These devices were built for the purpose of seeing the Others when they were inhabiting human hosts. At the execution room, there was a giant version of this device to show what seemed to be a leech attaching itself to the brain to control the human. All new potential recruits were made to look into them to see if they were capable of killing the enemy despite them looking human.
Detection Scope - Others

Detection Scopes in the Execution Room

The devices were soon made compact and put into helmets so that the soldiers on the field could easily differentiate between friend and foe on the battlefield. Should it hone in on someone who was an Other than the person would glow green. Every member of Squad 53 was given one to use on their first combat assignment.

Others detection devices

The devices placed in soldier helmets

However, it was found that these devices were fakes. What they displayed was a mere illusion. The mobile versions shined a person green if they didn't have a tracker injected into them in order to fool the troops into thinking that they were attacking an Other rather than a fellow surviving human.

It was shown that these devices could be fooled by having the trackers stuck onto their necks or put in their mouths.