GB2161953A

GB2161953A - Manufacturing a filter having a light transmission coefficient which varies over its surface

Info

Publication number: GB2161953A
Application number: GB08514818A
Authority: GB
Inventors: Pierluigi Testa
Original assignee: Videocolor
Current assignee: Videocolor
Priority date: 1984-06-22
Filing date: 1985-06-12
Publication date: 1986-01-22
Also published as: KR860000584A; GB8514818D0; FR2566542B1; FR2566542A1; IT1183899B; IT8567582A0; GB2161953B

Abstract

Process for the production of an attenuating filter 15 having a coefficient of transmission of light which varies over its surface, particularly for the manufacture of the screen 10 of a mask-type color television tube, involves using a writing device such as the stylus of an XY plotter, to plot lines (26) the width of which depends on the average required for the coefficient of transmission on a light medium, such as a sheet of paper. Each strip may be limited by a straight line (30) defined by the equation Y = Y0 and by a continuous curve (31) defined by the equation Y = Y0 + AF(x) and intermediate curves may be plotted having the following equation): Y = Y0 + AF(x> these curves are such that the coefficient A varies from one curve to another between 0 and 1 or between 1 and 0, the coefficient A being chosen according to the width of each inscribed line such that two successive lines overlap. <IMAGE>

Description

SPECIFICATION Process for manufacturing a filter having a light transmission coefficient which can vary over its surface The invention relates to a process for manufacturing a filter which attenuates the light passing through it and having a transmission coefficient which can vary over its surface. It relates more particularly, but not exclusiveLy, to the manufacture of such a filter which can be used for producing the screen of a masktype color television tube. -- It may be recalled here that a color television tube includes a glass plate on the internal surface of which are deposited, according to a specified configuration, cathodoluminescent substances, usually called phosphors, which, when they are excited, have the colors red, green and blue respectively. The excitation is usually obtained by means of three electron guns, one for each color.In order that a phosphor of a specified color is excited only by the electron gun intended for that color it is provided, most often, on the one hand that the beams of the three guns have different directions and, on the other hand, that each triad of phosphors is associated with a hole in a shadow mask arranged in front of the screen, the cross-section of the hole being such that the section of the beam which it allows through can only strike or excite a single phosphor.

It is therefore understood that the position of the mask with respect to the screen deposited on the plate must be determined with accuracy. For this purpose the mask is fixed to the plate of the tube before the phosphors are deposited on this plate and this mask is used in order to make the deposit of luminescent substances. Each of the substances is deposited as follows: first of all the inside face of the plate is covered with a solution of the luminescent substance of the specified color in a photo-sensitive material which hardens when it is illuminated by ultraviolet (UV) radiation; this solution covering the inside face of the plate is then illuminated through the mask using an optical system which includes a source of UV radiation and a lens simulating the deflector of the tube.The position of the optical system, particularly of the ultraviolet lamp, depends on the color of the phosphor in solution. In this way, only the locations provided for the specified color are illuminated and can therefore harden. The other locations are not hardened and do not become fixed on the glass; they can be cleaned off by washing with water or by use of another liquid.

But the surface area hardened depends on the intensity of the UV radiation; now this intensity is considerably greater along the optical axis, and therefore in the center of the screen, than at the edges; the ratio between these intensities is equal to about 2. In order for these surface areas to be approximately constant over the whole extent of the screen it is therefore necessary to attenuate the luminous radiation reaching the center of the screen and to keep the intensity approximately constant at the edges of the latter, the attenuation having to reduce progressively from the center towards the edges. For this purpose a filter performing this function is placed between the UV source and the screen.The best results are produced using a filter formed of a transparent plate covered with opaque strips the width of each of which varies from the center to the edges; the width of each opaque strip is greater at the center and smaller at the edges; thus the mean quantity of light passing through the filter at the center is smaller than that passing through this filter at its edges.

Also, the strips are not all identical to each other; for example the central sections of the outer strips are usually narrower than the corresponding sections of the central strips.

The known technique for producing such a filter consists in using a direct photographic process, starting with a transparent photographic plate entirely covered with an opaque photosensitive substance which can be eliminated after having been illuminated. In order to carry out selective illumination a device is used which includes a luminous source arranged in front of a slit of variable width depending on the width to be given to the opaque strip at the corresponding point and the plate is displaced with respect to the slit and, at the same time, in order to obtain the desired configuration, the corresponding width is given at all times to the slit. This device is very costly and is not easy to produce.

The invention overcomes this disadvantage.

It consists in replacing the luminous source and the variable width slit with a simple writing device, particularly an XY plotter, and in commanding the displacement of the device so that it draws the desired filter. Prefera bly the most common means are used, in particular opaque ink on white paper (or viceversa light ink on a dark paper). In order to manufacture the actual filter, the sheet of paper on which the plot has been produced is photographed, using ordinary equipment.

Thus the device which can be used for pro ducing the process according to the invention is significantly simpler and less costly than that used up to now.

Given that the lines plotted with the pen are not always very clean, it is preferable for the first stage of the process to make a plot on a larger scale than the filter to be produced. The scale is for example three to one. Then, in order to manufacture the actual filter, reduc ing photographic equipment is used.

The plot is made as in the known process by means of continuous parallel strips, parti cularly horizontal, of variable width along their x axis. For this purpose, each strip is limited by a straight line defined by the equation Y = Y0 and by a continuous curve defined by the equation Y = Y0 + F(x) and intermediate curves are plotted of equation: Y=Y0+AF(x) these curves being such that the coefficient A varies from one curve to another between 0 and 1 or between 1 and 0, the coefficient A being chosen according to the width of each inscribed line such that two successive lines overlap.

As a variant, the sheet on which the plot must be made is framed, i.e. divided into elementary zones each of which is associated with a coefficient of transmission and on each elementary zone there is placed an ink surface of area inversely proportional to the coefficient of transmission to be obtained. In each elementary zone the ink surface, which has a specified shape, occupies a random or quasirandom position. The positions of these surfaces thus have low or zero correlation which renders the cooperative or coherent effects of the light diffracted by the mask negligible.

Other characteristics and advantages of the invention will appear with the description of certain of its embodiments, this description being given with reference to the appended drawings in which: Figure 1 is a diagram of a plate of a color television tube, with its mask and the optical filter system used to manufacture the screen; Figure 2 is a partial view of a filter; and Figure 3 shows a process according to the invention for manufacturing the filter of Figure 2.

For the manufacture of the screen 10 of the shadow mask-type color television tube a solution is deposited on the inside surface of the plate 1 2 containing on the one hand a material which is photosensitive to ultraviolet radiation, and on the other hand a phosphor of a specified color: red, green or blue. After drying the deposit found on the inside face is subjected to ultraviolet radiation as shown in Figure 1. For this purpose an optical system is provided with an ultraviolet lamp 13, a twoelement lens 1 4 and an attenuating filter 1 5.

The lamp 1 3 has a position which corresponds with that of the specified color gun, for example the green one, and the lens 1 4 reproduces the deflection system which will subsequently be associated with the tube. In this way a luminous beam 1 6 emitted by the source 1 3 passes through a hole 1 7 in the mask 11 and reaches the inside face of the plate 12 at a location 1 8 which corresponds to that desired for the green phosphor associated with hole 1 7 of the mask 11.

The radiation from the source 1 3 does not have equal intensity in all directions but has a variable intensity, the emission diagram being represented by the lobe 20 in Figure 1. The intensity is a maximum in direction 21 which corresponds approximately with the axis of the tube and it progressively decreases as it moves away from this direction. Thus, near the edge 23 of the plate 12, the intensity 1D is about half the intensity 1o at the center of that plate.But as each of the various phosphors over the entire screen must extend over an approximately constant area and as the area of each element of phosphor obtained de pends on the luminous intensity, there is provided between the lens 14 and the mask 11 an attenuating filter 1 5 which approxi mately equalizes the luminous intensities reaching the screen at the various locations provided for the phosphors. The filter 1 5 is thus an attenuating filter or gray filter having a coefficient of transmission which is not uniform over its surface.

Such a filter 1 5 is composed of a transparent plate 25 on which there are black strips 26, 27, etc of variable widths as described for example in the Canadian Patent No 1,078,239. The coefficient of transmission in a zone 28 (Figure 2) depends on the opaque area in that zone 28; therefore it can be easily understood that this area can be varied by varying the widths of the strips 26, 27,..

According to the invention, in order to produce a filter 1 5 the traditional photographic process which consists in displacing with respect to each other a photographic plate and the assembly formed by a luminous source and a slit of variable width, is not used but, instead, opaque black strips are inscribed on a medium, for example white paper or a transparent sheet, particularly using a writing device such as the stylus of an XY plotter.

The displacement of the stylus of the XY plotter is for example controlled by a computer programmed to draw the filter having the desired distribution of coefficient of transmission, i.e. an appropriate shape for the outline of each variable width strip.

