Jewellers in Leicester have been fitting smoke security devices following a series of robberies.
The smoke screen fills shops with thick smoke, making it impossible for criminals to see anything.
Twice this year the devices have been activated at jewellery shops in Belgrave Road and on both occasions the raiders left empty-handed.
Leicestershire police recommended the devices to retailers following a number of recent jewellery store robberies.
Sgt Sarah Widdowson, from Belgrave neighbourhood policing team, said: "When the device goes off it absolutely fills the shop - you cannot see anything.
"We want would-be offenders to know that if they target Leicester jewellers, the chances are that they will get nothing.
Romil Patani, a spokesman for the jewellers, said many of the shops were using smoke devices.
He added: "When it goes off, you can see no more than an inch in front of you. It should be a good deterrent.
"All the shops have been upgrading their security - we are working with the police, and the police are working with us."