The emotion detector evaluates the emotional state of the person it is directed at. That way you can detect what a person is really feeling, when you suspect they are lying just to be polite.
Since the target person's emotional state is likely to change if they see they are being scanned (see the observer effect), the detector needs to be of a discreet size and must operate silently. And it must report its results in an unobtrusive manner, in a manner unobservable to the target. Perhaps different colored lights can arise above the target's head, with different colors indicating a different mood, much like mood rings of the late twentieth century, but more accurate.