William Schreiber is out to change how millions of Americans spend their leisure hours. He wants to improve that leisure-time lure, the TV set. Dr. Schreiber won't be on the screen itself. He will be rummaging around inside it, so to speak, tinkering with cameras, signals, tubes, electron guns, and the other components that bring a picture into your living room.
Dr. Schreiber, a silver-thatched professor of electrical engineering, heads up a newly formed research center at the Massachusetts Institute of Technology. Like other researchers around the world, the MIT group is involved in a high-stakes gambit: trying to create the ''ultimate'' television picture.
The Advanced Television Research Program, as it's called, is being backed by 10 US companies, including, in a rare display of unity, the three major commercial TV networks and PBS.
The project could have an impact on:
* Technology. By pooling resources, the industry will be able to explore some of the broad questions - from new transmission signals to picture displays - that are tough for individual companies to tackle. This effort also marks the kind of cooperative research venture, common in some other advanced technologies such as semiconductors, that many think needs to be done more if the United States is to compete with government-supported programs in foreign countries.
(The US Justice Department has given its blessing to the joint research effort. But MIT is only to hatch ideas - not products - to help guide broadcasters and manufacturers in building equipment.)
* Politics. By identifying what should go into tomorrow's TVs, the researchers could strengthen the US industry's hand in hammering out international technical standards governing new areas of broadcasting.
* Consumers. The prospect of a better TV picture sounds dandy, but will people buy it? Wading beyond the problems at hand, MIT scientists will also try to answer some of the more elusive questions about people who sit in front of the sets: Will they, for instance, be willing to pay the price necessary for changes in picture quality?
That more effort - in this case $2.5 million worth - should be put into studying the future of television technology shouldn't be surprising. The standard TV receiver, virtually unchanged since the advent of color some 30 years ago, is due for some updating. It is expected to undergo revolutionary changes in design over the next two decades. These could expand the role television will play in people's lives.
The broad outlines for tomorrow's sets have already been drawn. One wave of changes will be brought about by advances in satellite and computer-chip technology. Fitting sets with tiny semiconductors, for example, will lead to sharper pictures. Eventually these should allow the viewer to freeze frames and zoom in for close-ups.
But a second and perhaps further-reaching improvement, due perhaps in this decade, is known as ''high-definition television'' (HDTV) - a technology expected to make it possible to deliver wide-screen, movie-theater-like images into the home with stereo sound. It is to this area that MIT researchers devote much of their research.
TV-makers know how to build much of the equipment needed for sets with sharper-definition now. But a few technical questions remain, together with some sticky regulatory and economic issues.
To see how how HDTV will differ from today's televisions, some background information is necessary. Today's TV picture in the US is created by an electronic beam that scans 525 horizontal lines for each picture. When the 525 -line standard was adopted in the 1940s, it was fine for screen sizes of the day (under 16 inches). But with today's bigger sets (usually 23 inches or more) the image is less sharp, and often the individual lines are visible. TVs in Europe - which adopted a 625-line standard - produce a slightly better picture.
But the big improvements will come by pushing the technology further. Japan has done this. The Japanese state broadcasting organization, NHK, developed an experimental 1,125-line system in 1968. The density of scanning lines on a screen that is 25 percent wider than American screens produces images approaching the clarity of 35-mm film.
Not surprisingly, Japan wouldn't mind having its 1,125-line system become the world standard. But US electronics manufacturers have other ideas. One reason for the MIT study is to find what standard might be best.
Other questions loom. For instance, can the densely packed signal that higher-definition sets require be made compatible with current TV sets?
If finding the answers doesn't keep the MIT researchers busy, there are still more areas to explore. These include ways of employing extra signals, short of HDTV, to enhance picture clarity. ''We should be aiming for the quality of at least 35-mm motion pictures,'' Dr. Schreiber says, although he is the first to admit that the ideal picture is impossible to achieve.