Technology: Solved: the mystery of the speckled note

 作者:董最斌     |      日期:2019-02-28 01:14:01
By MICK HAMER There is a classic whodunit plot that tracks down the criminal by matching a typed ransom note or forged letter to the machine that produced it. In real life too, flaws on the keys of typewriters have been used by forensic scientists to trace the source of a document. Five years ago such evidence helped to convict a man accused of typing ransom notes after putting glass in baby foods. Now the technique has been brought up to date by the Metropolitan Police Forensic Science Laboratory in London, which has shown that microscopic marks on laser-printed documents can help to identify their source. Forensic scientists have had to keep up with technological change as laser printers have replaced typewriters, says Alan Filby, who heads the laboratory’s document section. The newly developed identification technique is expected to be used shortly by forensic scientists in British courts, and is likely to become increasingly common. The work is reported in the current issue of the Journal of the Forensic Science Society. Laser printers contain a rotating light-sensitive drum that transfers toner onto the paper. Any defect in the photosensitive coating on the drum’s surface results in tiny specks appearing on documents it prints. The drum in one common printer mechanism, used by manufacturers such as Canon and Hewlett-Packard, is 94 millimetres in circumferencel, so the drum revolves about 3.2 times while printing a standard 297-millimetre sheet of A4 paper. Any flaw on the surface of the drum will therefore be repeated three or four times on each sheet of paper in a vertical line. In the police tests, the size of the flaws ranged up to 0.6 millimetres, and so the chance of a defect turning up at any horizontal position on an A4 sheet (which is 210 millimetres wide) is about 1 in 350. Steve Day, who carried out the original research and is now head of the document service at the government’s Forensic Science Service in Huntingdon, says that with two or more faults on the drum the source of a document ‘is almost certain’, especially if the paper also has marks from the rollers. The technique can also establish whether all the pages came from the same printer: if a sheet in the middle of a document does not have the same flaws as the rest it could not have been printed in sequence. Tests carried out by Day and his colleagues show that dust and other dirt do not leave a mark on documents. Brushes in the printer stop such particles reaching the drum. Only about 60 per cent of laser printers have flaws on their drums, so the technique is not as universally applicable as imperfections in the type of a typewriter. But the absence of marks could help to eliminate a suspect printer. A further complication is that the drum in many laser printers forms part of the toner cartridge, which has to be replaced from time to time. In these machines, the telltale pattern of specks will change or disappear when a new toner cartridge is fitted. Day says that in a typical office printer the toner cartridge lasts about three months. If the old cartridge is thrown away, the evidence linking a document to the machine that produced it will be lost. Also, cartridges can be refilled with toner and reused: