Channel 4: After 25 years at the edge, Chairman Luke Johnson looks to the broadcaster's future

Channel 4 is a brilliant accident of history, and Britain's cultural life over the last quarter century has been all the richer for it. At its heart, the organisation exists to provide a forum for individual voices making one-off programmes that would otherwise never reach a national audience. And at its finest, Channel 4 can be sublime: stirring drama like It's a Free World, riotous comedy like The IT Crowd, compelling documentaries like Human Footprint, unbeatable current affairs like Dispatches. The BBC is a vast state-sponsored institution; ITV is a huge commercial business. Channel 4 sits between these goliaths, a quirky amalgam that aims to deliver public goods while retaining the verve of the private sector.

We regularly supply shock and awe to British citizens via the TV – and not always in the planned way. Channel 4 has always had an impressive capacity to infuriate and inspire. It has always been the broadcaster most reliant on independent producers, so they have frequently brought their most extreme and innovative ideas to us. We use over 300 programme makers every year – more than any other broadcaster – from companies both large and small. While this does not make for a peaceful existence, I suspect that Channel 4's integral policy of taking creative risks is the only way that an artistic body can renew itself on a regular basis. Otherwise, the schedule becomes clogged with long-running series and predictable formats, all produced in house, leaving little room for radical breakthroughs. Instead, we try more new programmes than any other channel every year.

As a launch pad for new talent, Channel 4 has a remarkable track record. It has given early breaks to an astounding array of comedians, directors, writers, actors and presenters – even TV executives. Many have gone on to considerable fame, from Stephen Frears to Jonathan Ross. And it is surely no coincidence that the bosses of the two largest TV broadcasters are also ex-Channel 4 chief executives. Part of the reason is that few people spend their careers at the organisation. They tend to give their creative best and then go on to to places like Hollywood, or perhaps to set up their own production company. It gives bright and ambitious individuals the opportunity to experiment and to be noticed. Perhaps that is why so many in the TV industry hold it in such affection.

Like its television sibling, Film4 has been a sparkling alternative, in this case to mainstream movie studios. It has backed British film more consistently than any other production house over recent decades. Not only did it produce My Beautiful Laundrette and Trainspotting, more recently it made The Last King of Scotland, Touching the Void, Brick Lane and will soon release The Lovely Bones. It has championed edgy projects by unknown scriptwriters and directors, using fresh performers – yet its films have still managed to win three Oscars in the last two years. And it continues to nurture ideas that would never otherwise reach the screen.

The creative economy has also benefited enormously from the huge success of Channel 4. PricewaterhouseCoopers estimates its contribution at £2bn a year; its activities support 22,000 jobs. Channel 4's investment in British films, and its policy of regional production, distinguish it from other broadcasters. It has achieved this without any direct cost to the taxpayer, save gifted spectrum. There can be no other part of the state that offers such enjoyment and value to the taxpayer for so little, while also boosting the economy.

In 1966, E B White, in an essay in support of public service broadcasting, wrote: "I think TV should be providing the visual counterpart of the literary essay, should arouse our dreams, satisfy our hunger for beauty, take us on journeys, enable us to participate in events, present great drama and must explore the sea and the sky and the wood and the hills... It should restate and clarify the social dilemma and the political pickle."

When it works, Channel 4 does all this and more. It can do so because it is independent: it sells its own advertising time to fund its programmes. I really believe that, overall, Channel 4 provides more distinctive entertainment and enlightenment, at less cost, to more citizens, than any equivalent body in Britain today. And because Channel 4 generates its own revenues, it is independent from shareholders, independent from in-house production, even independent from government – most of the time.

This allows Channel 4 to remain undeterred by controversy and still able to take risks. It is not afraid to break taboos, expose hypocrisy and lead campaigns like Jamie's School Dinners, or our current Lost for Words season about childhood literacy. From time to time, it is almost deliberately esoteric or provocative.

In a democracy, the media is a vital forum for legitimate debate. Only in totalitarian regimes are contrary opinions forbidden. We frequently challenge the consensus: not only did we broadcast The Great Global Warming Swindle, we have also bought An Inconvenient Truth and commissioned dozens of other "green" documentaries. Other broadcasters prefer to play it safe, but that is not Channel 4's mission. We plan to deliver original content with a social purpose to underserved audiences. And we succeed more often than not.

Critics suggest that we have compromised our remit in a chase for ratings. But there is no point in a worthy schedule that no one watches. Television is a mass medium and costs a lot to make. If we scrapped the popular shows in favour of obscurity, then our impact would diminish among audiences, advertisers and programme makers. Advertiser-funded TV broadcasting needs critical mass to really function well. We are able to spend £25m a year on the UK's only one-hour peak-time news show because we generate a surplus on other transmissions. We show the UK's only regular peak-time international current affairs programme, Unreported World, for 20 weeks a year, subsidised by more accessible lifestyle and reality shows.

For eight years, I wrote a newspaper column called "The Maverick". That helps qualify me to chair Channel 4. It should always lead, rather than follow, and be ready to arouse passions, and risk making enemies for a worthwhile cause. We should never get too comfortable as an organisation. We need to be disruptive – as long as there is a sense of purpose behind the excitement.