In a preferred embodiment, in order to plot the strip 26 of variable width along the x axis, first of all a line 30 is plotted parallel to the x axis and of ordinate Y0 which forms the lower limit of the strip 26. The upper limit of the strip 26 is the curve 31 defined by the following formula: Y=YO+ F(x) (1) After plotting the line 30 the curves are inscribed which obey the following formula: Y=Y0+AF(x) (2) This plot is performed in such a way that the coefficient A is close to 0 immediately after having plotted the line 30 and is close to 1 when the penultimate line, that preceding line 31 is plotted.

In order to pass from one curve to the next, the parameter A is increased in such a way that two successive intermediate lines overlap; the rate of overlapping is for example 20%. In other words we pass from one line to the next by a displacement in length less than the width of each line, a width which is determined by that of the plotting stylus. The various values of A between 0 and 1 do, of course, depend on the width of each inscribed line, i.e. on the diameter of the plotting stylus.

In this way a completely black strip 26 is obtained, modulated in width according to the desired law.

The plot can of course be performed in the reverse order: firstly the curve 31 is plotted and then intermediate curves while reducing A from 1 to 0.

Given that the borders of each strip, i.e.

lines 30 and 31, can be of insufficient definition, the plot is made on a larger scale than that of the required filter, for example a scale of 3 to 1. In order to obtain the definitive filter a photographic reduction of the plot produced is made on a transparent medium.

The maximum width of each strip 26, 27, ... is for example in the order of 1 millimeter.

The enlargement scale of the plot of strips depends on many factors, among others the accuracy of the filter which we wish to obtain, the diameter of the plotting stylus, the resolution of the photographic lens used for the reduction, the dimensions of the filter itself, etc.

As a variant (not represented), instead of plotting strips of variable widths and of the same average direction, the filter is produced as follows: the surface of the filter is devided into elementary zones preferably of constant area and of value chosen according to the desired accuracy, and a section of the surface of each elementary zone is darkened in proportion to the mean coefficient of transmission required in the zone.

This darkening or obscuring can be obtained in various ways. For example, there can be distributed over it characters, letters or symbols having no significance, the number of these being proportional to the area of obscured surface required. These obscuring elements can also be segments or simple spots. The model or pattern for such a filter is also produced, as in the previous example, using a plotter controlled by a computer. Also as in the previous example, good accuracy is obtained by first producing a plot on a large scale and then reducing.

The number of elements in each elementary surface is in the order of 1/ in order to obtain an accuracy of in the transmission of the filter.

It is also important to note that the positions of the various dark elements in each elementary zone will have to have little or no correlation with each other in order to render the cooperative or coherent effects of the light diffracted by the mask negligible. In this case even the diffraction produced individually by each element will have no effect.

Claims

1. Process for producing an attenuating filter with a coefficient of transmission of light which can vary over its surface, formed of continuous strips (26) but having a width which can vary according to the desired coefficient of transmission, characterized in that, using a writing device, opaque strips are plotted on a light medium, such as a sheet of paper or similar, or vice-versa, each one of which is limited by a straight line (30) defined by the equation Y = V0 and by a continuous curve (31) defined by the equation Y = V0 + F(x), and intermediate curves are plotted having the following equation:: Y = V0 + AF(x) these curves being such that the coefficient A varies from one curve to another between 0 and 1 or between 1 and 0, the coefficient A being chosen according to the width of each inscribed line such that two successive lines overlap.

2. Process according to Claim 1, characterized in that the overlapping of two successive lines is in the order of 20%.

3. Process according to Claim 1 or 2, characterized in that the manufacture of the filter is completed by photographing the medium bearing the said zones.

4. Process according to Claim 1 or 3, characterized in that the medium is a transparent sheet.

5. Process according to any one of Claims 1 to 4, characterized in that the plot has dimensions considerably larger than those of the filter to be produced and in that a reduction, particularly photographic is then made.

6. Process according to any one of the preceding claims, characterized in that the writing device is that of an XV plotter.

7. Process according to Claim 6, characterized in that the plotter is controlled by a computer.

8. Process for producing an attenuating filter with a coefficient of transmission of light which can vary over its surface, wherein a writing device is used for plotting on a light medium, such as a sheet of paper or similar, dark zones, or vice-versa, the area of these zones depending on the average desired for the coefficient of transmission, the surface of the filter being divided into elementary regions in each of which is plotted a distribution of figures of specified shape in order that the average coefficient of transmission in that zone corresponds with the desired coefficient, characterized in that in each elementary region the distribution of the positions of the said figures is random or quasi-random.

9. Application of the process according to any one of the preceding claims to the production of a filter for the manufacture of a mask-type color television tube.

10. Process for producing an attenuating filter substantially as hereinbefore described with reference to and as shown in the accom panying drawings.