Shadow Mask for Enhanced Resolution and Brightness in Color Cathode Ray Tubes

The present invention relates to a shadow mask for colour cathode ray tubes, and more particularly relates to an improvement in resolution and brightness of a shadow mask for colour cathode ray tubes in which magnetic field is formed in each mask aperture for passage of electron beams.

For example in U.S. Pat. No. 4,135,111, it is proposed to form magnetic field in each mask aperture for passage of electron beams in order to increase convergence of the electron beams and raise pass rate of the electron beams through a shadow mask for production of images with high brightness. For better formation of such magnetic field, the shadow mask of this type is conventionally made of magnetic material such as Cu-Ni-Co alloys and Cu-Ni-Fi alloys. Despite the increase pass rate of electron beams, 50 to 60% of them still impinge on the shadow mask and inevitably cause rise in temperature of the shadow mask. Since the above-described magnetic materials used for the shadow mask are rather high in thermal expansion, heating by electron beam impingement is apt to cause thermal deformation of the shadow mask. Such deformation causes a shift in position between the mask apertures and an associated luminescent screen of the picture tube, thereby developing doming which mars colour purity of images.

It is an object of the present invention to provide a shadow mask productive of images with high resolution and brightness on a screen of a colour cathode ray tube.

In accordance with a basic aspect of the present invention, a shadow mask is made of Invar of low thermal expansion and each mask aperture is surrounded by a magnet frame.

Suppressed thermal deformation of the shadow mask by use of Invar is well combined with intense convergence of electron beams by use of the magnet frame.

The invention will further be explained in more detail in reference to the embodiments shown in accompanying drawings, in which;

FIG. 1 is an enlarged front view of one embodiment of the shadow mask in accordance with the present invention, and

FIG. 2 is a section taken along a line II--II in FIG. 1.

In FIGS. 1 and 2, a shadow mask 1 is provided with a number of mask apertures 2 arranged in regular matrix orientation. Each matrix aperture 2 is surrounded by a magnet frame 3.

The shadow mask 1 is made of Invar which is nickel-steel. For example, it contains 0.20 wt% or less of C, 0.5 wt% of Mn, 36 wt% of Ni and Fe in balance. Its thermal expansion is in general very low. For example, its rate of thermal expansion is 1 to 2.times.10.sup.-6 /.degree.C. in the temperature range up to 40.degree. C.

The magnet frame 3 is made of Fe-Cr-Co spinodal decomposition type alloy magnet, Cu-Ni-Fe alloy magnet, rare earth magnet or ferrite magnet.

In production of such a magnet frame 3 from rare earth magnet or ferrite magnet, powdery magnet is dispersed in resin solution and the dispersion is applied to the peripheries of the mask apertures 2. It may also be applied by way of vacuum evaporation and spattering after proper local masking of the shadow mask face. When produced from Fe-Cr-Co spinodal decomposition type alloy magnet or Cu-Ni-Fe alloy magnet, a foil of magnet is attached to the total face of the shadow mask and unnecessary sections are removed by etching.

The thickness of the shadow mask should preferably be in a range from 0.15 to 0.2 mm. When square mask apertures 2 are employed as in the illustrated example, the length of one side should preferably be in a range from 0.2 to 0.3 mm. In the matrix orientation, the distance between adjacent mask apertures should preferably be in a range from 0.4 to 0.8 mm. The width of the magnet frame 3 should preferably be in a range from 30 to 50 .mu.m and its thickness in a range from 50 to 100 .mu.m. Same magnetic poles are located at opposite corner of the square.

Use of Invar for the shadow mask effectively suppresses its thermal deformation caused by electron beam impingement. The four pole magnetic field formed by the magnet frame causes increased convergence of electron beams which reduces impingement itself. Both effects concur to produce images with high resolution and brightness.

The following examples are illustrative of the present invention but not to be construed as limiting the same.

Using shadow masks shown in Table 1, colour cathode ray tubes of 20 inch size were prepared and their rate of thermal deformation, rate of passage of electron beams and rate of doming were recorded. Table 2 shows the data obtained. Samples 1 to 3 belongs to prior art and samples 4 to 7 to the present invention.

Rate of deformation was larger for the samples of the present invention. This is believed to be caused by the larger rate of thermal expansion of the magnet frames than Invar.

The data in Table 2 well warrants the fact that application of the present invention assures high brightness with less colour slip.