We face challenges ahead, as do all the so-called "legacy" media companies: newspaper and magazine publishers, radio and TV broadcasters. The advent of digital television has led to an explosion of choice for viewers, while the arrival of new competitors means that the price of content, such as imported shows, has risen. Traditionally, one of the secret ingredients in our magic formula was US series like Friends; those types of series have now become dramatically more expensive. Meanwhile, the online world is attracting an increasing share both of the advertising cake and of younger audience eyeballs – traditionally Channel 4's heartland.

But very few – if any – of these impressive new channels or websites are delivering what might be called public service broadcasting. Their objectives are purely commercial. By contrast, while Channel 4 is entrepreneurial and flexible, its overriding purpose is to show diverse, experimental and educative television – original British commissions from all over the country. In the four years since I joined the organisation, I have been constantly reminded of the power of TV: for example, the way Richard & Judy's Book Club has boosted reading and book sales. Great programmes can be uplifting and encouraging, and Channel 4 will be making more effort than ever to pursue those aims. Our vision must be to open minds, engage viewers and be a catalyst for positive change.

Moreover, Channel 4 is embracing the digital age, and expanding beyond a single channel. It already has successful offshoots like E4 and More4, together with Film4. It has a significant online presence, and is about to launch a raft of DAB radio stations. With 4OD, it has been at the forefront of developments in video on demand. Further digital media initiatives are planned. Long term, Channel 4 cannot rely on its core channel. It must diversify and broaden its appeal, as consumers obtain information and entertainment from an ever-wider array of platforms. The channel's remit will need updating to reflect the multiple ways in which its audiences can access its programmes and content. And, ultimately, it will require a new funding solution to replace its gifted analogue spectrum. But whatever formula is agreed, its editorial independence must be protected.

Channel 4 has been a pioneer from the start, and remains a huge force for good, despite its mistakes. It pushes the boundaries in every genre: from news, to documentaries, from comedy to film. It is staffed by outstanding people and supplied by inspired programme makers. It helps keep other public broadcasters up to the mark and generates enormous added value for the creative economy in Britain. I sincerely hope Channel 4 prospers, and is around to celebrate its 50th anniversary in 2032.

25 YEARS OF CHANNEL 4 CONTROVERSY

* Minipops, 1983

The fledgling channel was attacked for putting girls in adult clothes and make-up and getting them to perform erotic routines to chart-topping hits. After a public furore the second series was promptly shelved.

* Red Triangle, 1986

Channel 4 tried to screen 18-rated films late at night if they placed a content warning on screen. Films featured scenes of cannibalism and incest. The policy, which appeared to generate larger audiences, was attacked by sections of the media and by TV watchdog Mary Whitehouse.

* Brookside, 1993

The lesbian kiss storyline, above. The Liverpool-based soap caused a major controversy by showing the first lesbian on-screen kiss before the watershed. Channel 4 was heavily criticised, though the scene turned out to be the making of fledgling actress Anna Friel.

* Brookside 1996, The incest storyline. Brother and sister Nat and Georgia Simpson, above, were found in bed together by their younger brother. One MP called on viewers to "complain in their millions" but the soap's creator, Phil Redmond, said it had broken the "last television taboo".

* Queer as Folk, 1999

This ground-breaking drama caused a storm by featuring a 15-year-old boy being courted by a 29-year-old man. It drew 160 complaints – more than any other TV programme previously.

* Brass Eye, 2001

Satirist Chris Morris's 'Brass Eye' special, above, about paedophilia provoked hundreds of complaints. Some viewers believed that the programme condoned paedophilia, mocked victims and had duped celebrities Phil Collins and Kate Thornton by involving them in the show. The Independent Television Commission ordered Channel 4 to broadcast an apology.

* Russian Roulette, 2003

Three million viewers tuned into illusionist Derren Brown putting a gun to his head and pulling the trigger. Police claims that the channel had risked copycat incidents and promoted gun culture were rejected by the Broadcasting Standards Commission.

* The Séance, 2004

Derren Brown's live 'seance', above, became one of the most complained about shows in broadcasting history, inviting 12 people to contact one or more members of an alleged teenage suicide pact. The programme was cleared of any wrongdoing.

* Celebrity Big Brother, 2006

The racist bullying of Bollywood actress Shilpa Shetty, above, by her fellow celebrity housemates sparked an international incident and a record 54,000 complaints. Questions were raised in Parliament, effigies were burned in India and crowds were banned from attending Jade Goody's eviction. Ofcom, the media regulator, ruled that Channel 4 had been guilty of "serious editorial misjudgements" in its handling of the episode. Channel 4 has announced that it is axing the show for 2008.

* Diana: The Witnesses In The Tunnel, 2007

In spite of objections from Prince William and Prince Harry's saying it would be a 'gross disrespect' to their mother's memory, Channel 4 broadcast photographs taken after the fatal car crash.

* You Say, We Pay: Richard and Judy, 2007

Richard And Judy show systematically cheated viewers out of tens of thousands of pounds in a premium-rate phone quiz scam when 15,000 viewers were unwittingly encouraged to pay £1 a time to enter the daily competition after potential winners had been chosen. Punters paid out an estimated £1m.

* Time Team, 2007

Historical performer, Paul Allen is accidentally killed in a joust filmed for the programme 'Time Team'

Research by Robert Friedrich