure 19 illustrates a sub-lens that compensates for the beam incidence pattern in Figure 9; and Figure 20 illustrates the compensation pattern of the sub in Figure 19.

DETAILED DESCRIPTION OF THE INVENTION Now it will be described in the invention and the relevant prior art in relation to a CRT of the black stripe type, with reference to the accompanying drawings. Similar parts in different drawings are indicated by similar reference numbers. First, the exposure apparatus constituting the present invention and the prior art will be described. With reference to Figure IA, the prior art apparatus comprises a light source 1, a shutter 2, a filter 3 optical, a lens 4 of definition angle and a main lens 5 and a cover 6 of rotating glass. The light source 1 can be moved in the indicated x direction to simulate the positions of different electron guns. When the shutter 2 is opened, the light of the light source 1 is irradiated on the bottom 7 of a CRT, through the slots in a color separation mask 8. The optical filter 3 controls the flat condition of the distribution intensity and the wavelength distribution of the light on the bottom surface. The lens 4 of the angle of definition adjusts the light paths from the light source 1 so that corrected trajectories are obtained from the light source 1 in different positions. The defining angle lens 4 has a cross-sectional shape that is tapered in the direction of the x-axis. The direction of the x-axis is the horizontal direction in both the drawing and the bottom 7, when the CRT is observed in its usual orientation, as shown below. The direction of the y-axis is the vertical direction on the bottom 7 in its usual orientation. The direction of the z axis is the direction of the optical axis of the apparatus, generally perpendicular to the bottom 7 of the CRT. The main lens 5 has a spherical curved surface 5a oriented towards the bottom 7, and a flat surface 5b oriented towards the light source 1. The curved surface 5a is described by a mathematical function z (x, y) which is designed especially for simulation of electron beam trajectories, and does not have a simple analytical expression. The rotating glass cover 6 is a flat glass plate which, upon rotating, averages the effects of the powder and the like during the exposure process. With reference to Figure 2A, in the prior art, the main lens 5 is held fixed in a lens holder 9.

The letter "d" indicates the thickness of the flat part of the main lens 5, below the curved surface 5a. The full thickness of the main lens 5 is z (x, y) + d. With reference to Figure 3A, in the prior art, a sub-lens 10 can be inserted between the definition angle lens 4 and the main lens 5 to compensate for the beam incidence error. The details will be provided later. With reference to Figure IB, the invented apparatus comprises a light source 1, a shutter 2, an optical filter 3, a defining angle lens 4 and a rotating glass cover 6 as in the prior art, but in Instead of a single fixed main lens 5, the invented apparatus has a main lens 11 comprising a comparatively thin fixed HA lens and replaceable flat transparent plates 11B, where n is a positive integer. For purposes of brevity, flat transparent plates 11B will be referred to below as flat lenses. The fixed HA lens has a curved surface 11Aa and a flat surface IlAb. With reference to Figure 2B, the fixed HA lens is held in the fixed lens holder 12. The flat lens is held in a replaceable lens holder 13, which is normally mounted in contact with the fixed lens holder 12, but can be detached from the fixed lens holder 12 by lens replacement. In Figure 2B, there are also two flat lenses 11B-1, 11B-2 (n = 2). There is a small air gap between the flat lenses 11B-1, 11B-2 and between the fixed HA lens and the first planar lens 11B-1. Each plane lens has two parallel flat surfaces, HBa, HBb. The flat surface HBa has a zero curvature, and is described by the constant function z (x, y) = 0. Each flat lens has the same constant thickness "c" at all points. The letter "e" indicates the thickness of the flat part of the fixed lens 11A, below the curved surface 11Aa. The thickness of the fixed lens HA is z (x, y) + e, where z (x, y) is the same function as that used in the main lens 5 of the prior art.

