microfilter^tm Technology

Exploring New Frontiers in Computer Monitor Design By Toshiba Corporation

Although multimedia has been slower to take off than earlier predicted, it is strongly positioned to become the next major application. However, its success will require major changes in the way computer manufacturers approach the market. While early adopters will tolerate slow systems, software bugs and bad screen technology for the privilege of experiencing a new technology, mainstream users will not. They want reliable hardware and software that produce images at least as good as those they are accustomed to viewing on their televisions. In addition, few will want to pay a high premium for the added performance.

In some cases, multimedia functionality will require a completely new approach to product development, especially computer monitors. Television screens require a high brightness characteristic to produce clear motion images, while personal computer displays require a high-resolution rate to produce clear, fine character images. Until quite recently, the need for both high resolution and high brightness was mutually exclusive. However as multimedia applications began to emerge, new requirements were placed on monitors and manufacturers were faced with new challenges. The immediate solution was that manufacturers developed displays optimized for either text or graphics, but not both. That choice determined whether shadow mask or aperture grill technology was used.

Monitor manufacturers are also faced with power restraints. With the emergence of the "green PC," today's monitors not only need to be high performance but also energy efficient. As the monitor becomes one of the most critical components of the system, manufacturers are relying on new technologies to boost performance, without increasing power requirements or cost.

Shadow Mask vs. Aperture Grills

A shadow mask is an opaque metal sheet with dot holes etched into its surface. This mask is placed between the screen's surface and an electron gun. The electron gun emits three electron beams (one red, one green and one blue) to illuminate the screen surface. The electron beams travel from the gun to the screen through the shadow mask holes and land on the respective red, green and blue phosphor dots. The shadow mask tube is primarily used for text-oriented and price-sensitive markets.

Aperture grill technology, on the other hand, is designed specifically for graphic-intensive applications. In an aperture grill, thin vertical wires rather than an opaque sheet with holes are used. The wires allow more light through than the solid mask. Therefore, the beam can be less intense, resulting in a clearer, truer color image. Sony's Trinitron is perhaps the best known example of this technology. When Sony's patent expired a few years ago, it opened the way for new technical developments.

The Next Frontier

Companies like Mitsubishi and NEC have introduced products that combine aspects of both shadow mask and aperture grill technologies. While combining these technologies appears to have overcome some of the problems, it remains expensive. This cost has added momentum to another alternative technology that combines ultrafine particle technology/filter technology with conventional shadow mask. Called microfilter^tm technology, it offers performance equal to the aperture grill at a lower cost.

Click here to see Figure 1.

Phosphor Screen Structure

Toshiba's new microfilter technology combines shadow mask with filter technology as shown in Figure 2. Figure 1 shows cross-sectional views of a microfilter and conventional phosphor screen, respectively. In the microfilter tube, blue, green and red filters are placed between the corresponding color-emitting phosphor stripes and the face glass. These filters have absorption spectrum curves designed to selectively pass light, as shown by the curves to the left of Figure 2.

Click here to see Figure 2.

Using an ultrafine particle dispersion liquid composed of inorganic pigments, the blue, green and red filter patterning is performed by photolithography. In addition to the chromatic characteristics, the pigments must have two extra properties. First the particle size should be less than 0.1mm to ensure transparency. Larger sized particles cause hazing which limits filtering characteristics. Figure 3 shows an example of the particle size distribution of the pigments in the dispersion; in this case, the blue, green and red pigments have a mean particle diameter of 107nm, 105nm and 75nm, respectively. Secondly, the filter must be able to tolerate high temperatures, since a CRT is baked at 430° C during the manufacturing process.

Click here to see Figure 3.

Only a small number of inorganic pigments meet the requirements mentioned above. Table 1 shows the properties of candidate pigments for the M filter. Ultramarine (blue) and cadmium selenide (red) pigments have good chromatic properties as color filters. However, fine particles are not obtained for use in the M filter. Toshiba currently uses iron oxide for the red filter and cobalt pigments for the blue and green filters.

Click here to see Table 1.

BE Filter

The BE (Black Enhancer) filter is used on TV grade tubes in the Japanese market. It is described here to give you an idea of performance improvements when microfilter technology (inside surface) and BE (outside surface) filter technology are applied to a CRT. For color display tubes, the microfilter technology is used with ARECS and ARECS-575 and Super ARCAS coatings.

The BE filter is formed during the final manufacturing process. Ultrafine organic pigment particles are suspended in a solution for the Sol-Gel method and sprayed over the face glass using the spin coating technique. The CRT is then heated to 170 degrees Celsius for approximately ten minutes. This hardens the coating without degrading the organic pigment, thereby maintaining its superior spectral characteristics.

As the optical density of the BE filter increases, the faceplate color shifts to purple under ambient light and stains the achromatic picture with a purplish hue. This limits the pigment concentration. As a result, it is necessary to use semi-tint glass in order to obtain sufficiently low screen reflectance.

Characteristics Of A microfilter Color CRT

The microfilter CRT uses an inorganic pigment filter (M filters) for each primary color on the inside surface and an organic pigment color filter (BE filter) on the outside surface of the face glass. This combination provides better results in phosphor luminescence color purity than that of either filter alone (Figure 2, right side). The improved color coordinates for each color are outlined in Figure 4. In particular, the results for red are quite impressive, providing an extremely pure and bright color.

Click here to see Figure 4.

Figure 5 shows brightness as a function of the screen's reflectance for various CRTs. The brightness and screen reflectance are compared against a clear glass faceplate with transmittance of 85 percent. Contrast is defined by the ratio of brightness to reflectance. The contrast is better when the curve shifts up and to the right. Curve "a" shows the case for a conventional CRT, where the reflectance varies depending on the light transmittance of the face glass.

Click here to see Figure 5.

Semi-tint glass with transmittance of 53 percent is most commonly used at present to achieve good contrast, and is shown as the mark ">" in the figure. Curve "b" shows better performance with a BE filter. However, a BE filter cannot be used at low screen reflectance region on the clear glass because it brings about a purplish hue as explained above. The mark "X" shows the limiting point. The mark "o" indicates a point where a BE filter is applied to a semi-tint CRT similar to those presently being used. Curve "c" for the M filter gives more brightness than "a" or "b" in the whole reflectance region. However, the brightness increment tends to saturate at reflectance regions below 40 percent. Curve "d" shows the effect brought about by combining the M filter with the BE filter. This provides the highest brightness and contrast at practical reflectance region. The mark "o" indicates the designated point on the microfilter.

Click here to see Table 2.

Summary

Table 2 compares the brightness, screen reflectance, contrast and color purity of CRTs. When compared to Black Enhancer and conventional CRTs, microfilter technology offers an improvement in color purity of seven percent and 12 percent respectively, and an improvement in contrast of 36 percent and 74 percent respectively. In fact, by employing a slurry coating process in combination with adjustments in glass transmittance, phosphor design and outer-coating, it is possible to achieve increased contrast of up to 30 percent, increased brightness of up to 30 percent, increased color fidelity of up to 10 percent, and increased power savings of up to 25 percent.

Toshiba America Electronic Components, Inc., 1 Parkway North, Deerfield, Illinois 60015. Tel: (847) 945-1500; Fax: (847) 945-1044.