But no architect is as famous as all that. The two policemen who did their best to save Kahn after finding him near death in a lavatory on the station's lower level had no idea who he was. Later, when they checked his wallet, they found that he had scratched out his address in his passport and any other details that would have helped them contact his family. His body was taken to the public morgue, where it lay for three days until it was claimed.

Louis Kahn's son, Nathaniel, remembers reading the story in the New York Times as an 11 year old, and looking for his own name. It wasn't there. As far as the newspaper was concerned, his father had only one child, a daughter. The young Nathaniel Kahn knew dimly that his father had three families. There was Esther Israeli, the wife who wouldn't let Kahn go, or perhaps he couldn't bring himself to leave her, and Sue Ann, their daughter, who got the name check in the Times. There was Anne Tyng, an architect who had worked in Kahn's office, with whom he had a second daughter, Alexandra. And there was Nathaniel's mother, Harriet Pattison, a landscape architect who fell in love with his father when she was working in his office.

Almost a quarter of a century later, Nathaniel Kahn started working on his first feature-length film, My Architect. It's the remarkable story of the father he knew only as a series of disconnected memories. In America, it has been the surprise documentary hit of the year - nominated for an Oscar - and it is certainly the most personal.

Kahn hadn't intended the film to be so directly autobiographical at the start. 'It began by not being about my father. It was any son's search for his father; that was the compass needle, the thread. There are lots of ways to tell that kind of story,' he says as we sit in the lobby of the Best Western Hotel in Venice Beach, Los Angeles. 'It got more personal as I got more confident.' The film took five years to finish and involved an endless round of grant applications and arm-twisting to raise the money.

The making of the film, produced with his partner, Susan Rose Behr, was a testing process for Kahn and his family. 'It's a little embarrassing for a man in his mid-thirties to go around asking about his father. They were all polite, but it was only when I got to Bangladesh that people really understood.' In Bangladesh, Kahn is a kind of national hero. His monumental capital building, with its massive brick and concrete walls, is uplifting as well as imposing. It helped the fragile, newly emerging state acquire a sense of itself.

My Architect is a sharply observed, witty film that scrutinises his father's life with a forgiving, sad, sweet eye, assessing his shortcomings and his creative achievements. It is also an emotional rollercoaster as Kahn asks his two half-sisters, his mother, his father's surviving lover, his aunts and an assortment of taxi drivers, rabbis, former employees, clients, critics, famous architects and, most of all himself, a series of searching, impossible questions. Bob Richman's camera records the results with graceful discretion.

Kahn previously worked in the theatre and the film has an improvisational flow that makes it feel like a dramatic performance of unusual emotional honesty. Individual scenes are allowed to grow out of their context and were never reshaped by later discoveries. 'I only met people once; that was a rule the editor and I set ourselves. If I went to a place, something would come out of it.'

Louis Kahn had a complex relationship with Harriet Pattison, Nathaniel's mother. He would arrive, announced only by a last-minute phone call, at her house once a week. He would play with his son on the lawn, stay for lunch and dinner, and drink a chilled martini or two. Then Harriet would drive him into town and drop him at the end of a darkened street, with Nathaniel wrapped under a blanket, watching as his father vanished into the night, back to his wife. Kahn does not spare us his mother's humiliation at his father's hands. The door to her office at the studio would be locked when Kahn's wife came visiting. She had to cajole Kahn's secretary to find out where he was. She was crossed off the guest list for the opening of the Kimbell Art Museum in Fort Worth, one of the greatest triumphs of Kahn's career, where she was responsible for the landscaping.

After leaving Yale, where he studied philosophy, and finding himself continually exploring the monumental spaces of the two art galleries his father designed for the campus, Nathaniel Kahn became an actor. He recognises his father's self-dramatising tendencies in himself. 'He was always playing the part of an architect; his outfit was a bit of a costume,' he says of his father's trademark floppy bowtie and occasional cape. '"You know, even when I get a haircut, I'm an architect," he would say. That says so much about my father's sense of identity. Architecture was more than a profession for him. It was, in the romantic sense, a calling. And, in a practical sense, it helped with the ladies; it's an enormously attractive profession to have.'