With reference to Figure 3B, when it is necessary to compensate for a beam incident error in the manufacturing process, the invented apparatus replaces one or more of the flat lenses 11B with a sub-lens 14. In general, k of lenses 11B planes can be replaced by sub-lenses 14, so that the k sub-lenses 14 and the (n-k) lenses 11B are mounted on the replaceable lens holder 13 in Figure 2B, where k is a whole number non-negative arbitrary that does not exceed n. For example, all n of the flat lenses 11B can be replaced by sub-lenses 14, so that the main lens 11 comprises only the fixed HA lens, without a flat lens 11B. Regardless of how many flat lenses 11B are replaced by the sub-lenses 14, the total thickness of the lens group including the fixed lens 11A, the flat lenses 11B (if any) and the sub-lenses 14 (if any) and the 4 angle of definition angle, remain essentially the same. Subsequently, the structure and formation of a phosphorescent screen 20 on the inner surface of the CRT bottom 7 will be described. With reference to figure 4A, after the formation of the phosphorescent screen, the bottom 7 has a vertically scratched appearance. With reference to Figure 4B, the stripes comprise a black matrix 21 and a repeated series of stripes 22 of red phosphorus, stripes 23 of green phosphorus and stripes 24 of blue phosphorus. The black matrix 21 separates the matches of different colors from each other. § The color separation mask 8 is mounted at a fixed position at a certain distance behind the bottom 7. The color separation mask 8 is an electrode plate with equally spaced vertical grooves. The CRT has three electron guns (not visible), which are mounted in line, aligned side by side in the x direction. During the operation of the CRT, the electron beam of a barrel passes through the slots of the color separation mask 8 and strikes the red phosphor stripes 22. The electron beams of the other two electron guns, they pass through the same slots in the color separation mask 8, and strike the green phosphor stripes 23 and the blue phosphor stripes 24, respectively. The process of forming the phosphorescent screen 20 comprises the steps of mounting the color separation mask 8, forming a black matrix 21, forming the red phosphor stripes 22, forming the green phosphor stripes 23 and forming the blue phosphor stripes 24 . To form the black matrix 21, after the color separation mask 28 has been mounted, the inner surface of the background 7 is coated with a photoresist. The fountain 1 of the light of the exposure apparatus is moved to the position of the first electron gun and the shutter 2 is opened. The lens k * -12 -main of the apparatus refracts the light from the light source 1 in a manner substantially equal to the electron gun of the first electron beam could refract the magnetic or electrostatic fields during the operation of the CRT, so that, after passing through the color separation mask 8, the light illuminates part of the bottom 7 on which will strike the electron beam, this is the part where the red phosphorus stripes 22 will be formed. The shutter 2 is then closed and the light source 1 is moved to the position of the second electron gun. When the shutter 2 is opened, the light of the light source 1 illuminates part of the bottom 7 on which the green phosphor stripes 23 are to be formed. The shutter 2 is then closed, and the light source 1 is moved to the position of the third electron gun, and the shutter 2 is opened again to illuminate the bottom 7 at the positions where the blue phosphor stripes 24 will be formed. When photoprotection develops, the parts that have been illuminated remain, while the unlighted parts are removed. The inner surface of the bottom 7 is now coated with a non-phosphorescent black matrix material, and the remaining photoprotection streaks are removed, taking the unnecessary matrix material and leaving the black matrix 21. To form the strips 22 of red phosphorus, the inner surface of the bottom 7, including the black matrix 21, is coated with a photosensitive red phosphor material. The light source 1 moves to the position of the first electron gun, the shutter 2 is opened and the bottom 7 is illuminated through the slots in the color separation mask 8. Refracted by the main lenses, the light again illuminates the electron beam incident sites from the first electron gun. The photosensitive red phosphor material is then revealed with a chemical agent that removes the phosphor parts that have not been illuminated, but does not remove the illuminated parts. The red phosphor stripes 22 in this manner are left in their proper positions. The green phosphor stripes 23 are formed in a similar manner, using a photosensitive green phosphor material, the light source 1 is moved to the position of the second electron gun. The blue phosphor stripes 24 are also formed in this manner, using a photosensitive blue phosphor material, moving the light source 1 to the position of the third electron gun. When the completed CRT is used for image display, to the extent that the electron beams are scanned through the bottom 7, the beam of each electron gun, following substantially the same path to that followed by the light from the source 1 light during the manufacturing process, striking on phosphor stripes of the appropriate color. Figure 5A compares the trajectory of the electron beam 25 and the path of the light 26 of the light source 1. The light source 1, the defining angle lens 4 and the main lens 5 (or main lens 11) are of course not present in the completed CRT. For various reasons, as indicated above, the light path 26 may not simulate the path 25 of the electron beam with perfect precision, in which case errors in beam incidence occur, as shown in FIG. 5B. Only the incident error of the beam in the x direction needs to be considered; the beam incidence error in the direction and can be ignored because the colors of the phosphor stripes, 22, 23, 24 do not change in this direction. The incidence error d (x, y) is a function of the position in the bottom 7. Experience has shown that most of the incidence error patterns due to factors in the manufacturing process can be expressed approximately by a equation of the next type, in which J,., h2, h3 and h4 are constants and ux is the unit vector in the positive x direction. d (x, yju ^ = hxxyux + h ^ u * + h3x2ux + h ^ yu Figures 6 to 9 illustrate the type of beam incident error caused by each of the four terms in the above equation. The coordinates are expressed in a Cartesian coordinate system with its origin in the center of the background, the positive x direction extends to the right and the positive direction extends upwards. The bottom edges are at x = ± 240 mm and y = +. 