WHEN the Governor, Chris Patten, dots the ceremonial lion's eyes and unveils a plaque to open the Tsuen Wan Central Library at 3 pm today, it will mark a major stage in the Regional Council's expansion plans for its library services. The Tsuen Wan library is using the world's first major multi-lingual automated public library system. It is the third central library in the Regional Council's area, and the first new library to provide fully computerised services from day one. Its floor area of 3,800 square metres allows the library to provide the full range of facilities expected of a modern library. They include books, newspapers, periodicals and audio-visual materials in Chinese and English. The brightly coloured junior library has 52,000 books in both languages for children under 12 years old. A centrepiece of the room is a large wooden train which doubles as a rack for books and a seating area for children. Bean bags in corners of the room encourage young people to sit and explore the contents of the superb selection. At the opposite end of the fourth floor is a students' study room with 162 seats and an exit leading to an open-air patio, with further seating space. On the same floor is the audio cassette library with 6,000 tapes for lending. The floor above is the adult lending library with its stock of 130,000 books in the ratio of three Chinese to one English. The third floor houses the reference library, which is divided into an open section where the public can browse for information, and the closed stack section. The books are displayed on shelves which slide together when not in use - making full use of the capacity of the room. These books can be seen on request. Businessmen will find the collection of about 10,000 volumes of technical standards published by the British Standards Institution of particular interest. Because the library is computerised, borrowers must change their old-style cards for a computerised one which looks like an MTR ticket. It reduces the time spent checking in and out books to a matter of seconds. The official opening ceremony is today, but the library first opened for business last Saturday. More than 4,000 books and audio cassettes were taken out, and many more people checked out the facilities and browsed through the books on display. ''We were particularly busy on that day,'' said librarian Tsu Fu-shing. ''Nearby residents turned out in large numbers to visit the library, and readers were genuinely interested to see the computer terminals.'' Mr Tse was pleased with the lending activity on the opening day, which compared favourably with the Tuen Mun Central Library, where, despite a slightly larger stock of books, only 3,000 items were loaned out on the same day. The number was also four times more than the normal number lent on an average Friday from the Fuk Loi Estate.