By Patrick Jackson
The US military revealed a heat-ray gun, the Active Denial System (ADS), to reporters this week.
The heat gun's dish could make a target for an RPG
The technology brings a new, more disorientating dimension to crowd control.
Rioters know where they are with a water cannon: they can see where the cooling is coming from.
Likewise, tear gas smokes before it stings and baton rounds are meant to bounce before they hit the crowd.
A millimetre-wave beam is different: a hot blast which, at a maximum range the Pentagon says is 10 times greater than that of other "non-lethal weapons", effectively comes out of nowhere, silently and invisibly.
Longer, lighter, simpler
"Imagine you're a marine guarding your post and you see some suspicious-looking people coming towards you at a distance," said Susan LeVine, principal deputy of the Joint Non-Lethal Weapons (JNLW) Directorate which tested the system.
RIOT CONTROL MILESTONES
1958: British Army use CS tear gas in Cyprus
1960s: Lorry-mounted water cannon used in US
1960s: UK uses baton rounds - wood, rubber, finally plastic
1980s: Pepper spray - a bear repellent - adopted by US police forces
"You will be able to engage them at a point well beyond small-arms range so that you can give them a clear signal to stop," she told the BBC News website.
Bill Sweetman, technology and aerospace editor for Jane's Information Group, believes the primary purpose of the heat-ray gun will be to disperse a crowd which could be concealing gunmen.
The beam, he says, has advantages over existing non-lethal weapons other than range:
- it is more economical, as you can keep generating power pulses in different directions while there is petrol in the generator
- it is less indiscriminate than tear gas and less cumbersome than water cannon
- it is more accurate as it travels at the speed of light and is not subject to the effect of wind
'Not to be trusted'
The heat beam may be an advance on the water jet but it is causing alarm for other reasons.
"What happens when people are in the first rows of a dense crowd and cannot flee?" asks Dr Steve Wright, associate director of Leeds Metropolitan University's Praxis Centre, which studies conflict resolution technology.
"How do subjects exposed from a distance know where to flee from the beam?
"People hit the pain waves and don't know which way to run."
Such a weapon also has the potential to cause panic and deadly stampedes, Dr Wright says.
He is also concerned that America is developing weapons of "tuneable lethality" whereby "you can tune in the amount of pain the weapon provides, from heating to death".
Put to the test
Alan Fischer, media relations manager of Raytheon, which built the ADS as well as making its own commercial version Silent Guardian, is concerned that some people have been likening the technology to a microwave oven.
Some of the confusion may arise from the fact that Raytheon built the first microwave oven back in 1947.
The millimetre wave may, like microwaves and radars, operate in the radio frequency spectrum but it is
"only designed to go a very shallow distance into the skin", Mr Fischer told the BBC News website.
"This has nothing to do with microwaves or microwave cooking or anything like that," he says.
Dr Wright asks if Pentagon tests on healthy service volunteers adequately reflect the potential effect on pregnant women, children and babies.
Ms LeVine, one of the 600-odd people exposed to the beam in tests, says that health tests have been rigorous:
"We've looked at the risk of injuries, at the risk of skin cancer, birth defects, impact on fertility and everything has proved to be negative."
Chinks in the armour?
But how vulnerable might it be in the field to what the Pentagon calls "counter-measures"?
Dr Wright suggests that something as simple as household foil and "a fine metal mesh in front of the eyes" could counteract it.
Attempts to get around the beam would only prove its value, Ms LeVine argues.
"The point of ADS is to assess intent so if somebody is coming at you and they have knocked up something that clearly shows they are going to try and get by this beam, the system has already done its job," she says.
Bill Sweetman questions whether the Humvee-mounted version of the ADS - a "pretty obvious target" - would be vulnerable to a rocket-propelled grenade.
As far as Ms LeVine is concerned, "a lot of vehicles would be vulnerable to an RPG".
But the Jane's editor is not convinced the heat-ray gun will prove a decisive weapon.
"It is a bit of a uni-tasker and my feeling is that uni-taskers of one kind or another seldom cause military revolutions," he says.
It may serve its military purpose well enough, Mr Sweetman adds, but law enforcement is a different story.
"I don't think you would use this unless you thought there was a risk of the other side escalating it into lethal force," he says.
"I don't think you would use this against a bunch of Millwall football fans on the rampage."
HOW HEAT-RAY GUN WORKS
1 360-degree operation for maximum effect
Antenna, linked to transmitter unit, can be mounted on vehicle
Automatic target tracking
2 Antenna sealed against dust and can withstand bullet fire
3 Invisible beam of millimetre-wave energy can travel over 500m
4 Heat energy up to 54C (130F) penetrates less than 0.5mm of skin
Manufacturers say this avoids injury, although long-term effects are not known