140 mm The beam incidence error vectors are indicated by arrows with exaggerated lengths by a factor of 1000, for visibility. With reference to figure 6, the first term hjXVUx (h? <0) produces a symmetric inclined incidence error pattern in which the error vectors point inward in the upper half of the screen and outward in the lower half of the screen. With reference to Figure 7, the second term h2ux (h2> 0) produces a uniform horizontal displacement. All beam incident points are displaced by the same amount in the same direction. With reference to figure 8, the third term h3x2ux (h3 < 0) produces a non-uniform displacement pattern. All the incident points are displaced in the same direction, but the magnitude of the incident error increases from the center to the left and right edges of the screen. The magnitude of the error changes only with the horizontal position and not with the vertical position. With reference to figure 9, the fourth term h4yux (h4 < 0) produces a cut pattern. The incidence error is positive in the lower half of the screen, zero in the center of the screen and negative in the upper half of the screen, increasing in magnitude towards the upper and lower edges. The magnitude of the error changes only with the vertical position and not with the horizontal position. Although the beam incidence error can be caused by small variations in the manufacturing equipment and other factors that are difficult to predict in advance, once it is observed that a beam incidence error occurs due to the factors in the process of manufacture, can generally be analyzed as a combination of a limited number of patterns such as the above, and can be corrected by using a combination of sub-lenses, each designed to correct one of these patterns. A set of sub-lenses can be prepared in advance, for use as the need indicates. Figures 10 and 11 illustrate the sub-lenses described in the prior art mentioned above. Both sub-lenses have the same average thickness "c". The sub-lenses 10A in Figure 10 have a spherical curved surface lOAa and a flat surface lOAb. The curved surface lOAa is described by the following equation, in which A is a constant. z (x, y) = a3x2y The sub-lens 10B in FIG. 11 has a spherical curved surface LOBa and a planar surface 10Bb. The surface lOAa is described by the following equation in which a4 is a constant. z (x, y) = a4xy The thickness of sub-lens 10A at an arbitrary point (x, y) is a3x2y + c and the thickness of sub-lens 10B is a4xy + c. The sub-lens 10A can be placed in different orientations to compensate for different beam incidence error patterns. Figure 12 shows an example of the calculated effect of the sub-lens 10A when its axes x and y are oriented in the same directions as the x and y axes of the bottom 7 of the CRT. The axes x and y in figure 12 have the same meaning as in figures 6 to 9, the bottom edges 7 of the CRT are at x = + 240 mm and y =; +140 mm. The lengths of the compensation vectors are exaggerated by a factor of 1000. In this orientation, the sub-lens 10A compensates for the first symmetric inclined beam incidence error pattern of the first term, which is shown in Figure 6. The figure 13 shows an example of the calculated effect of the sub-lens 10A as it rotates through an angle of thirty-five degrees (35") about the z axis.The x and y axes in Figure 13 are still parallel to the edges of the bottom 7 of CRT, but they are no longer parallel to the x and y axes of the 10A sub-lens. In this orientation, sub-lens 10A approximately compensates for the beam incidence error pattern of the third term shown in Figure 8. Figure 14 shows an example of the calculated effect of sub-lens l.OB when its axes x and y they are parallel to the corresponding edges of the bottom 7 of a CRT. In this orientation, the sub-lens 10B approximately compensates for the beam incidence pattern of the fourth term shown in Figure 9. In the prior art, the compensation for a beam incidence error pattern combining the effects of the first , third and fourth terms of the equation provided above is obtained by inserting a corresponding combination of sub-lenses of the types shown in Figures 10 and 11, rotated through suitable angles about the z-axis. The compensation for the effect of the second term of the equation, which shifts all beam incident points equally in the same direction, is obtained by shifting the position of the light source 1. However, a comparison of Figures 9 and 14 shows a greater inaccuracy in the compensation of the prior art for the incidence error pattern of the fourth term or cut pattern. In Figure 9, the incidence error at the upper and lower edges of the bottom 7 (y = 140 mm) have a uniform magnitude, but the magnitude of compensation in Figure 14 increases from the mean edges (x = 0) from the edges towards the points (x = +200 mm) near the corners. The compensation values calculated on the upper edge of the bottom 7 (y = +140 mm) are shown in Figure 15. The horizontal axis in Figure 15 indicates the position of the top edge in millimeters. The vertical axis indicates the compensation calculated in micrometers (μm). There is a difference of approximately four micrometers (4 μm) between the compensation value in the center of the upper edge (x = 0) and the value in the points of 180 millimeters (where x = + _180 mm). This difference of 4 μm, although it is smaller than the width of the vertical black bars that form the black matrix 21, is not so small that it is negligible. The inventors have found that sometimes a beam incidence error occurs in a pattern which can not be corrected by any combination of the sub-lenses 10A, 10B used in the prior art. With reference to figure 16, this is a pattern in which the direction of the error is inverted as the beam moves out of the center, towards the right and left edges of the CRT bottom. In Figure 16, the error vectors point outwards in the neighborhood of x = + _100 mm, but point inwards in the neighborhood of x = + _150 mm. The pattern is bilaterally symmetric with respect to x = 0, and the magnitude of error varies only with the x coordinate, and not with the y coordinate. The error vectors in Figure 16 are also exaggerated by a factor of one thousand.

