A local expert has challenged video evidence used in the speeding case of tycoon Peter Lam Kin-ngok, saying flaws it highlighted would not occur if guidelines on the use of laser guns were followed. The video of British experts testing a laser gun showed the devices could make mistakes if misaligned or the laser beams were reflected. Tam Wing-yim, a Hong Kong University of Science and Technology laser-gun expert who tests them for police, described the flaws as 'scary' after watching the video in the Legislative Council transport panel yesterday. 'A misalignment like the one shown in the video clip should have been identified by the officer and the laser gun returned for maintenance,' he said. In the video, a laser gun is pointed at a wall and a reading suggests the wall is moving. Instead, the reading is a result of the officer moving from his initial position. Inaccurate readings were also recorded when the gun was used 400 metres from the target. 'Several tests in the video may have been set up for a certain purpose, but it did not show what happens when the gun is used properly,' Dr Tam said. He said that in Hong Kong, laser guns were mostly used within 100 metres of their targets. The guns are also returned to the manufacturer for maintenance every six months. Questions about the guns were raised after Mr Lam successful challenged their accuracy in court. He was allegedly clocked doing 114km/h in a 50km/h zone on Lantau, but pleaded guilty to driving at 79km/h. Since then, 417 motorists have asked police to review their cases. Traffic Chief Superintendent Michael Demaid-Groves said more than 60 per cent of review requests had been processed, with two tickets withdrawn due to clerical errors. In another 104 requests, for which the fixed-penalty fines had been paid, the six-month period to appeal against the penalty in court had passed. Legal advice on whether there was any basis to proceed further with these cases was expected by the end of the month. A working group set up to review procedures on the use of laser guns had established recommendations, Mr Demaid-Groves said. The log sheet format, together with the use of tripods and the procedures on laser gun storage, have been standardised, while all tests will be conducted in accordance with the manufacturers' manual. 'Officers will have to receive sufficient training before using the laser guns, and they must now return the guns to the gun storage room after operations,' he said. Lawmaker Wong Kwok-hing said tickets issued on the same day based on the same laser gun used in Mr Lam's case should be waived, while Civic Party lawmaker Audrey Eu Yuet-mee said the force must ensure officers follow the guidelines.