Kahn brings his mother to the edge of tears when he asks her why she never remarried. He sits down with his two half-sisters to talk about their father's funeral, at which they met for the first time, and from which Kahn's wife tried to exclude Kahn's two illegitimate children and their mothers. 'I wonder if that really is true,' muses her daughter, hinting at years of anger and betrayal.

There are moments of almost unbearable poignancy. Nathaniel Kahn meets the site architect of a research laboratory his father designed for Jonas Salk, the discoverer of the polio vaccine. 'Did you know my father well?' asks Kahn lightly. 'Oh yes, he used to spend Christmases at home with us, playing with my kids.' The camera stays on Kahn's face. You see him jolted, white-faced, as if reeling from a slap, but not missing a beat.

At one point, Nathaniel reads to camera what he calls the first letter that he has written to his father after all these years. 'Did you ever really mean to come up to Maine to spend the holidays with us, or was it just something that you said to get my mother off the phone. Because I have to tell you, Lou, that we waited for you.'

Kahn was expecting a lot of his family. Neither his sisters nor is mother got to see the film until it went on release. 'Both my sisters have been very supportive. Anne Tyng was enormously trusting. On some level, it's been good for us all. The story has been told; now we can get on with our lives.'

Despite everything, My Architect is neither a bitter, nor an angry film and nor are the people in it. It paints a lyrical and affectionate picture of Louis Kahn, just 5ft 6in tall and terribly disfigured by burn scars from his childhood in Estonia, his fingers black with charcoal from his drawings. 'It's a cautionary tale about me and my sisters, rather than a rage against my father, which, in any case, is an emotion that I don't feel,' says Kahn. 'There are reasons for what happened. There are things that this man could do and things that he couldn't. It's not as if he didn't try. He did care, but there were problems.'

Nor is Nathaniel Kahn angry with his father's architecture. In the film, at first he keeps his distance. He is sceptical about one university building his father designed in Philadelphia, introducing the voices of occupants, mocking the streams of architectural pilgrims that still come flocking. 'As a child, my father's architecture seemed to me to be industrial in a way. It seemed harsh and kind of chilly; I didn't respond to it. I responded to older, colonial architecture. I liked the warmth of a fireplace. I wanted a traditional home; modern was not my world. Modern seemed remote.' He might as well be talking about the stunted family life that was all his father could offer him.

But as the film unrolls, Kahn visibly warms to his father's magic. In one memorable sequence, he goes roller-blading in languid, effortless loops across the sublime courtyard of the Salk Institute, hanging over the lip of the Pacific, water trickling across, like a small boy showing off a new skill to his father.

'What does not work is interviewing people and then shooting buildings,' says Kahn. 'It becomes artificial and narrative, all the things I don't like in a documentary.' The technique he chose instead - putting people into specific places - presents his father's buildings with force and intelligence and results in a film both beautiful and engaging even to those without the least interest in architecture.

Nathaniel Kahn makes a lucid case for his father's work. Like Frank Gehry, who appears in the film, Louis Kahn was an architect who designed nothing that attracted any attention until he was past 50, but who was responsible for half-a-dozen masterpieces. His work was, Kahn points out, an attempt to give architecture back the soul that it seemed to have lost in the 20th century, using massive, simplified, monumental forms that seem to recall the silence and the emptiness of the ruins of ancient Rome that had so impressed Louis Kahn.

'Lou was not very well read, but he knew a little about a lot. He was not an intellectual, but he liked to be thought of as a thinker. He reflected about architecture, he played with words and ideas until they felt right for him.' The film shows archive footage of Louis Kahn with his students, sitting on a table, as they hang on his every word as he delivers his most famous aphorism about the need for an architect to listen to the brick and ask it what it wants to be: 'Hello brick.'

'The brick business was embarrassing for me. The kids in my class in high school, when they really wanted to annoy me, they would go and put an ear to a wall and go, "Shhh, I'm listening to the bricks,"' says Kahn. 'But if you think about it, it's a very poetic way of saying something very practical. Lou's world was not that of the great philosophers, not Hegel or Heidegger. The brick comment was a poetic way of saying know your material, don't push it to do something it doesn't do well.'

The surprise success of My Architect is going to be a hard act for Nathaniel Kahn to follow. 'He left me this fabulous story to tell and without that sense of mystery about him, I'd never have had the chance to do so much.' He isn't ready to talk about his next project. 'I take something from Lou on this. He said, "With each building, you reinvent everything from the ground up."'