The foregoing are not the only inaccuracies found in the prior art. The insertion of the sub-lens 10, which has a thickness of several millimeters each, between the defining angle lens 4 and the main lens 5, deflects the light paths from the light source 1 in a non-designed manner before that the light reaches the flat surface 5b of the main lens 5. This displacement occurs because the light, in general, does not move parallel to the optical axis of the apparatus, an effect due in part to the lens 4 of definition angle, and necessary for the simulation of electron beam trajectories . The displacement is caused by the thickness of the sub-lens 10, and not by the curved surfaces of the sub-lenses; the insertion of a flat lens of the same thickness "c" could lead to the same displacement. As a result of the displacement, the calculated configuration with caution of the curved surface 5A of the main lens 5 is invalidated; the main lens 5 no longer simulates the refraction of the electron beam in the manner calculated with precision. This problem can be solved by placing the sub-lenses between the main lens 5 and the rotating glass cover 6, but then the light paths would be distorted by the main lens 5 before the light reaches the curved surfaces of the sub-glasses. lenses, and the sub-lenses would no longer work as precisely as desired.

In the present invention, this problem is eliminated by replacing the sub-lenses 14 by the main lens 11, as indicated in Figure 3B instead of inserting their lenses in front or behind the main lens. Referring again to FIG. 2B, when flat lenses 11B are mounted on the replaceable lens holder 13, the flat lenses and the fixed HA lenses function as if they were a single lens with a thickness equal to the sum of the thicknesses of the lenses. constituent lenses. The total effective thickness of the flat part of the main lens 11 in this case is the same as in the prior art (d = e + nc). The height of the surface 11Aa of the fixed lens HA is also the same as the height of the 5a curve surface of the main lens 5 in the prior art. The lower surface of the lower planar lens 11B-2 is smaller than the lower surface of the main lens 5 in the prior art, but the position of the definition angle lens 4 is the same as in the prior art. That is, the distance from the lower surface of the planar lens 11B-2 to the defining angle lens 4 in the invented apparatus is reduced, as compared to the distance from the lower surface 5b of the main lens 5 to the definition angle lens 4. in the invented apparatus which can be derived from the group of lenses comprising the main lens and the definition angle lens 4 in the prior art by sliding the lower part of the main lens 5 of the prior art into n transparent transparent plates, and moving these parallel plates slightly downward in various amounts, parallel to the optical axis. The geometric optdescribes the parallel movement of a parallel plate in a group of lenses does not change the optical properties of the lens group as a whole; the light paths remain of the same length, and the light rays that come out of the groups of lenses in the same position and direction in comparison to if the parallel plate had not moved. Therefore, when n flat lenses 11B are mounted on the replaceable lens holder 13, the main lens 11 of the invented apparatus is an exact optical equivalent for the main lens 5 in the prior art. If it is found that the beam incidence error occurs, the invented apparatus, like the prior art, uses one or more sub-lenses to compensate for the error. The replaceable lens holder 13 is separated from the fixed lens holder 12, one or more flat lenses 11B are removed and the sub-lenses 14 are installed in place., then the replaceable lens holder 13 is reattached to the fixed lens assembly 12. The sub-lenses 14 are selected from a set of sub-lenses that indicate both the sub-lenses of a type used in the prior art and other sub-lenses that will be described later. The different sub-lenses have different curved surfaces, but each sub-lens 14 has the same average thickness "c" as the flat lenses 11B. The replacement of lenses 14 by flat lenses 11B alters the light paths in the group of lenses comprising the main lens 11, the sub-lenses 14 and the lens 14 of definition angle, but the alteration is almost entirely due to the Curved surfaces of the sub-lenses 14. There is no additional displacement of light paths due to the average thickness "c" of the sub-lenses, because this thickness is substantially equal to the thickness of the flat lenses 11B replaced by the sub-lenses. 