"Five quid on the clown"

It's reminiscent of the feudal knight taking on Death at chess in Ingmar Bergman's The Seventh Seal ... only with noughts and crosses, in colour, after the pubs shut.

This tale of chalk, sweat and leers will be familiar to any Briton who has ever switched on a TV.

Test Card F - an eye-baffling arrangement of lines and coloured bars with the photo of Carole Hersee mid-game at its centre - has graced British TV screens since 1967.

The BBC card has clocked up a serious chunk of airtime. Transmitted when TV stations closed down for the night and during the working day when no programmes were broadcast, Test Card F has been shown for an estimated 70,000 hours.

F for ever

Even the longest running TV soap operas can only boast a fraction of this airtime.

That dear old F has lasted for 34 years is somewhat surprising. The preceding five test cards displayed all the staying power of a Spinal Tap drummer.

Test Card F, one of TV's innocent pleasures

Malcolm O'Neil, of fan club The Test Card Circle, says F has become a British icon to rival the double-decker London bus and red telephone box.

Not bad for something intended to check the synchronisation, saturation, linearity and convergence of your TV picture.

Prior to the advent of the colour signal, professionals installing TV sets happily made do with coolly scientific test cards for fine-tuning reception.

Old grey TV test

Colour TV presented new problems. It was deemed necessary to include a picture of a person on the new test card so adjustments could be made for flesh tones.

BBC engineer George Hersee dashed off some snapshots of his daughters, Carole and Gillian, for a mock-up of Test Card F, not thinking either of the girls would actually make it to the screen.

"No flares?!? I'll show them!"

But replacing the girls with an adult model was deemed too risky. The fickleness of grown-up fashions could mean costly updating if, God forbid, flares and caftans were ever consigned to the back of the wardrobe.

So back into the photographer's studio went Carole. The result was the now famous noughts and crosses picture.

Charming though they are, the props surrounding Carole - the stuffed toy and blackboard - serve a purpose.

Central issue

The chalk cross was cunningly located to help engineers centre your TV picture. Mr Hersee once admitted the BBC considered using an Asian model with a bindi forehead marking to fulfil this role.

However, the white cross on its black background proved more versatile. With colour images made up of overlapping red, green and blue, poor "convergence" of these shows up as "colour fringing" on the cross.

Even the somewhat unsettling clown has a technical purpose. Colour TVs filter red, green and blue signals from the black and white ones and decode them separately.

"It's a chrominance delay error, look at the buttons"

This process causes the colours to be delayed. If black and white aren't properly held back too your TV suffers a dreaded chrominance/luminance delay error.

If that is the case, the yellow of the clown's buttons jumps to the right, leaving the buttons themselves plain white.

Test Card F has travelled around the world - with some foreign broadcasters choosing to replace Carole with a local child.

Carole Hersee, now a grown-up mother of two, has long wished her youthful image would vanish from British screens too. The photo earned her a mere £100 (about 0.01p per hour of transmission) and the scorn of her schoolmates.

Cross girl

"I became fed up hearing about it. It should have gone years ago."

Though Test Card F has been adapted for use in the digital, widescreen age, it is slowly disappearing from our screens thanks to our evolving 24-hour culture and the video recorder.

"Uh-oh, the screen's gone blue. Get the test card!"

With BBC One and Two running programmes through the night, the test card has been squeezed from schedules. It is now only used as an emergency stopgap.

"The BBC is in the business of broadcasting programmes not the test card," says a long-serving corporation engineer.

Acerbic TV critic Victor Lewis-Smith is not so sure Test Card F's passing is a sign of progress. He says the quality of some of today's TV offerings "frequently make me long for its return".