The bigger picture

thousands of tiny transparent balls could soon make it easy to manufacture television screens the size of an average living-room wall. The technique has been developed and perfected by the French Atomic Energy Commission in Grenoble, France.

The conventional cathode-ray tubes are far too bulky to be used in widescreen televisions -- they need depth so that the electron gun at the rear of the tube can scan the entire screen. One of the most promising methods of making screens that measure more than a metre from corner to corner is to replace the electron gun with thousands of tiny cones. Each cone is about a micrometre across the base and sits in small pit with its tip poking towards the screen. In an electric field the cones emit electrons. Printed on a plate above a group of 10 or more cones is a fluorescent dot that lights up when struck by electrons. The problem with such displays is making the thousands of tiny holes that the cone poke through.Typically, the holes in a large screen of this type are made in sections. However, because the mask used to produce the holes degrades each time it is used, the end result can be a display that looks as though it is made up of a series of separate rectangles. Where the holes have become ragged, the picture on the screen may be distorted or have bright or dark spots. If the holes are too big, the voltage required to encourage the electrons to appear has to be increased too. Now, Michael Ida at the commission's electronics and instrumentation laboratory has found a way to make holes of consistent size and shape.

The technique uses thousands of tiny spheres that range in size from less than on micrometre up to a few micrometers in diameter. These spheres are spread as a single layer on the surface of the screen. Researchers then shine ultraviolet light on the layer of balls, which act as lenses and focus radiation onto the light-sensitive layer below, burning a small hole in it. Microballs of varying sizes are used to create holes of different sizes, and the whole screen is treated at the same time rather than in a series of sections. Ida says that the spheres can virtually be made of any material as long as that material transmits ultraviolet light. "It works even with plastic or glass," Ida says, "the only condition is that you get materials with good ultraviolet transmission."Using the microballs to make holes has proved successful in tests. Once the balls have done their job and started holes in the right positions, they are removed and pits for the cones are etched conventionally (New Scientist , Vol 157, No 2119).

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