14. It is also important that the sub-lenses 14 are placed closer to the upper face 11A of the main lens 11 than in the prior art, as can be seen by comparing Figures 3A and 3B. For all these reasons, the rays of light refracted by a sub-lens in the invented apparatus deviate less from its horizontal position on the surface of the main lens 11 in comparison to as would the same light rays refracted by the same sub-lens. lens in the prior art. Therefore the accuracy of total compensation is improved. Figure 17 shows a novel sub-lens 14A used in the invented apparatus. The upper surface 14Aa of this sub-lens 14A has a bilateral symmetric configuration described by the following cosine function, in which a.x and h are constants. z (x, y) = a-L (1 - cos (bxx)) The lower surface 14Ab is flat. The thickness of sub-lenses 14A at one point (x, y) is ax (l-cos (bxx)) + c. Figure 18 shows the calculated compensation pattern produced by the sub-lens 14A when its axes x and y are parallel to the x and y axes of the CRT background 7. This compensation pattern compensates for beam incidence error of the type shown in Figure 16, which can not be corrected in the prior art. In both the right half (x> 0) and the left half (x <0) of the background, the compensation direction is reversed at some distance m from the origin (x = + _m), where m depends on the constant bx The constant a.? determines the magnitude of the compensation. Figure 19 shows another novel sub-lens 14B used in the invented apparatus. The upper surface 14Ba of this sub-lens 14B is described by the following function, in which a2 and b2 are constants. This function replaces the term x in the formula of sub-lens 10B of the prior art with sin (b2x). z (x, y) = a2 (sin (b2x)) and This function is the product of a sine function in x and a non-constant linear function (a2y) in y. The lower surface 14Bb is flat. The thickness of the sub-lens 14B at the point (x, y) is a2 (sin (b2x)) + c. Figure 20 shows the calculated compensation pattern produced by the sub-lens 14B when its axes x and y are parallel to the x and y axes of the CRT background 7. The pattern in Figure 20 compensates for the cut beam incidence error pattern shown in Figure 9 with greater precision than in the prior art, due to an appropriate selection of the constant b2, the magnitude of the compensation is substantially uniform across the upper and lower edges of the bottom 7. The magnitude of the compensation is determined by the constant a2. The compensation for the incidence error patterns that combine the patterns shown in Figures 6, 8, 9 and 16 can be obtained by combinations of sub-lenses of the novel types 14A, 14B and 10A of the prior art type. described before. The compensation for the type of incident error shown in Figure 7 can be obtained by shifting the position of the light source 1, as in the prior art. The invented apparatus improves the compensation accuracy in two ways: by replacing the sub-lenses with parts of the main lens, instead of simply inserting sub-lenses where lenses or flat elements were not present before, so that the total thickness of the lens the optical elements in the lens group remain substantially unchanged; and by using sub-lenses with surface figures given by the previous sine and cosine functions, which better compensate for certain beam incidence error patterns. These improvements lead to an improvement in the quality of the phosphorescent screen and therefore in the quality of the image produced by the CRT. The invention is not limited to the use of sub-lenses with specific surface configurations given above. The concept of substitution of sub-lenses for part of the main lens is applicable to any set of sub-lenses. The invented sub-lenses, shown in Figures 17 and 19, can also be used in appliances that simply insert sub-lenses in line with a main lens and a light source, instead of replacing part of the main lens. The sub-lenses can be inserted either in the front or behind the main lens. The invention is not limited to phosphorescent screens of the black stripe type, but is applicable to the manufacture of any type of phosphorescent screen. The group of lenses are not limited to the configuration shown in the drawings. The definition angle lens may be absent, or other optical elements may not be present.

