Even before a picture comes onto the screen, you hear a bite of Chopin's A Flat Polonaise and you know instantly who the subject of the program must be. No one else but Arthur Rubinstein can make that kind of sound come out of a piano - that uncannily rich, vivid, plangent sonority.

The 90-minute special, which airs on Channel 26 tonight (with a stero simulcast on WETA-FM. 90.9) at 9, is titled "Rubstein at 90". Correspondent Robert MacNeil interviews him at his Paris home, and he performs two concerts- the Grieg A Minor and the Saint-Saens G Minor - with the London Sympathy under the baton o Andre Previn.

The conclusion one draws is that Rubinstein at 90 is not much different in any essential way from Rubinstein at 80, or 60, or 40 for that matter. Here he is, close to blindness, unable to read or write, and he's the same dauntless, robust, effervescent character he's always been.

"I discovered suddenly a new beauty of life - a new beauty of life, " he exclaims to MacNeil. "When I could see, I was reading too much some books that I shouldn't have read, not too intelligent, that took away my time." Now, he goes on to explain, with his impaired vision and his curtailed public appearances he can go to concerts, listen to records ("endless records," he says) and familiar and unfamiliar beauties of music.

There are new discoveries in the perfomances, too - even the much-trammelled tunes of the Grieg score seem to reveal new melodic enticements in Rubinstein's hands. Yes, there are signs of age, blurred runs, faltering trills, here and there. But the things that matter, that make Rubinstein Rubinstein - the magical cantabile and the magnanimous spirit and so much more - nare never absent. And the visual aspect of this Unitel production is managed with a great deal of sensitively and intelligence.

As the credits scroll down at the end. Rubinstein's voice is heard once again: "I'm a free person; I feel terribly free. They could put me in chains and I still would be free because my thoughts would be mine - and that's all I want to have." What else is there to say?