The Active Denial System weapon, classified as "less lethal" by the Pentagon, fires a 95-gigahertz microwave beam at rioters to cause heating and intolerable pain in less than five seconds.
The idea is people caught in the beam will rapidly try to move out of it and therefore break up the crowd.
But New Scientist magazine reported on Wednesday that during tests carried out at Kirtland Air Force Base in New Mexico, participants playing the part of rioters were told to remove glasses and contact lenses to protect their eyes.
In another test they were also told to remove metal objects like coins from their clothing to avoid local hot spots developing on their skin.
"What happens if someone in a crowd is unable for whatever reason to move away from the beam," asked Neil Davison, coordinator of the non-lethal weapons research project at Britain's Bradford University.
"How do you ensure that the dose doesn't cross the threshold for permanent damage? Does the weapon cut out to prevent overexposure?" he added.
The magazine said a vehicle-mounted version of the weapon named Sheriff was scheduled for service in Iraq in 2006 and that US Marines and police were both working on portable versions.