Those skilled in the art will recognize that additional variations are possible within the scope claimed below.

Claims (11)

CLAIMS i 1

1. An apparatus to expose the bottom of a cathode ray tube to light, following simulated electron beam trajectories, so that a phosphorescent screen is modeled at the bottom that has a light source and a group of lenses, the group of Lenses are placed between the light source and the background, the group of lenses comprises: a fixed lens having a curved surface that produces the simulated electron beam trajectories; and at least one replaceable optical element that is selected from a flat transparent plate and a sub-lens having a curved surface, the flat transparent plate and the sub-lens have a substantially identical thickness, the sub-lens compensates for any inaccuracy of the simulated electron beam trajectories produced by the fixed lens.

2. The apparatus as described in claim 1, further comprising: a fixed lens holder, which holds a fixed lens; and a replaceable lens holder that holds the replaceable optical element, which allows the replaceable optical element to move for replacement.

3. The apparatus as described in claim 2, wherein the replaceable lens holder simultaneously holds a plurality of replaceable optical elements, as described in claim 1, which allows compensation for a combination of different sub-lenses for imprecision of the simulated electron beam trajectories, produced by the fixed lens.

. The apparatus as described in claim 1, wherein the curved surface of the sub-lens has a symmetrical cosine profile bilaterally.

5. The apparatus as described in claim 4, wherein the curved surface of the sub-lens is describable in a Cartesian coordinate system with coordinates x, y and z, by a mathematical function of the following form, in which ax and bx are constants : z (x, y) = ax (l - cos (bxx))

6. The apparatus as described in claim 1, wherein the curved surface of the sub-lens is generated as a product of a sine function and a non-constant linear function.

7. The apparatus as described in claim 6, wherein the curved surface of the sub-lens is describable in a Cartesian coordinate system with coordinates x, y and z, by a mathematical function of the following form, in which a2 and b2 are constants : z (x, y) = a2 (sin (b2x)) and

8. An apparatus for exposing the bottom of a cathode ray tube to light, so a phosphorescent screen is modeled on the background, which has a light source to produce the light, and a main lens that refracts the light to follow trajectories simulated electron beam from the light source to the bottom, comprising: at least one sub-lens insertable in line with the main lens and the light source, which has a curved surface that compensates for inaccuracies of the simulated beam paths of electrons produced by the main lens, the curved surface of the sub-lens has a symmetrical cosine profile bilaterally.

9. The apparatus as described in claim 8, wherein the curved surface of the sub-lens is describable in a Cartesian coordinate system with x, y and z coordinates by a mathematical function of the following formula, in which ax and b are constants: (x, y) = a ^ l - cos (bxx)) t - * t fc »-4 * * - • '*** ^ yes - 31 -

10. An apparatus for exposing light to the bottom of a cathode ray tube, so that a screen is modeled? I phosphorescent on the background, which has a light source to produce the light, and a main lens that refracts the light to follow simulated electron beam trajectories from the light source to the background, which comprises: at least one sub- Insertable lens in line with the main lens and the light source, which has a curved surface that compensates for inaccuracies of the simulated electron beam trajectories produced by the main lens, the curved surface of the sub-lens is generated as a product of a sine function and a non-constant linear function.

11. The apparatus as described in claim 10, wherein the curved surface of the sub-lens is describable in a Cartesian coordinate system with coordinates x, y and z, by a mathematical function of the following form, in which a2 and b2 are constants : z (x, y) = a2 (sin (b2x)) y.