Are you afraid of vampires? Me too. That's why I invented this detector.
Here is how it works:
You know that the vampire's image has no reflection, so they are invisible when you look them in the mirror.
Now look at my drawing - this is a small box, but here it is drawn without the cover, to show the principle of operation.
You look through the hole A and you see the scene which is reflected from mirrors D and E (blue arrow, A-B). Vampire is, of course, invisible for you, but when you press the button F, then you see the raw scene without the mirrors (red arrow C), and you see the vampire! So all you have to do is to press and release the button F repeatedly (there is the spring, not visible here) and all the people will stand steady, except the vampire, which will blink!
(For those who took this too seriously: I am joking)
(Or am I?)