A modern state needs to identify its citizens constantly. Many of them are a potential threat — benefit fraudsters, terrorists, drug pushers, bogus asylum-seekers, élitists, smokers. The citizens even need to identify themselves, both to the authorities and, in their financial dealings, to each other. Yet identity cards are easily stolen, fingerprinting is messy, DNA sampling is intrusive, and none of them work at a distance. Daedalus now has a new idea.

Portraits taken with small cheap cameras are often spoiled by ‘red-eye’. The light from the flash enters the eyes of the subject, is reflected from the retinal blood-vessels, and makes his or her pupils appear red in the photograph. What a splendid way, says Daedalus, of obtaining the blood spectrum of a distant subject! DREADCO opticians are devising a camera to maximize the red-eye effect, and to record its spectrum over all the wavelengths that can enter the eye, from near ultraviolet to near infrared. One team is combining a frequency-swept flash with time-resolved charge-coupled-device imaging; another favours a broad-band flash source and a dispersive or Fourier-transform spectrometric detector. Meanwhile, the company's biochemists are exploring the individuality of a blood spectrum.

For a start, it should encode detailed blood-group data: not merely A, B and O, but all the dozens of lesser blood antigens. The plasma polysaccharides and proteins will also tell their story, partly hereditary and partly medical. Indeed, direct indices of criminality, such as metabolites of alcohol, cocaine or nicotine, should show up usefully in the blood spectrum. While not as specific as DNA analysis, red-eye spectrophotometry should still be a powerful identifier. The blood spectrum of every citizen will rapidly be acquired from normal passport or identification-card photography. The resulting database will transform surveillance.

Motorists in speeding cars, rioters in the street, burglars entering protected premises, all will be literally identified in a flash. In daylight the flash might not even be noticed. Counter-measures are possible; but a modern data-dictatorship is used to outflanking them. Just as the police stop any car without a licence plate, and the British government, in its plans to intercept the whole nation's e-mail, will demand that we decrypt for it any message it can't manage to decrypt for itself, so the authorities will arrest anyone wearing dark green glasses.