SCOTLAND YARD issued an artist's impression last week of a woman wanted for questioning about the recent terrorist bombing outside the Israeli embassy in Kensington. Many people vaguely recognised her. I vaguely recognised her - and then realised that the picture did not resemble anyone but was a mixture of Middle Eastern features.

The suspect is said to be aged between 55 and 60. The picture appears to be of someone younger, though perhaps suffering from a serious illness. Scotland Yard says it is 'very pleased' with the response to the picture and will comment no further.

Artists' impressions are seldom a great success, for this is a visual art dependent on verbal skills. The veracity of the image depends on interaction between the artist and the person describing the suspect. The artist must try to interpret a description without making suggestions that will influence that description.

Photofit is an updated version of the artist's impression, with photographed features, eyes, mouths, noses and face shapes that can be pieced together.

Introduced by the Home Office in 1970, it has some 800 pieces, giving a fairly limited range of possibilities. With few sets of women's features, and hardly any for people of Oriental, Asian or Latin appearance, some police forces still prefer to use artists.

Some of the most notable recent successes with images have been due to Electronic Facial Identification Technique - E-Fit - systems, otherwise known as videofit. The pounds 7,000 system, developed by a computer company working with Aberdeen University and the Home Office, generates likenesses on a computer screen.

Its great advantage is that it is based on witnesses' verbal descriptions. Details such as 'big nose, staring eyes, mean mouth and lantern jaw' are typed in: the computer shows a finished picture. After this, minute adjustments can be made. The system has a much larger set of features, carrying 400 hairstyles alone.

The E-Fit of the serial killer of homosexuals, Colin Ireland, was almost completely accurate. The head shape, hairline, eyes and mouth are clearly recognisable. Only the nose is a little too wide.

So concerned was Ireland by the E-Fit and a security camera video picture of him walking with one of the victims that he turned himself in to the police in the hope of brazening it out.

Among the E-Fit system's other successes was an image that helped to track down the abductor of baby Alexandra Griffiths. Another image led to a suspect in Cambridge recently giving himself up after seeing an E-fit of himself on television.

The results using traditional methods have been less impressive. The old-fashioned photofit of the 'Yorkshire Ripper', Peter Sutcliffe, is a more handsome, less saturnine prospect than Sutcliffe turned out to be when arrested. His hair was completely different and Sutcliffe sported a beard where the photofit had none.

The photofit closest to Julie Kelley, 22, released on bail last week charged with abducting baby Abbie Humphries, looked more like the late actor Richard Beckinsale than the alleged kidnapper.

Twenty-six of the country's 44 police forces now use E-Fit. Sergeant John Royle, of the Greater Manchester police, who is one of Britain's most experienced users of the system and an adviser to the Association of Chief Police Officers' identification working party, said: 'There have been lots of cases where E-Fit has worked out very well.

'Officers can now take a laptop and printer, carry out a cognitive interview with a victim and print an image there and then.

'Police artists are being phased out except in exceptional circumstances.'

No independent study has been made of the effectiveness of such pictures in crime appeals, but Professor Peter Vanezis, head of the forensic science unit at Glasgow University, says it is time for a study of such systems.

'We're able to go into court and give reasonable evidence on security video images,' he said. 'Facial recognition is a different matter entirely. There you are looking at being able to get an accurate picture out of a witness. You've got to be able to extract information without leading the witness on.'

(Photographs omitted)