It has become an iconic image of the coronavirus outbreak in China: a masked official aiming what appears to be a small white pistol at a traveller’s forehead.

For weeks, this ominous-looking device has been deployed at checkpoints across China — tollbooths, apartment complexes, hotels, grocery stores, train stations — as government officials and private citizens screen people for fevers in an effort to prevent the spread of the coronavirus.

Sometimes described as a “thermometer gun,” the device is equipped with an infrared sensor that can quickly measure surface temperature without making any contact with a person’s skin.

But for all of its powerful sensing technology, the thermometer has ultimately proved to be an ineffective defence mechanism, according to medical officials and experts on infrared devices. Like the surgical masks that have become ubiquitous in China, thermometer guns tend to be unreliable outside carefully controlled health care settings.